Biomolecules and Polymers, Class 12, Chemistry Detailed Chapter Notes PDF Download

Carbohydrates and their Classification - Biomolecules, CBSE, Class 12, Chemistry

CARBOHYDRATES:

1. Introduction:

Carbohydrates received their name becasue of their general formula C_x(H₂O)_y, according to which they appear to be hydrates of carbon.

xCO₂  yH₂O  C_x(H₂O)_y  xO₂

 

Photosynthesis

6CO₂  6H₂O 18 ATP  C₆H₁₂O₆  6O₂

 

Cellular Respiration

C₆H₁₂O₆  6O₂  6CO₂  6H₂O 38 ATP (36 ATP net gain)

 

 

2. Classification and structure of Carbohydrates:

Carbohydrats are polyhydroxy aldehydes and ketones and substances which hydrolyse to polyhydroxy aldehydes and ketones.

The simplest carbohydrates are called sugars or saccharides, (Latin: Saccharum, sugar). Carbohydrates can be classified as monosaccharides, oligosaccharides and polysacchaides.

 

3. General Characteristic of Monosaccharides:

The important characteristics of monosaccharides as follows:

(i) All monosaccahrides are water soluble due to the presence of hydrogen bonding between the different OH groups and surrounding water molecules.

(ii) Monosaccharides have sweet taste and upon heating they get charred and give the smell of burning sugar.

(iii) Monosaccharides are optically active in nature due to the presence of chiral carbon atoms.

(iv) The chemical characteristics of monosaccharides are due to OH groups and carbonyl group which may be either aldehydic or ketonic group.

Glyceraldehyde contains one asymmetric carbon atom (marked by an astrisk) and can thus exist in two optically active forms, called the D-form and the L-form. Clearly, the two forms are mirror images that cannot be superimposed, that is they are enantiomers.

 

D-Glyceraldehyde L-Glyceraldehyde

All four isomers have been prepared synthetically. The D-and L-erythrose are mirror images, that is, they are enantiomers. They have exactly the same degree of rotation but in opposite directions. Equal amounts of the two would constitute a racemic mixture, that is a mixture that would allow a plane-polarised light to pass through the solution unchanged.

Supplying hydrogen atoms to the five carbon atoms to satisfy their tetravalency, following structure (open chain) may be assigned to glucose: (* indicates assymetric carbon atom).

====================================================== 

Structure of Glucose - Biomolecules, CBSE, Class 12, Chemistry

6. Configuration* of Glucose:

Since the above structure possesses four asymmetric carbon atoms (shown by asterisks), it an exist in 2⁴ = 16 optically active forms, i.e., eight pairs of enantiomers. All these are known and correspond to the D- and L-forms of glucose, mannose, galactose, allose, glucose, idose and talose.

The naturally occurring dextrorotatory glucose ( )-glucose is only one of the 16-stereoisomers.

 

D( )Gucose ( )-Glyceradehyde

Notations D- and L- for denoting configuration were given by Rosanof; according to this convention any compound whose bottom asymmetric carbon atoms has the configuration similar to the configuration of dextrorotatory glyceraldehyde (drawn above, i.e. the bottom carbon atom has -OH to the left and H to the right is given L-configuration. Remember that the symbols D-and L- have no relation with the specific rotation value, i.e., with ( ) or (-) value. For example, the natural (-) fructose belongs to D-series, i.e., it is D(-)-fructose)

 

7. Objections to open-chain structure of glucose:

Even through open chain structure of ( ) glucose explains most of its reactions, it fails to explain the following facts about it.

i) Glucose does not restore Schiff's reagent colour.

ii) Glucose does not form a bisulphite and aldehyde-ammonia compound.

iii) Glucose forms two isomeric penta-acetates neither of which reacts with carbonyl reagents.

iv) The existence of the two isomeric glucoses and the change in specific rotation (mutarotation) is not explained by an open-chain formula.

v) Glucose reacts with methanol in presence of dry HCl gas to form two isomeric glucosides.

Since glucose is less soluble in ethanol, it separates out on cooling the reaction mixture. Commercially, it is obtained by the hydrolysis of starch which is available from relatively inexpensive source such as maize, potatoes and rice.

 

Constitution of Glucose:

1. Molecular Formula: By the usual analytical methods, the molecular formula glucose is found to be C₆H₁₂O₆.

2. Straight Chain of six carbon atoms:

i) Reduction of glucose with conc. HI and phosphorus gives 2-iodohexane and n-hexane. This indicates that six carbon atoms in glucose are present in a straigth chain

C₆H₁₂O₆ CH₃ - (CH₂)₄ - CH₃

Glucose n-hexane

ii) Glucose when oxidized with bromine water gives gluconic acid which when reduced with excesss of HI gives n-hexanoic acid, CH₃.(CH₂)₄.COOH confirming the presence of a straight chain of six carbon atoms in glucose.

 

3. Presence of five hydroxyl groups: When treats with acetic anhydride, glucose forms penta-acetate indicating the presence of 5 _ OH groups and since glucose is a stable compound, the five -OH groups must be attached to 5 different carbon atoms.

 

4. Presence of an aldehydic groups:

i) Glucose forms a cyanohdrin with hydrogen cyaide and a mono-oxime with hydroxylamine suggesting the presence of a carbonyl group.

ii) Glucose reduces Fehling solution and Tollen's reagent indicating that the carbonyl group is aldehydic in nature.

iii) The presence of aldehydic gruop in glucose is confirmed by its oxidation to gluconic acid having the same number of carbon atoms.

Now since aldehydic group is monovalent, it must be present on the end of the chain.

 

5. Open chain structure: On the basis of the above points, glucose may be assigned following prt structure orientation shown in the a anomer has the -OH trans to the -CH₂OH group and the b anomer has the -OH cis to the -CH₂OH group.

===========================================================

Structure Formulas for Monosaccharides - Biomolecules, Chemistry, Class 12

Structure formulas for monosaccharides:

Although many of the properties of D( )-glucose can be explained in terms of an open-chain strcuture (1, 2, or 3), a considerable body of evidence indicates that the open-chain strcuture exists, primarily, in equilibrium with two cyclic forms. These can be represented by structures 4 and 5 or 6 and 7. The cyclic forms of D( )-glucose are hemiacetals formed by an intramolecular reaction of the -OH group at C5 with the aldehyde group. Cycliation creates a new stereogenic centre at C1, and this stereogenic centre explains how two cyclic forms are possible. These two cyclic forms are diastereomers that different only in the configuration of C1. In carbohydrate cheistry diastereomers of this type of called anomers, and the hemiacetal carbn atom is called the anomeric carbon atom.

Structures 4 and 5 for the glucose anomers are called Haworth formulas and, although they do not give an accuate picture of the shap of the six-membered ring, they have many practical uses. Demonstrates how the representation of each stereogenic centre of the open-chain form can be correlated with its representation in the Haworth formula.

Each glucose anomer is designated as an a anomer or a b anomer depending on the location of the -OH gruop of Cl. When we draw the cyclic forms of a D sugar in the

 

Mutarotation:

Ordinary D( )-glucose has a melting point of 146°C. However, when D( )-glucose is crystallized by evaporating an aqueous solution kept above 98°C, a second form of D( )-glucose with a melting point of 150°C can be obtained. When the optical rotations of these two forms are measured, they are found to be significantly differnt, but when an aqueous solutin of either form is allowed to stand, its rotation changes. The specific rotation of one form decreases and the rotation of the other increases, until both solutions.

show the same value. A solution of original D-( )glucose (mp 146°C) has an initial specific rotaiton of 112°, but, ultimately, the speciic rotation of this solution falls to 52.7°. A solution of second form of D( ) glucose (mp 150°C) has an initial specific rotation of 18.7°, but slowly, the specific rotation of this solution rises to 52.7°. This change in rotation towards an equilibrium value is called mutarotation.

The explanation for this mutarotation lies in th existence of an equilibrium between the open-chain form of D( ) gucose an the a and b forms of the cyclic hemiacetals.

 

Conversion to esters:

Treating a monosaccharide with excess acetic anhydride and a weak base (such as pyridine or sodium acetate) converts all of the hydroxyl groups, including the anomeric hydroxyl, to ester groups. If the reaction is carried out at a low temperature (e.g., 0°C), the reaction occurs stereospecifically; the a anomer gives the a-acetate and the b anomer gives the b-acetate. Acetate esters are common protecting groups for carbohydrate hydroxyls.

 

BENEDICT'S OR TOLLENS' REAGENTS: REDUCING SUGARS

Benedict's reagent (A alkaline solution containing a cupric citrate complex ion) and Tollen's solution [Ag (NH₃)₂OH] ^_ oxidize and thus give positive tests with aldoses and ketoses. The tests are positive even though aldoses and ketoses exist primarily as cyclic hemiacetals.

