Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Organic Chemistry

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Chemistry : Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

The document Carbohydrate -Bio-Molecules Chemistry Notes | EduRev is a part of the Chemistry Course Organic Chemistry.
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Introduction to Bio-Molecules

We are aware that our body, plants and other animals are made up of many chemical substances. There are certain complex organic molecules which form the basis of life. These build up living organisms and are also required for their growth and maintenance. Such molecules are called biomolecules. The main classes of bio-molecules are

1. Carbohydrates

2. Nucleic acids

3. Amino Acids

4. Peptides

5. Terpenes

6. Lipids

7. Alkaloids


Carbohydrate

Carbohydrates form a very large group of naturally occurring organic compounds which play a vital role in daily life. They are produced in plants by the process of photosynthesis. After eating plant foods, humans convert the carbohydrates into glucose.

The most common carbohydrates are glucose, fructose, sucrose, starch, cellulose etc. Chemically, the carbohydrates may be defined as polyhydroxy aldehydes or ketones or substances which give such molecules on hydrolysis. Many carbohydrates are sweet in taste and all sweet carbohydrates are called as sugars. The chemical name of the most commonly used sugar in our homes is sucrose.

Functions of Carbohydrate

1. Provide energy

2. Maintain blood glucose

3. Spare protein

4. Prevents ketosis

Classification of Carbohydrates:


A. Depending upon their behaviour on hydrolysis-

(1) Monosaccharides: polyhydroxy aldehyde or ketone which cannot be hydrolysed further to a smaller molecule containing these functional groups, is known as a monosaccharide. About 20 monosaccharides occur in nature and glucose is the most common amongst them.

Monosaccharides are further classified on the basis of the functional group present in them. If a monosaccharide contains an aldehyde group, it is known as an aldose and if it contains a keto group, it is known as a ketose.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

(2) Disacccharides: Carbohydrates which give two monosaccharide molecules on hydrolysis are called disaccharides e.g. sucrose, maltose, lactose etc.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

(2). Polysaccharides: Carbohydrates which yield a large number of monosaccharide units on hydrolysis e.g. starch, glycogen, cellulose etc.

Amylose: It is a component of starch.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Amylopectin: It is another polysaccharide component of starch that has a branched structure.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

The Comparison of Starch and Glycogen Molecules

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

(4). Oligosaccharides: Similar in length to simple carbohydrates and in makeup to polysaccharides. Humans lack the enzymes necessary to digest them. Intestinal microflora digest and ferment them and cause bloating, discomfort, and flatulence. Food sources are Legumes, beans, cabbage, brussels sprouts, broccoli.

The Structure of an Oligosaccharide

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

B. Depending upon functional group

1. Aldoses: They are aldehyde containing carbohydrates. At C1, carbonyl is an aldehyde.

2. Ketoses: They are ketone containing carbohydrates. At any other carbon, carbonyl present would be ketonic group.

C. Depending upon no. of carbon atoms

(a) Aldoses

(i) Three carbons: Aldotriose

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

(ii) Four carbons: Aldotetrose

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

(iii)Five carbons:Aldopentose

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

(iv) Six carbons: Aldohexose

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

(b) Ketoses

Ketoses are less common than aldoses.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Fructofuranose and Fructopyranose

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

 Cyclic Forms of Carbohydrates: Furanose Forms

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Cyclic Structure of Monosaccharides Hemiacetal Formation

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Cyclization of carbohydrates to the hemiacetal creates a new chiral center. The hemiacetal or hemiketal carbon of the cyclic form of carbohydrates is the anomeric carbon. Carbohydrate isomers that differ only in the stereochemistry of the anomeric carbon are called anomers.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Haworth projections: The structures of cyclic sugars are best represented by the Haworth projections. Haworth projections allow us to see the relative orientation of the OH groups in the ring.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Converting Fischer Projections to Haworth formulas

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Note: The pyranose forms of carbohydrates adopt chair conformations.


Mutarotation

The α- and β-anomers are in equilibrium, and interconvert through the open form. The pure anomers can be isolated by crystallization. When the pure anomers are dissolved in water they undergo mutarotation, the process by which they return to an equilibrium mixture of the anomer.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Osazone Formation: Aldoses and ketoses react with three equivalents of phenylhydrazine.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

The C-2 epimers of aldoses form identical osazones.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Reaction of Ketoses with Phenylhydrazine.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Deoxy Sugars

Carbohydrates that do not contain hydroxy group.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Amino Sugars

Carbohydrates in which a hydroxyl group is replaced with an —NH2 or —NHAc group

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Oxidation of Monosaccharides

C1 of aldoses can be selectively oxidized to the carboxylic acid (aldonic acids) with Br2 or Ag(I) (Tollen’s test).

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Reducing sugars: Carbohydrates that can be oxidized to aldonic acids. For example, oxidation of aldoses to aldaric acids with HNO3.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Reduction of Monosaccharides

C1 of aldoses are reduced with sodium borohydride to the 1° alcohol (alditols)

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Chain Elongation:

The carbon chain of an aldose can be increased by one carbon by Kiliani–Fischer synthesis method.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Chain Shortening

The Ruff degradation shortens an aldose chain by one carbon.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

Preparation of the Calcium D-Gluconate for the Ruff Degradation

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Acylation of Monosaccharides: The OH groups of monosaccharides show the chemistry of typical alcohols.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Alkylation of the OH Groups

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Formation of Glycosides

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Mechanism:

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


Formation of an N-Glycoside

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev


The Anomeric Effect

The anomeric effect refers to the tendency of a group X at C(1) of a pyranose ring to assume the axial rather than the equatorial orientation. This phenomenon is important in carbohydrate chemistry since it influences the composition of isomeric mixtures and hence their reactivities. It has been suggested that the effect is caused mainly by a stabilizing interaction between the axial lone pair of electrons on the ring oxygen atom and the anti-periplanar, antibonding orbital of the C-X bond. This leads to a shortening of the bond between the ring oxygen and the anomeric carbon and a lengthening of the C-X bond. Alternatively, electrostatic repulsive forces between the dipoles due to the ring oxygen lone pairs and the exocyclic oxygen or halogen may account in part for the observed axial preferences.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

The anomeric effect is solvent and substituent dependent and decreases in the following order: C1> OAc > OMe > OH, as exemplified by the equilibrium concentrations of the alpha and beta anomers of substituted D-glucose in various protic solvents at 250C.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

The formation of a glycoside favors the α-glucoside product: the anomeric effect.

Carbohydrate -Bio-Molecules Chemistry Notes | EduRev

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