 

Sugars that give positive tests with Tollen's or Benedict's solutions are known as readucing sugars, and all carbohydrates that contain a hemiacetal group given positive tests. In aqueous solution these hemiacetals exist in equilibrium with relatively small, but not insignificant, concentratiosn of noncyclic aldehydes or a-hydroxy ketones. It is the latter two that undergoes the oxidation until one reactant is exhausted.

Carbohydrates that contain only acetal groups do not given positive tests with Benedict's or Tollen's solutions, and they are called non-reducing sugars. Acetals do not exist in equilibrium with aldehydes or a-hydroxy ketones in the basic aqueous media of the test reagents.

 

Bromine Water : THe synthesis of Aldonic Acid

Monosaccharides do not undergo isomerization and fragmentation reactions in mildly acids solution. Thus, a useful oxidizing reagent for preparative purposes is bromine in water (pH 6.0). Bromine water is a general reagent that selectively oxidizes the -CHO group to a -CO₂H group. It converts an aldose to an aldonic acid.

Experiments with aldopyranoses have shown that the actual course of the reaction is somewhat more complex than we have indicated above. Bromine water specifically oxidizes the b anomer, and the initial product that forms is a d-aldonolactone. This compound may then hydrolyze to an aldonic acid, and the aldonic acid may undergo a subsequent rign closure to form a g-aldonolactone:

 

NITRIC ACID OXIDATION: ALDARIC ACID

Dilute nitric acid _ a stronger oxidizing agent than bromine water oxidizes the both -CHO group and the terminal -CH₂OH group of an aldose to -CO₂H groups. These dicarboxylic acids are known as aldaric acids:

   or 

This aldaric acid obtained from D-glucose is called D-glucoaric acid.

 

PERIODATE OXIDATIOSN: OXIDATIVE CLEAVAGE OF POLYHYDROXY COMPOUNDS

Compounds that have hydroxyl groups on adjacent atoms undergo oxidative cleavage when they are treated with aqueous periodic acid (HIO₄). The reaction breaks carbon-carbon bonds and produces carbonyl compounds (aldehydes, ketones, or acids).

  HIO₄ ¾¾¾® 2  HIO₃  H₂O

Since the reaction usually takes place in quantitative yield, valuable information can often be gained by measuring the nubmer of molar equivalents of periodic acid that is consumed in the reaction as well as by identifying the carbonyl products.

Periodate oxidations are though to take place through a cyclic intermediate:

 

Before we discuss the use of periodic acid in carbohydrate chemistry, we should illustrate the course of the reaction with several simple examples. Notice in these periodate oxidations that for every C - C bond broken, a C - O bond is formed at each carbon.

1. When three or more -CHOH groups are continuous, the internal ones are obtained as formic acid. Periodate oxidation of glycerol, for example, gives two molar equivalents of formuladehyde and one molar equivalent of formic acid;

 

2. Oxidative cleavage also take place when an - OH group is adjacent to the carbonyl group of an aldehyde or ketone (but not that of an acid or an ester). Glyceradehyde yields two molar equivalents of formic acid and one molar equivalent of formaldehyde, while dihydroxyacetone gives two molar equivalents of formaldehyde and one molar equivalent of carbon dioxide.

Periodic acid does not cleave compound in which th hydroxyl gruops are separated by an intervening _CH₂ group, nor those in which a hydroxyl group is adjacent to an ether or acetal.

 

REDUCTION OF MONOSACCHARIDES: ALDITOLS

Aldoses (and ketoses) can be reduced with sodium borohydride to compounds called alditols:

Reduction of D-glucose, for example, yields D-glucitol.

 

REACTIONS OF MONOSACCHARDIES WITH PHENYLHYDRAZINE: OSAZONES

The aldehyde group of an aldose reacts with such carbonyl reagents as hydroxylamine and phenylhydrazine. With hydroxylamine, the product is the expected oxime. With enought phenylhydrazine, however, three molar equivalents of phenylhydrazine are consumed and a second phenylhydrazeone group is introduced at C2. The product is called a phenylosazone. Phenylosazones crystallize readily (unlike sugars) and are useful derivatives for identifying sugars.

The mechanism for osazone formation probably depends on a series of reaction in which behaves very much like  in giving a nitrogen version of an enol.

 

A Mechanism for the Reaction

Phenylosazone Formation

   

 

Osazone formation result in a loss of the stereogenic centre at C2 but does not affect other stereogenic carbons; D-glucose and D-mannose, for example, yield the same phenylosazone:

 

 

D-Glucose Same Phenylosazone D-Mannose

This experiment, first done by Emil Fischer, established that D-glucose and D-mannose have the same configuration about C3, C4 and C5. Diastereomeric aldoses that differ in configuration at only one carbon (such as D-glucose and D-mannose) are called epimers. In genral, any pair of diastereomers that differ in configuration at only a single tetrahedral stereogenic carbon can be called epimers.

 

Epimers:

Many common sugars are closely related, differing only by the stereochemistry at a single carbon atom. For example, glucose and mannose differ only at C2, the first asymetric carbon atom. Sugars that differs only by the stereochemistry at a single carbon are called epimers. and the carbon atom where they differ is generally stated. If the number of a carbon atom is not specified, it is assumed to be C2. Therefore, glucose and mannose are "C2 epimers" or simply "epimers". The C4 epimer of glucose is galactose and the C2 epimer of erythrose is threose.

 

=========================================== 

Cyclic structure of Fructose - Biomolecules, Chemistry, Class 12

Cyclic structure of Fructose:

Like glucose, fructose also has a cyclic structure. Since fructose contains a keto group, it forms an intramolecular hemiketal. In the hemiketal formation, C5_OH of the fructose combins with C2-keto group. As a result, C2 becomes chiral and thus has two possible arrangements of CH₂OH and OH group around it. Thus,

D-fructose exists in two stereoisomeric forms, i.e., a-fructopyranose and b-D fructopyranose. However in the combined state (such as sucrose), fructose exists in furanose form as shown below:

 

Hydroylsis of Sucrose:

(Invert Sugar or Invertose). Hydrolysis of sucrose with hot dilute acid yields D-glucose and D-fructose.

C₁₂H₂₂O₁₁  H₂O  C₆H₁₂O₆  C₆H₁₂O₆

Sucrose D( )-glucose D(-)-Fructose

[a]_D = 66.5° [a]_D = 53° [a]_D = -92°

Invert Sugar

[a]_D = ( 53°) - (-92°) = 39°

Surose is dextrorotatory, its specific rotation being 66.5%, D-glucose is also dextrorortatory, [a]_D = 53°, but D-fructose has a large negative rotation, [a]_D = -92°. Since D-fructose has a greater specific rotation than D-glucose, the resulting mixture is laevorotatory. Because of this the hydrolysis of sucrose is known as the inversion of sucrose, and the equimolecular mixture of glucose and fructose is known as invert sugar or invertose.

===================================================================

Polysaccharides - Starch,Cellulose and Glycogen - Biomolecules, CBSE, Class 12, Chemistry

POLYSACCHARIDES

Polysacchardies are the polymers of monosacchardies. The natural polysacchardies generally contain about 100-3000 monosacchardie units. The three most abundant natural polysaccharides-cellulose, starch and glycogen are derived from the same monomer, i.e., glucose.

Starch: It is a polymer of glucose. Its molecular formular is (C₆H₁₀O₅)_n where the value of n(200 - 1000) varies from source to source. It is the chief food reserve material or storage polysacchardie of plants and is found mainly in seeds, roots, tubers, etc. Wheat, rice, potatoes, corn, bannans etc., are rich source of starch.

Starch is not a single compound but is a mixture of two components - a water soluble component called amylose (20%) and a water insoluble component called amylopectin (80%). Both amylose and amylopectin are polymers of a-D glucose.

Amylose is a linear polymer of a-D glucose. It contains about 200 glucose units which are linked to one another through a-linkage involving C₁ of one gluose unit with C₄ of the other.

Amlopectin, on the other hand, is a highly branched polymer. It consists of a large number (several branches) of short chains each containnig 20-25 glucose units which are joined together through a-linkages involving C1 of one glucose unit with C4 of the other. The C1 of terminal glucose unit in each chain is further linked to C6 of the other glucose.

unit in the next chain through C1-C6 a-linkage. This gives amylopectine a highly branched structure as shown below:

 

 

Hydrolysis:

Hydrolysis of starch with hot dilute acids or by enzmyes give dextrins of varying complexity, maltose and finally D-glucose. Starch does not reduce Tollen's reagent and Fehling's solution.

Uses:

It is used as a food. It is encountered daily in the form of potatoes, bread, cakes, rice etc. It is used in coating and sizing paper to improve the writing qualities. Strch is used to treat textile fibres before they are woven into cloth so that they can be woven without breaking. It is used in manufacture of dextrins, glucose and ethyl alcohol. Starch is also used in manufacture of starch nitrate, which is used as an explosive.

Cellulose:

Cellulose is the chief component of wood and plane fibres; cotton, for instance, is nearly pure cellulose. It is insoluble in watre and tasteless; it is a non-reducing carbohdrate. These properties, in part at least, are due to extremely high molecular weight.

Cellulose

Cellulose has the formula (C₆H₁₀O₅)_n . Complete hydrolysis by acid yields D( )-glucose as the monosaccharide. Hydrolysis of completely methylated cellulose gives a high yield of 2, 3, 6-tri-O-methyl-D-glucose. Like starch, therefore, cellulose is made up of chains of D-glucose units, each unit joined by a glycoside linkage of C-4 of the next.

Cellulose differs from starch, however, in the configuration of the glycoside linkage. Upon treatment with acetic anhydride and sulfuric acid, cellulose yields octa-O-acetylecllobiose, there is evidence that all glycoside linkages in cellulose, like the one in ( ) cellobiose, are beta linkages.

Physical methods give molecular weights for cellulose ranging from 250000 to 1000000 or more; it seems likely that there are at least 1500 glucose units per molecule. End group analysis by both methylation and periodic acid oxidation gives a chain legnth of 1000 glucose units or more. X-ray analysis and electron microscopy indicate that these long chains lie side by side in bundles, undoubtedly held together by hydrogen bonds between the numerous neighbouring _OH groups. These bundles are twisted togethr to form.

Rope like structure which themselvs are grouped to from the fibers we can see. In wood these cellulose "ropes" are embedded in lignin to give a structure that has been likened to reinforced concrete.

 

Properties of Cellulose:

We have seen that the glycoside linkages of cellulose are broken by the action of acid, each cellulose molecule yielding many molecules of D( )-glucose. Now let us look briefly at reactions of cellulose in which the chain remains essentially intact. EAch glucose unit in cellulose contains three free _ OH groups; these are the positions at which reactions occurs.

These reactions of cellulose, carried out to odify the properties of a cheap, available ready-made polymer, are of tremendous industrial importance.

Like any alcohol, cellulose forms esters. Treatment with a mixture of nitric and sulfuric acid converts cellulose into cellulose nitrate. The properties and uses of the product depend upon the extent of nitration. Guncotton, which is used in making smokeless powder, is very nearly completely nitrated cellulose, and is often called cellulose trinitrate (three nitrate groups per glucose unit). Pyroxylin is less highly nitrated material containing between two and three nitrate groups per glucose unit. It is used in the manufacture of plastics like celluloid and collodion, in photographic film, and in lacquers. It has the disadvantage of being flammable, and forms hihgly toxic nitrogen oxides upon burning.

Industrially, cellulose is alkylated to ethers by the action of alkyl chlorides (cheaper than sulfates) in the presence of alkali. Considerable degradation of the long chain is unavoidable in these reactions. Methyl, ethyl, and benzyl others of cellulose are important in the production of textiles, films, and various plastic objects.

=======================================================

Amino Acids and their Classification - Biomolecules, CBSE, Class 12, Chemistry

 

Amino Acids

1. Intorudction:

Amino acids are the compounds which contain both an amino group and a carboxy group in their molecules. They constitute a particularly imortant class of difunctional compounds as they are the building blocks of proteins.

While several hundred different amino acids are known to occur naturally, 20 of them deserve special mention as they ae preesent in proteins. These amino acids are listed in Table. As given in this table, for amino acids trivial names are common. The convention to use a three letter code, as an abbreviation, for each amino acid is also given in the table. These abbreviations are particularly useful in designating the sequence of amino acids in peptides and proteins which your will study.

 

E = essential amino acid NE = Non essential amino acid

 

AMINO ACID AS DIPOLAR IONS:

Amino Acids contain both a basic group (_NH₂) and an acidic group (_COOH). In the dry solid state, amino acids exist as dipolar ions, a form in which the carboxyl group is present as a carboxylate ion, _CO₂^_, and the amino group is present as an aminium ion, _NH₃  (Dipolar ions are also called zwitterions.) In aqueous solution, an equilibrium exists between the dipolar ion and the anionic and cationic forms of an amino acids.

If alanine is dissolved in a strongly acidic solution (e.g. pH 0), it is present in mainly a net cationic form. In this state the amine group is protonoted (bears a formal 1 charge) and the carboxylic acid group is neutral (has no formal charge). As is typical of a-amino acids, the pK_a for the carboxylic acid hydrogen of alanine is considerably lower (2.3) than the pK_a of an ordinary carboxylic acid (e.g., propanoic acid, pK_a 4.89):

The reason for this enhanced acidity of the carboxyl group in an a-amino acid is the inductive effect of the neighboring aminium cation, which helps to stablize the carboxylate anion formed when it loses a proton. Loss of proton from the carboxyl group in a cationic a-amino acid leaves the molecule electrically neutral (in the form of a dipolar ion). This equilibrium is shown in the red-shaded portion of the equation below.

The protonated amine group of an a-amino acid is also acidic, but less so that the carboxylic acid group. The pK_a of the animium group in alanine is 9.7. The equilibrium for loss of an aminium proton is shown in the blue-shaded portion of the equation below. The carboxylic acid proton is always lost before a proton from the aminium group in an a-amino acid.

The state of an a-amino acid at any given pH is governed by a combination of two euilibrium, as shown in the above equation for alanine. The isoelectric point (pI) of an amino acid such as alanine is the average of pKa₁ and pKa₂;

pI = ½ (2.3 9.7) = 6.0 (isoelectric point of alanine)

When a base is added to a solution of the net cationic form of alanine (initially at pH 0, for example), the first proton removed is the carboxylic acid proton, as we have said. In the case of alanine, when a pH of 2.3 is reached, the acid proton will have been removed from half of the molecules. This pH represents the pK_a of the alanine carboxylic acid proton, as can be demonstrated using the Henderson-Hasselbalch equation. The Henderson - Hasselbalch equation shows that for an acid (HA) and its conjugate base (A^_),

pK_a = pH log 

When the acid is half neutralized,

 

b) Co-polymers are another tye of polymer. These contains more than one sub-unit (or monomer).

Example:

In the above example styrene and maleic anhydride monomers laternate. Co-polymer can be a block co-polymer.

 

Example:

Co-polymers can be random as well.

— B - A - A - B - A - B - B - A - B - A -B - B - A —

A and B are monomers.

 

6. There are many polymers in nature.

Example: Cellulose, starch, pepsin, insulin, egg albumin, rubber, DNA (Deoxyribonucleic acid) etc. These are called Biopolymers.

Man made polymers are, Nylon, Terylene, Polythene, Polystyrene, PVC (Polyvinyl chloride), Bakelite, Perspex, Polysiloxane etc.

 

7. The propertis of a polymer solution are strikingly different from those of a true solution. For example, when polyvinyl alcohol is added to water, it swells.

a) Its shape gets distorted and after a long time it dissolves.

b) When more of polymer is added to a given solvent, saturation point is not reached. The mixture of polymer and solvent assumes a soft dough-like consistency.

 

8. Addition polymers and condensation polymers are two important types of polymers.

 

9. Polymer can be described as linear, branched and network.

==============================================================

Overview of Polymers (Addition, Polymerization) - Polymers, CBSE, Class 12, Chemistry

POLYMERS AND POLYMERIZATION:

Macromolecules, both natural and man-made, own their great size to the fact they are polymers (Greek: many parts); that is, each one is made up of a graet many simpler unit - identical to each other or at least chemically similar - joined together in a regular way. Theyare formed by a process we touched on earlier: polymerizatioin, the joining together of many small molecules to form very large molecules. The simple compounds from which polymers are made are called monomers.

 

Petides and Proteins:

In the last section, you studied the polymers of monosaccharides which act as structural components in plants and serve as energy storage in animals. In this section, you will study another kind of natural polymers called peptides and proteins.

Peptides are biologically important polymers in which 2-amino acids are joined by the amide linkages, formed by the reaction of the carbóxy group of one amino acid with the amino group of another amino acid. These amide linkages are also called peptide bonds. The general structure of a peptide is shown below:

Peptides can be classified as dipeptides, tripeptides and tetrapeptides, depending on whether the number of amino acids, two, three or four, respectively. Peptides containing upto 50 amino acids are called polypeptides. Bradykinin is an important naturally occurring nonapeptide which is present in blood plasma and is involved in the regulation of blood pressure.

Arg—Pro — Pro — Gly — Phe — Ser — Pro — Phe — Arg

Bradykinin

 

Configuration of proteins :

(a) Biological nature or function of protein was confirmed by its conformation.

(b) This conformation is of 4 types

 

Primary Structure :

This type of structure was given by FriedrichSanger in 1953 in Insulin (of one chain)

• Primary structure is conformed by a single polypeptide chain in a linear manner.
• All amino acid are attached in a straight chain by peptide bond.
• No biological importance & soon changed to other forms.

 

Secondary Structure :

• In it structure of straight chain from irregular changes to form coils.
• H-bond peptide bond present in secondary. structure.
• This H bond is present between hydrogen of Amino group and oxygen atom carboxylic acid group.
• This structure is of two types
• 

 

(i) a-helix

• Chain is spiral
• 3.7 atoms in one coiling
• Right handed circular.

Eg. Myosin, Keratin etc.

 

(ii) b-pleated sheet

• Structure of protein is not arranged in a sequence.
• Polypeptide chain are parallel to each other
• H - bond form by near chains

Eg. Silk fibres.

 

Tertiary structure :

In this structure of protein atoms are highly coiled and form a spherical form

Ex. Albumin

 

This structure is formed by 4 regular hydrogen bonds which makes a regularity in it

(i) Hydrogen bond :

 = O …….. H _ 

Hydrogen bond

They are formed between oxygen of acidic amino acid and H of basic amino acid.

 

(ii) Hydrophobic bond -

• Non - polar side chains of neutral amino acid tends to be closely associated with one another in proteins.
• Present in between the amino Acid.
• These are not true bonds.

(iii) lonic bond :

-COO^_.....H₃  N-

Ionic bond

These are salt bonds formed between oppositely charged groups in side chains of Amino acids

Eg. Aspartic acid

Glutamic acid

 

(iv) Disulphide bonds :

| ——— S - S —— |

• Relatively stable bond and thus is not broken readily under usual conditions of denaturation.
• Formed between the -SH group of Amino acid Ex. Cystine and Methionine .

 

Quaternary structure :

• When 2 or more polypeptide chains united by forces other than covalent bonds (i.e. not peptide and disulphide bonds) are called Quaternary structure.
• It is most stable structure.

Ex. Haemoglobin

 

Types of proteins

Classification of protein is based upon three general properties shape, Solubility and Chemical composition.

 

Simple proteins

It is formed of only Amino Acids

Þ 

 

(A) Fibrous :

• It is insoluble
• It is of elongated shape.
• It is highly resistant to digestion by proteolytic enzymes.
• Their main function - Protection.

Ex. Collagen, Keratin etc

 

(B) Globular :

• These are spherical and oval in shape. Chains are highly coiled
• These are soluble.

Ex. Albumin

 

Conjugated Proteins

• These are complex proteins in which protein molecule is combined with characteristic non-amino acid substance.
• Non-amino acid or Non - Protein part is called as prosthetic group

Ex. Nucleoproteins

(Protein Nucleic acid),

 

Phosphoproteins (Protein (PO₃)^2_)

Eg. ® Casein of milk., Vitelline of egg - yolk

 

 

Derived proteins :

(a) These are obtained as a result of partial hydrolysis of natural proteins.

Eg. ® Proteose, Metaproteins, Peptones

 

(b) Denaturation of Proteins

When a protein in its native form, is subjected to a physical change like change in temperature, or a chemical change like change in pH, the native conformation of the molecule is disrupted and proteins so formed are called denaturated proteins.

The denaturation may be reversible or irreversible. The coagulation of egg on boiling is an example of irreversible protein denaturation.

However, it has been shown now that in some cases, the process is actually reversible. The reverse process is called renaturation.

 

Test of Protein :

(a) With conc. HNO₃ on heating give yellow ppt. Which on more heating give solution On adding NH₄OH Red colour appears. It is Xanthoprotic test.

(b) (NH₄OH) dil. CuSO₄ protein give Blue violet colour. It is a biurete test.

(c) Millon reaction. Proteins on adding Millon's reagent (a solution of mercuric and mercurous nitrates in nitric acid containing a little nitrous acid) followed by heating the solution give red precipitate or colour.

(d) Ninhydrin reaction. Proteins, peptides and a-amino acids give a characteristic blue colour on treatment with ninhydrin.

 

Biological Importance of protein :

(a) Component of plasma membrane.

(b) All enzymes are protein.

(c) Many hormones are protien.

(d) Antigen and antibody are protein.

(e) Actin and myosin protein are important in muscle contraction.

(f) Proteins are important in growth, regeneration and repairing.

(g) Calorific value 4.0 kcal.

Lipids

(a) Lipids words is derived from greek word lipos which means fat.

(b) Lipids are heterogeneous group of substances which have common property of being relatively insoluble in water and soluble in non-polar solvents such as ether, Chloroform etc.

(c) Form 3-5% part of protoplasm.

(d) H₂O ¹ 2 : 1 (different from water)

(e) Ratio of oxygen is less.

(f) Specific gravity < 1

Simple lipid :

Triglycerides

(a) These are esters of fatty acids with glycerol.

Ester bond is present

(b) Synthesis is of following type-

(c) Fatty acids which occur in natural fats usually contain an even number of carbon atoms(4 to 30) in straight chains.

(d) Simplest fatty acid HCOOH.

(e) More complex fatty acid are formed by successive addition of -CH₂ groups.

 

(i) Saturated :

⇒ Only single bond is present in them.

⇒ First member is CH₃COOH.

Other examples :

⇒ Palmitic acid - C₁₅H₃₁COOH

→ CH₃(CH₂)₁₄ COOH

⇒ Stearic acid - C₁₇H₃₅COOH

→ CH₃(CH₂)₁₆COOH

⇒ Palmitic and stearic acid is found in fats of animals in less amount.

⇒ These are solid and are found in fats.

 

(ii) Unsaturated :

⇒ Double bond is present in these fatty acid chain.

⇒ These are liquids at room temperature. Found in Oils.

⇒ These are of two types

 

Monounsaturated - 1 Double bond is present

Eg. Oleic acid.

⇒ Oleic acid is present in more amount in nature.

 

Polyunsaturated - More than two double bond

Eg. Linoleic acid with two double bonds

Linoleinic acid with three double bonds

Arachidonic acid with four double bonds (Groundnut)

 

Wax :

⇒ These are esters of other alcohols of high

molecular weight instead of glycerols.

⇒These are insoluble in water.

⇒ These are monohydric alcohols.

⇒ Some examples of waxes -

Myricye palmitate (Honeybee wax) Cetyl palmitate (Dolphin and whale wax)

Cerumen (ear wax)

 

Compound Lipid- Are of 4 types :

(a) Phospholipids. (b) Glycolipids.

 

Phospholipids :

Phosphorous is present.

ex. cell wall

 

Glycolipids

⇒ Lipid Sugar = Glycolipids

⇒ Present in brain, Adrenal glands, kidney,

WBC liver, thymus, Spleen, Lungs, egg yolk

⇒ Glycolipids = 2 Fatty acid 1 sphinocine

1 galactose.

 

Derived lipids

⇒ By hydrolysis of fats they are obtained

 

Steroids :

⇒ These are different from other fats.

⇒ It is insoluble in water.

 

(i) Bile acids :

⇒ Present in secretion of liver.

 

(ii) Sex hormones :

⇒ These are androsterones.

 

(iii) Adrenal hormone- Eg : Aldosterone

 

Sterols :

⇒ They have -OH groups.

⇒ They are complex monohydroxy alcohols.

(i) Cholesterol - It is widely distributed in all cells of body.

 

Biological importance of Fat :

⇒ It is source of energy.

⇒ It is important for absorption of vitamin A, D, E and K.

⇒ It is important component of plasma membrane.

⇒ It act as shock absorber of body.

⇒ Calorific value 9.3 kcal.

====================================================================

Nucleic Acids-Chemical Composition, Structure - Biomolecules, CBSE, Class 12, Chemistry

Nucleic Acid

(a) These are special type of acids which are present in nucleus & cytoplasm.

(b) Control the metabolic activities of cell.

(c) They are also found in Mitochondria, centriole and chloroplast.

Types → These are of 2 types

(d) Fischer discovered Nitrogen bases in 1888

(e) Levan found sugar

 

Deoxyribosenuclic Acid (D.N.A.) :

(a) It is found in Nucleus.

(b) They on pneumococcus bacteria.

(c) DNA made up of 3 units-

(i) Thymine (i) Adenine

(ii) Cytosine (ii) Guanine

 

(d) Nucleoside

When nitrogen base combined with deoxyribose sugar it constitute a nucleoside.

 

S.No. Deoxyribonucleoside

1 Adenine Deoxyribose → Deoxyadenosine

2 Guanine Deoxyribose → Deoxyguanosine

3 Cytosine Deoxyribose → Deoxycytidine

4 Thymine Deoxyribose → Deoxythymidine

 

(i) Nucleotide

(a) Nitrogen base Sugar Phosphate → Nucleotide

(b) Nucleotide is a unit of DNA

(c) All nucleotides combined and form a chain called polynucleotides by which RNA and DNA formed.

 

Structure of DNA

(a) Double Helical model of DNA was proposed by biochemist J.D.Watson, British chemist FHC Crick in 1953.

(b) DNA in double stranded structure is made up of two chains of polynucleotides.

(c) DNA is a polymer of Nucleotide.

(d) Nucleotide are joined by 3' → 5' phosphodiester bonds. 
(e) Sugar and phosphorous are alternately arranged.

(f) In both chains, in between A and T, 2 Hydrogen bonds are present while in C and G 3H bonds are present.

(A = T) (C º G)

(g) A always attach with T while C always attaches with G.

(h) Purine and pyrimidine are found in ratio 1 : 1. cells.

(i) DNA is attached with histone protein.

(j) In prokaryotic cell and mitochondria circular DNA is present.
 

 

Function of DNA

(i) Self - Replication or self -Duplication

DNA has the property of self - replication . It is therefore a reproducing molecule. This unique property of DNA is at the root of all reproduction. Through its replication, DNA is acts as the key to heredity. In the replication of DNA, the two strands of a double helix unwind and separate as a template for the formation of a new complementary strand.

 

(ii) Protein Synthesis

The specific sequence of base pair in DNA represents coded information for the manufacture of specific proteins. These code instructions first are transcribed into the matching nitrogen- base sequences within mRNA and the instructions in such RNA subsequently are translated into particular sequence of amino acid units within the polypeptide chains and proteins.

The major steps in the utilization of the genetic information can be represented as :

D N A  D N A R N A Protein

 

Ribonucleic Acid (RNA) :

⇒ Found in cytoplasm as well as in nucleus.

Cytoplasm → In the ribosome (heigher amount)

 

Chemical Nature :

⇒ Ribonucleic acid is a polymer of purine and pyrimidine ribonucleotides linked by 3' → 5' phosphodiester bridges. The number of nucleotides in RNA ranges from asfew as 75 to many thousands. Although sharing many features with DNA, RNA possesses several specific difference.

⇒ As indicated by its name, sugar in RNA to which the phosphate and nitrogen- bases are attached is ribose rather than the deoxyribose of DNA.

⇒ Although RNA contains the ribonucleotides of adenine, guanine, and cytosine, it does not posses thymine. Instead of thymine, RNA contains the ribonucleotides of uracil. Thus the pyrimidine components of RNA differs from those of DNA.

⇒ RNA exists basically as a single-stranded molecule rather than as a double -stranded helical molecule, as does DNA. However the single strand of RNA is capable of folding back on itself like a hairpin and thus acquiring double-stranded characteristics. In these regions. A pairs with U and G pairs with C.

Thus a given segment of a long RNA molecule might, for example, be represented as follows.

P-R-P-R-P-R-P-R-P-R

|      |        |        |      |

A    U      G      G    C

⇒ where R stands for ribose ; A, U, G, and C for Adenine, Uracil, Guanine and Cytosine respectively.

 

Types of RNA and their Functions :

There are 3 main types of RNA molecules

(i) Messenger RNA (mRNA) (ii) Transfer RNA (tRNA) (iii) Ribosomal RNA (rRNA)

 

(i) Messenger RNA (mRNA)

⇒ This type of RNA consists of single strand of variable length and serves as a template for protein synthesis. Codon in the chromosomes.

⇒ mRNA forms complementary copy of DNA as it carries chemical messages in the form of nitorgen-base sequence from the nucleus to the ribosomes, i.e. fromDNA to cytoplasm where proteins are synthesized. Therefore, it is called messenger RNA or mRNA

⇒ mRNA is sythesised from DNA in the nucleus.

⇒ It is called transcription.

 

ii) Ribosomal RNA

⇒ A ribosome is a cytoplasmic nuucleoprotein structure which serves as the organellar machinery for protein synthesis from mRNA templates.

⇒ On the ribosome, the mRNA and tRNA molecules interact to translate into a specific protein molecule the information transcribed from the DNA.

⇒ rRNA constitutes the largest part of total RNA (Highest) - 80%

 

(iii) Transfer RNA (RNA) :

Þ These are also called Soluble RNA.

Þ Single stranded.

Þ 10-15% of the total RNA.

Þ Size - Smallest ® 75 - 80 nucleotides only.

Þ Synthesis - Within nucleus from DNA.

Function- It transport amino acid from cyto plasm to the site of protein synthesis.

========================================================================

Enzymes and Mechanism of Enzyme Action - Biomolecules, CBSE, Class 12, Chemistry 

Enzymes

Proteins which are used as a catalyst in biochemical reaction is known as biocatalysts.

 

Specific cheracteristics

Enzymes have following two specific character as :

(i) Specificity (ii) Efficiency

 

Specificity of enzymes

(a) Generaly one enzyme can catalyze only one biochemical reaction.

(b) It can increases rate of reaction upto 10²⁰ times.

(c) In some cases one enzyme can catalyzes more than one reaction and one reaction can be catalyzed by more than one enzyme. eg. Enzyme present in Yeast (Zymase) can ferment both glucose and fructose into alcohol and also cane-sugar can be hydrolyses by invertase and sucrase enzymes.

 

Efficiency of enzymes

(a) One molecule of enzyme can convert millions of substrate molecules into product per second.

eg. Carbonic anhydrase enzyme present in red blood cells has a highest turn over number.

(b) With having tertiary structure it can be collected as crystals.

Enzymes are denatured at higher temperature.

(c) Enzyme can be stored at low temperature as they are inactivated.

 

Importance of enzymes

In the thousands of enzymes presents in body if even a single enzyme would be absent or damaged than complex disease in results.

eg. Scarcity of Phenylalanine hydroxylase enzyme in human body is result inPhenylketonuria disease.

 

Factors affecting enzyme action :

(i) Optimum temperature and pH. Enzyme catalysed reactions have maximum rate at physiological pH of around 7.4 and human body temperature of 37ºC (310 K) under one atmosphere pressure.

In fact, as the temperatue or pH is increased, the rate rises to a maximum (at 37ºC or pH = 7.4) and then falls off.

(ii) Enzyme activators (co-enzymes). The

activity of certain enzymes is increased in the presence of certain substances, called co-enzymes. It has been observed that if a protein contains a small amount of vitamin as the non-protein part, its activity is enhanced considerably. The activators are generally metal ions such as Na , Mn² , Cu² , Co²  etc. These metal ions are weakly bonded to the enzyme molecules and increase their catalytic activity. For example, the enzyme, amylase, in presence of NaCl, which provides Na  ion, shows a very high catalytic activity.

(iii) Enzyme inhibitors and poisons. Just as in the case of catalysts, the activity of enzyme is slowed down in the presence of certain substance. Such substances are called inhibitors or poisons. They act by combining with the active functional group thereby reducing or completely destroying the catalytic activity of the enzymes. The use of many drugs is on account of thier action as enzyme inhibitors in our body.

 

 

Nutrients

Sodium, Potassium and Chlorine

(i) Na  is the principal mineral cation in the extracellular fluid.
(ii) K  is the principal cation inside the cell.

(iii) Cl^_ is the principal mineral anion in the ECF.

(iv) Na  and K  are essential to the maintenance of water balance and acid-base balance.

(v) Na  and K  are important in nerve impulse transmission.

 

Calcium and Phosphorus

(i) Calcium and phosphorus are deposited in bones and teeth to give them strength and rigidity.

(ii) Ca²  is also essential for blood coagulation, neuromuscular function, cardiac function and actions of many enzymes and hormones.

(iii) Phosphorus enters into many compounds such as nucleic acids and phospholipids, many coenzymes and high energy compounds like ATP.

(iv) Calcium plays an essential role in sustaining intestinal peristalsis and growth of body tissues.

Iron

(i) Iron is required for haemoglobin synthesis.

(ii) Iron is essential both for transportation of oxygen to tissues and for operation of oxidative systems within the tissue cells.

 

Magnesium

(i) Magnesium is required as a catalyst for many intracellular enzymatic reactions, particularly those relating to carbohydrate metabolism.

(ii) Mg is the central metal atom in chlorophyll

 

Iodine

Iodine is used in the synthesis of thyroid hormones.

 

Zinc

(i) Zinc is a constituent of carbonic anhydrase, present in RBCs helping in CO₂transport.

(ii) Zinc is a component to lactic dyhydrogenase, important for the interconversion between pyruvic acid and lactic acid

(iii) Zinc is a component part of some peptidases and therefore is important for digestion of proteins in the alimentary canal

 

Cobalt

(i) Cobalt helps in erythropoiesis and in the activities of some enzymes.

(ii) It is present in vitamin B₁₂ ^.

 

Copper

(i) Copper helps in the utilisation of iron.

(ii) Copper deficiency may produce anaemia because of failure in iron utilisation.

 

Molybdenum

(i) Molybdenum is a constituent of oxidase enzymes (xanthine oxidase)

(ii) Molybdenum plays an important role in biological nitrogen fixation

 

Fluorine

(i) Fluorine maintains normal dental enamel and prevents dental caries.

(ii) Exessive intake of fluorine cause fluorosis characterized by mottled teeth and enlarged bones.

 

Vitamins

It has been observed that certain organic compounds are required in small amounts in our diet but their deficiency causes specific diseases. These compounds are called vitamins.

Classification of Vitamins

Vitamins are classified into two groups depending upon their solubility in water or fat.

(i) Fat soluble vitamins:

Vitamins which are soluble in fat and oils. But insoluble in water are kept in this group. These are vitamins A, D, E and K. They are stored in liver and adipose (fat storing) tissues.

(ii) Water soluble vitamins:

B group vitamins and vitamin C are soluble in water so they are grouped together. Water soluble vitamins must be supplied regularly in diet because they are readily excreted in urine and can not be stored (except vitamin B₁₂) in our body.

Some important vitamins, their sources and diseases caused by their deficiency are listed in table.

==========================================================================

Classification of Polymers - Polymers, CBSE, Class 12, Chemistry

POLYMERS

The term polymer is used to describe a very large molecule that is made up of many repeating small molecular units. These small molecular units from which the polymer is formed are called monomers. The chemical reaction that joins the monomers together is called polymerisation. Starting from n moleculer of a compound M, linking in a linear manner will form polymer x–M–(M)n–2–M–y. The nature of linkages at the terminal units i.e. M–x and M–y depends upon the mode of reaction used in making the polymers. Homopolymers and Copolymers Polymers which are formed by only one type of monomer are called Homopolymers. Some examples of homopolymers and their monomers are given below :

Homopolymers and Copolymers

Polymers which are formed by only one type of monomer are called Homopolymers. Some examples of homopolymers and their monomers are given below :

Homopolymer Monomer

​

Polymers, which are formed by more than one type of monomers are known as copolymers. Some examples are given below in the table:

 

Types of copolymers

Depending upon the distribution of monomer units, the following types of copolymers are possible.

(1) Random Copolymer

If the monomer units have random distribution throughout the chain, it is called random copolymer. For example, if the monomer A and monomer B undergo copolymerisation then the structure of the random copolymer is:

 

(2) Alternating Copolymer

If the two monomer units occur alternatively throughout the polymer chain, it is said to be alternating copolymer. For example,

The exact distribution depends upon the proportion of the two reactant monomers and their relative reactivities. In practice neither perfectly random nor perfectly alternating copolymers are usually formed. However, most copolymers tend more towards alternating type but have many random imperfections.

 

(3) Block Copolymer

Polymers in which different blocks of identical monomer units alternate with each other are called block copolymers.

For example,

Block copolymer can be prepared by initiating the radical polymerisation of one monomer to grow homopolymer chains, followed by addition of an excess of the second monomer.

 

(4) Graft Copolymer

Polymers in which homopolymer branches of one monomer unit are grafted onto a homopolymer chain of another monomer unit are called graft co_polymers. For example:

Graft copolymers are prepared by g-irradiation of a homopolymer chain in the presence of a second monomer. The high energy radiation knock out H-atoms from the homopolymer chain at random points thus generating radical sites that can initiate polymerisation of the second monomer.

 

CLASSIFICATION OF POLYMERS

Polymers are classified in following ways:

(I) CLASSIFICATION BASED UPON SOURCE

(1) Natural polymers

Polymers which are obtained from animals and plants are known as natural polymers, Examples of natural polymers are given below.

Natural polymer                                                       Monomers

1. Polysaccharide                                                    Monosaccharide

2. Proteins                                                                 a-L-Amino acids

3. Nucleic acid                                                          Nucleotide

4. Silk                                                                        Amino acids

5. Natural Rubber (cis polyisoprene)                 Isoprene (2-Methyl-1, 3-butadiene)

6. Gutta purcha (trans polyisoprene)                 Isoprene

 

Natural polymers which take part in metabolic processes are known as bioplymers. Examples are polysaccharides, proteins, RNA and DNA.

 

(2) Semisynthetic polymers

Polymers which are prepared from natural polymers are known as semisynthetic polymers. Most of the semisynthetic polymers are prepared from cellulose.

Examples are: cellulose acetate, cellulose nitrate, cellulose xanthate and Rayon.

 

(3) Synthetic polymers

Man-made polymers, i.e. polymers prepared in laboratory are known as synthetic polymers.

Example are : PVC, polyethylene, polystyrene, nylon-6, nylon-66, nylon-610, terylene, synthetic rubbers etc.

 

(II) CLASSIFICATION BASED UPON SHAPE

(1) Linear polymers

Polymer whose structure is linear is known as linear polymer. The various linear polymeric chains are stacked over one another to give a well packed structure.

The chains are highly ordered with respect to one another. The structure is close packed in nature, due to which they have high densities, high melting point and high tensile (pulling) strength. Linear polymers can be converted into fibres.

 

Note :

(i) All fibers are linear polymers. Examples are cellulose, silk, nylon, terylene etc.

(ii) Linear polymers may be condensation as well as addition polymers. Examples are cellulose, Polypeptide, nucleic acid, nylon, terylene etc.

 

(2) Branched chain polymers

Branched chain polymers are those in which the monomeric units constitute a branched chain. Due to the presence of branches, these polymers do not pack well. As a result branched chain polymers have lower melting points, low densities and tensile strength as compared to linear polymers.

 

(3) Cross-linked or Three Dimensional network polymers

In these polymers the initially formed linear polymeric chains are joined together to form a three dimensional network structure. These polymers are hard, rigid and brittle. Cross-linked polymers are always condensation polymers. Resins are corss linked polymers.

 

CLASSIFICATION BASED UPON SYNTHESIS

(1) Condensation polymerisation

(i) They are formed due to condensation reactions.

(ii) Condensation polymerisation is also known as step growth polymerisation.

(iii) For condensation polymerisation, monomers should have at least two functional groups. Both functional groups may be same or different.

(iv) Monomers having only two functional group always give linear polymer.

For example,

(v) Condensation polymers do not contain all atoms initially present in the monomers.

Some atoms are lost in the form of small molecules.

(vi) Monomer having three functional groups always gives cross-linked polymer.

Examples are : Urea-formaldehyde resin, phenol-formaldehyde resin.

==========================================================================

Addition Polymerisation or Chain - Growth - Polymers, CBSE, Class 12, Chemistry 

(3) Addition polymerisation

(i) Polymers which are formed by addition reaction are knowns as addition polymers.

(ii) If monomer is ethylene or its derivative, then addition polymer is either linear polymer of branch-chain polymer.

Examples are : polystyrene, polytetrafluoroethylene, polyacrylonitrile etc.

(iii) If monomer is 1, 3-butadiene or 2-substituted-1, 3-butadiene, then polymer is always branched chain polymer.

(iv) Addition polymers retain all the atoms of the monomer units in the polymer.

(v) Addition polymerisation takes place in three steps:

Initiation, chain propagation and chain termination.

(vi) Addition polymers are called as chain growth polymers.

 

Types of Addition Polymerization

(A) Radical Polymerisation:

(i) Radical polymerisation takes place in the presence of radical initiators. The radical initiator may be any of the following :

(ii) Reaction intermediate of radical polymerization is a free radical.

(iii) Radical polymerization has more chance for those monomers whose free radicals are more stable.

Examples are:

 

 

 

In vinylic polymerization, various other reaction of free radicals with some other compounds present may compete with the parent addition chain reactions. One such reaction takes place with molecules that can react with the growing chain to interrupt the further growth. This leads to the lowering of the average molecular mass of the polymer. Such reagents are called as chain transfer agents and include , CBr₄etc.

For example, in the presence of CCl₄, styrene polymerises to form plystyrene of a lower average molecular mass which also contains some chlorine what happens here is that growing polystyrene radical which normally would add on a monomer reacts with the chain transfer agent to end the original chain and produces a new radical. The latter initiates a new polymerization chain and thereby forms a new polymer as depicted below.

If the chain transfer agent a radical, which is highly unreactive, the reaction chain gets terminated such a compound thus inhibits or arrests polymerisation. Many amines, phenols, quinones etc. act as inhibitors. So, even traces of certain impurities, which can act as chain transfer agent or an inhibitor can interfere with the original polymerisation chain reaction. Hence, the monomers chould be free from such inhibitors.

In case the alkene is a diene, the following kinds of polymerisation is possible:

 

(1) 1, 4-polymerisation

When the polymerisation takes place at C₁ and C₄ of butadiene, an unbranched polymer is formed. This product is different from that formed from an alkene having a double bond, which at each of its carbons is substituted by different groups and hence can exist either as trans-polybutadiene or

cis-polybutadiene or a mixture as shown below.

 

(2) 1, 2-polymerisation

Alternatively, 1, 3-butadiene can undergo polymerisation at C₁ and C₂ to yield the polymeric product, polyvinly polythene.

The double bonds in these initial polymers can be linked by further treatment with chemicals to modify the properties of the polymers. These reactions form the basis of the formation of rubber.

 

(B) Cationic Polymerisation :

(i) Polymerisation which is initiated by an electrophile is known as cationic polymerisation.

(ii) Reaction intermediate of cationic polymerisation is a carbocation.

(iii) Carbocations can undergo rearrangement leading to the formation of a more stable carbocation.

(iv) The electrophile commonly used for initiation is BF₃.OEt₂.

(v) Monomers that are best able to undergo polymersation by a cationic mechanism are those with electron - donating substituents that can stabilise the carboncation. Some examples are:

(v) It is terminated by a base.

Thus, when the initiator is cationic in nature, it would generate a cationic intermediate on addition to the double bond for propagating the addition chain process and is termed as catioic addition polymerisation. The process is initiated by an acid. The stages of polymerisation are depicted below.

 

Chain initiation step :

 

Chain propagating step :

 

Chain terminating step :

Cationic polymerisation is facilitated in monomers containing electron - releasing groups. Thus, isobutylene undergoes cationic polymerisation easily as it has two electron releasing _CH₃ groups that will stabilize the intermediate carbo cation.

 

(C) Anionic Polymerisation:

(i) Anionic polymerisation takes place in the presence of base or nucleopile, which is initiator in this polymerisation.

(ii) Reaction intermediate in propagation steps are carboanion.

(iii) the suitable initiator can be NaNH₂ or RLi.

(iv) Those monomers undergo anionic polymerisation reaction whose anion is stable.

 

Example of monomers are:

(v) Anionic polymerisation always give linear polymer.

(iv) Anionic polymerisation termnated by an acid.

The formation of polystyrene from styrene in the presene of potassium amide is an important example of this category of polymerisation. The mode of anionic polymerisation is depicted below:

 

Chain initiation step :

Chain propagating step :

Chain terminating step :

 

(D) Ziegler- Natta polymerisation :

(i) Addition polymerisation which takes place in the presence of Ziegler- Natta catalyst [(C₂H₅)₃Al and TiCl₄] is known as Ziegler- Natta polymerisation or co-ordination polymerisation.

(ii) Ziegler- Natta polymerisation always gives linear, stereo-regular polymers.

(iii) Ziegler- Natta catalyst revolutionised the field of polymer chemistry because they allow the synthesis of stronger and stiffer polymers (due to linear geometry) that have greater resistance to cracking and heat. High density polyethylene is prepared using a Ziegler- Natta catalyst.

 

CLASSIFICATION BASED ON INTERMOLECULAR FORCES (SECONDARY FORCES)

(i) Intermolecular forces present between polymeric chains are (a) Van der waals forces (b)Hydrogen bonds and (c) Dipole - dipole attractions.

(ii) Mechanical properties such as tensile strength, elasticity, toughness etc. depend upon the secondary force present between the polymeric chains.

(iii) Magnitude of secondary forces depends upon the size of the molecule and the number of functional groups along the polymeric chains.

Magnitude of secondary forces is directly proportional to the length of the polymeric chain. On the basis of magnitude of secondary forces, polymers can be diveided into the following

 

Five categories.

(1) Elastomes

An elastomer is a plastic that stretches and then reverts back to its original shape. It is randomly oriented amorphous polymer. It must have some cross-links so that the chains do not slip over one another. Very weak Van der waal forces are present in between polymeric chains.

When elastomers are stretched, the random chains stretch out, but there are insufficient Van der Waal forces to maintain them in that configuration and position. When the stretching force is removed, they go back to their random shape. Elastomers have the ability to stretch out over ten times their normal length.

Important examples are vulcanized rubbers.

Note : Addition polymers obtained from butadiene and its derivatives are elastomers.

 

(2) Fibres

Fibres are linear polymers in which the individual chains of a polymer are held together by hydrogen bonds and / or dipole-dipole attraction. In the fibres, the polymeric chains are highly ordered with respect to one another.

Due to strong intermolecular forces of attraction and highly ordered geometry, fibres have high tensile strength and least elasticity. They have crystalline character and have high melting points and low solubility. Examples are cellulose, nylon, terylene, wool, silk etc.

Note: (i) Condensation polymers formed from bifunctional monomers are fibres in character.

(ii) Addition polymers of alkene derivatives having strong-I group are fibres in character.

 

(3) Thermoplastic Polymers

Thermoplastic polymers are polymers that have both ordered crystaline regions (the regions of the polymer in which the chains are highly ordered with respect to one another) and amorphous, non crystalline regions (the regions of the polymer in which the chains are randomly oriented).

The intermolecular forces of attraction are in between elastomers and fibres. There are no cross-links between the polymeric chains. Thermoplastic polymers are hard at room temperature, but when they are heated, the individual chains can slip past one another and the polymer become soft and viscus. This soft and viscous material become rigid on cooling. The process of heating softening and cooling can be repeated as many times as desired without any change in chemical composition and mechanical properties of the plastic. As a result, these plastics can be moulded into toys, buckets, telephone and television cases.

Some common examples are : polyethene polypropylene, polystyrene, polyvinylchloride, teflon etc.

Note : Addition polymers obtained from ethylene and ethylene derivatives are thermoplastic polymers.

 

(4) Thermosetting Polymers

Polymers which become hard on heating are called thermosetting polymers. Thermosetting polymers can be heated only once when it permanently sets into a solid, which cannot be remelted by heating.

Thermosetting polymers are cross-linked polymers. Greater the degree of cross- linking that exist, the more rigid is the polymer. Cross-linking reduces the mobility of the polymer chains, causing them to be relatively brittle materials, the hardening on heating is due to the extensive cross-linking between different polymer chains to give a three dimensional network solid. Examples are : phenol formaldehyde resin, urea-formaldehyde resin, melamine- formaldehyde resin.

 

DIFFERENCE BETWEEN THERMOPLASTIC AND THERMOSETTING POLYMERS

S.No. Thermoplatic polymers                                                                                               Themosetting polymers

1. Soften and melt on heating and become                                                                         Become hard on heating and process is

hard on cooling i.e. process is reversible                                                                             irreversible.

2. Can be moulded and remoulded and                                                                               They can be moulded once and cannot

reshaped.                                                                                                                                    be remoulded or reshaped.

3. They are addition polymers                                                                                                 They are condensation polymers.

4. Structure is generally linear                                                                                                  Structure is cross - linked.

=====================================================================

Copolymerisation and Rubber - Polymers, CBSE, Class 12, Chemistry

RUBBER

1. Natural Rubber

Natural rubber is obtained from nearly five hundred different plants but the main source is a braziliensis tree. It is obtained in the form of milky sap known as latex. This latex is coagulated with acetic acid and formic acid. The coagulated mass is then squeezed.

The raw natural rubber is a soft gummy and sticky mass. It is insoluble in water, dilute acids and alkalies but soluble in non-polar solvents. It has low elasticity and low tensile strength. Natural rubberis a polymer of 2-methyl-1, 3-butadiene (isoprene). On average, a molecule of rubber contains 5000 isoprene units held together by head to tail. All the double bonds in rubber are cis, hence natural rubber is cis-polyisoprene.

Gutta - percha is a naturally occurring isomer of rubber in which all the double bonds trans. Thus, gutta-percha is trans-polyisoprene.

 

It is harder and more brittle than rubber. It is filling material that dentists use in root canal treatment. In order to give strength and elasticity to natural rubber, it is vulcanized. Heating of rubber with sulphur or sulphur containing compound at 150^oCfor few hours is known as vulcanisation. The essential feature of the vulcanisation is the formation of cross-linking between the polymeric chains. This cross-linking gives mechanical strength to the rubber. Vulcanisation process can be enhanced in the presence of certain organic compounds known as accelerator. The common accelerators are:

In addition, fillers such as carbon black and zinc oxide are usually added to the crude rubber before vulcanisation in order to imporve its wearing characteristics.

Natural rubber is used for making shoes, water - proof coats and golf balls. Vulcanised rubber is used for manufacture of rubber bands, gloves tubing and car tyres.

 

SYNTHETIC RUBBER OR POLYMERISATION OF DIENES

Polymers of 1, 3 - butadienes are called synthetic rubbers because they have some of the properties of natural rubbers including the fact that they are water proof and elastic.

Synthetic rubbers have some improved properties. They are more flexible, tougher and more durable than natural rubber.

 

1. Homopolymers

Monomer of this class is 2-substituted-1, 3-butadienes.

 where G=H, CH₃ or Cl.

polymerisation is always carried out in the presence of Zieglar-natta catalyst which gives stereo regular polymers.

Neoprene was the first synthetic rubber manufactured on large scale. It is also called dieprene. Its monomer, chloroprene (2-chlorobutadiene) is prepared from acetylene.

Chloroprene undergoes free radical polymerisation to form neoprene (polychloroprene).

Many of the properties of neoprene are similar to natural rubber but neoprene is more resistant to action of oils, gasoline and other hydrocarbons. It is also resistant to sunlight, oxygen, ozone and heat. It is non-inflammable.

It is used for making automobile and refrigerator parts, hoses for petrol and oil containers, insulation of electric wires and conveyor belts.

 

2. Copolymers

The following synthetic rubbers are example of copolymers.

 

(c) Buna-N : It is obtained by copolymerisation of butadiene and acrylonitirile (General purpose Rubber acrylonitirle or GRA).

It is very rigid and is very resistant to action of petrol, lubricating oil and many organic solvents. It is mainly used for making fuel tanks.

 

(d) Cold Rubber : Cold rubber is obtained by polymerization of butadiene and styrene at -18^o to 5^oC temperature in the presence of redox system. Cold rubber has a greater tensile strength and greater resistance to abrasion than SBR.

================================================================

Polymers of Commercial Importance - Polymers, CBSE, Class 12, Chemistry

 

NYLON

Nylon is used as a general name for all synthetic fibres forming polyamides, i.e., having a protein like structure. A number is usually suffixed with the name `nylon' which refers to the number of carbon atoms present in the monomers.

(1) NYLON -66 (Nylon six, six)

It is obtained by the condensation polymerzation of hexamethylenediamine (a diamine with six carbon atoms) and adipic acid (a dibasic acid having 6 carbon atoms).

 

(2) NYON-6, 10 (Nylon six, ten)

It is obtained by condensation polymerisation of hexamethylenediamine (six carbon atoms) and sebacic acid (a dibasic acid with 10 carbon atoms).

Nylon fibres are stronger than natural fibres and so are used in making cords and ropes. The fibres are elastic, light, very strong and flexible. They have drip dry property and retain creases. It is inert towards chemicals and biological agents. It can be blended with wool. Nylon fibres are used in making garments, carpets, fabrics, tyre cords, ropes etc.

 

(3) NYON-6 (Perlon L)

A polyamide closely related to nylon is known as perlon L (Germany) or Nylon-6 (USA). It is prepared by prolonged heating of caprolactum at 260^o-270^oC. It is formed by self condensation of a large number of molecules of amino caproic acid. Since, caprolactum is more easily available, it is used for polymerization, with is carried out in the presence of H₂O that first hydrolyses the lactam to amino acid. Subsequently, the amino acid can react with the lactam and the process goes on and into form the polyamide polymer.

Carpolactam is obtained by Backmann rearrangement of cyclohexanone oxime.

 

(4) NYON-2 - NYLON-6

It is in alternating polyamide copolymerof glycine and amino caproic acid and is biodegradable.

 

POLYETHYLENE

Polyethylene is of two types:

(a) Low Density Poly Ethylene (LDPE) : It is manufactured by heating ethylene at 200^oC under a pressure of 1500 atmospheres and in the presence of traces of oxygen. This polymerisation is a free radical polymerisation.

nCH₂=CH₂

The polyethylene produced has a molecular mass of about 20,000 and has a branched structure. Due to this, polyethylene has a low density (0.92) and low melting point (110^oC). That is why polyethylene prepared by free radical polymerisation is called low density polyethylene. It is a transparent polymer of moderate tensile strength and high toughness. It is widely used as a packing material and as insulation for electrical wires and cables.

 

(b) High Density Poly Ethylene (HDPE) : It is prepared by the use of Zieglar - Natta catalyst at 160ºC under pressure of 6 to 7 atmosphere.

The polymer is linear chain, hence it has high density (0.97) and has high melting point (130^oC). That is why it is called high density polyethylene. It is a translucent polymer. It has greater toughness, hardness and tensile strength than low density polyethylene. It is used in the manufacture of containers (buckets, tubes), house wares, bottles and toys.

 

PLASTICISER

A plasticiser is an organic compound that dissolves in the polymer and allows the polymer chains to slide past one another. This makes polymer more flexible. Dibutylphthalate is a commonly use plasticiser.

 

MELAMINE - FORMALDEHYDE RESIN

This resin is formed by condensation polymerisation of melamine and formaldehyde.

It is a quite hard polymer and is used widely for making plastic crockery under the name melamine. The articles made from this polymer do not break even when dropped from considerable height.

 

BAKELITE

Phenol-formaldehyde resins are obtained by the reaction of phenol and formaldehyde in the presence of either an acid or a basic catalyst. The reaction starts with the initial formation of ortho and para-hydroxymethyl phenol derivatives, which further react with phenol to form compouds where rings are joined to each other with _CH₂groups. The reaction involves the formation of methylene bridges in ortho, para or both ortho and para positions. Linear or cross - linded materials are obtained depending on the conditions of the reaction.

 

 

POLYESTERS

Dacron is a common polyester, prepared using ethylene glycol and terephthalic acid. The reaction is carried out at 140^o to 180^o C in the presence of zinc acetate and Sb₂O₃ as catalyst.

nHOCH₂CH₂OH nHO₂C

The terylene fibre (Dacron) is crease resistant and has low moisture absorption. It has high tensile strength. It is mainly used in making wash and wear garments, in blending with wood to provide better crease and wrinkle resistance.

=================================================================

Molecular Mass of Polymers and Biodegradable Polymers - Polymers, CBSE, Class 12, Chemistry

BIODEGRADABLE POLYMERS

By far the largest use of synthetic polymers is as plastic. A major portion of it is used as throwaway containers and packing materials. Since plastics do not disintergrate by themselves, they are not biodegradable over a period of time. Non-biodegradability is due the carbon-carbon bonds of

addition polymers which are inert to enzyme catalysed reaction. These polymers create pollution problem.

Biodegradable polymers are the polymers that can be broken into small segments by enzyme catalysed reactions using enzymes produced by microorganisms. In biodegradable polymers, bonds that can be broken by the enzymes are inserted into the polymers. Therefore, when they are buried as waste, enzymes present in the ground can degrade the polymer.

One method involves inserting hydrolysable ester group into the polymer. For example, when acetal (I) is added during the polymerzation of alkene, ester group is inserted into the polymeric chains.

Aliphatic polyesters are important class of biodegradable polymers. Some examples are described below:
 

(1) Poly - Hydroxybutyrate-CO-b-Hydroxyvalerate (PHBV)

It is a copolymer of 3-hydroxybutanoic acid and 3 hydroxypentanoic acid, in which the monomer units are connected by ester linkages.

CH₃_CH(OH)_CH₂_COOH CH₃_CH₂_CH(OH)_CH₂ _COOH →  where R = CH₃,C₂ H₅

The properties of PHBV very according to the ratio of both the acids. 3-Hydroxybutanoic acid provides stiffness and 3-hydroxypentanoic acid imparts flexibility to the co-polymer. It is used in specialty packaging, orthopaedic devices and even in controlled drug relase. When a drug is put in a capsule of PHBV. It is released only after the polymer is degraded. PHBV also undergoes bacterial degradation in the environment.

 

(2) POLY (GLYCOLIC ACID) AND POLY (LACTIC ACID)

They constitute commercially successful biodegradable polymers such as sutures. Dextron was the first bioadsorbable suture made for biodegradable polyesters for post - operative stitches.

 

MOLECULAR MASS OF POLYMER

Normally, a polymer contains chains of varying lengths and therefore, its molecular mass is always expressed as an average. In contrast, natural polymers such as protein contains chain of identical length and hence, have definite molecular mass.

The molecular mass of a polymer is expressed as

(a) Number average molecular mass (_n)

Where N_i is the number of molecules of molecular mass M_i

 

(b) Weight average molecular mass (_w)

Where  is the number of molecules of molecular mass M_i. Methods such as light scattering and ultracentrifuge depend on the mass of individual molecules and yield weight average molecular masses. is determined by employing methods which depend upon the number of molecules present in the polymer sample viz. Colligative properties like osmotic pressure.

The ratio of the weight and number average molecular masses (_w/_n) is called Poly Dispersity Index (PDI). Some natural polymers, which are generally monodisperesed, the PDI is unity (i.e._w=).

In synthetic polymers, which are always polydispersed, PDI > 1 because is always higher than _n.

 

COMMON POLYMERS