Hydrocarbons, Class 11, Chemistry Detailed Chapter Notes PDF Download

Alkanes (Properties and Nomenclature) - Hydrocarbons, Class 11, Chemistry

Hydrocarbons

Alkane

Alkane are the saturated non polar hydrocarbon having general formula C_nH_2n+2.

Hydrocarbon - Those organic compounds which contain only carbon and hydrogen atoms are known as hydrocarbons.

1.2 General method of preparation

1.2.1 By catalytic reduction of alkenes and alkynes

R - C º ≡ C - R'  R - CH₂ - CH₂ - R'

R - CH = CH - R'  R - CH₂ - CH₂ - R'

Hydrogenation → Addition of H₂ to unsaturated bond.

 

Hydrogenation is of two kind

(a) Heterogeneous and (b) Homogeneous

(a) Heterogeneous It is two phase hydrogenation the catalyst is finely divided metal like Ni, Pt or Pd and a solution of alkene.

(b) Homogeneous It is one phase hydrogenation both catalyst and alkenes are solution. In this hydrogenation catalyst are organic complex of transition metal like Rh or Ir.

Hydrogenation is exothermic, qualitative and during the hydrogenation, total heat evolved to hydrogenate one mole of unsaturated compound is called heat of hydrogenation. Heat of hydrogenation is the measurement of stability of isomeric alkenes.

stability of alkene 

 

1.2.2 From alkyl halide

(A) From organometallic compound  compound having bond. (M  metal)

(i) By wurtz reaction

2R - X  + 2Na R → R + 2NaX

R - X + R' - X  R - R, R- R', R' - R'

Mechanism  Two mechanisms are suggested

 

(a) Ionic mechanism

2Na  2Na 2e

(b) Free radical mechanism

Na  

 

Note : The alkyl halide should be 1º or 2º, with 3º R - X SN² and free radical coupling is not possible due to steric hinderance so in that case elimination or disproportionation is possible.

In the ionic mechanism alkyl sodium  gives  strong base as well as nucleophile which gives SN² with R - X, ether should be dry otherwise, if moisture is present then  forms R - H instead of R - R with H₂O.

 

(ii) By G.R. 

 

 

(iii) By corey house alkane synthesis

 

Mechanism

R₂CuLi is the source of 

R₂ CuLi do not reacts with -NO₂, - CN, > C = O etc.

Ex.1 

if C is CH₃ - CH₂ - (CH₂)₅ - CH₃, than what is Y.

Ans. CH₃ - (CH₂)₆ - Br

 

Q.1

 

 

(iv) By Franklande reagent

R - X Zn R - X  R - R Zn X₂

Mechanism

 

R - X 

 

(B) By reduction of alkyl halides

 

(i) with metal-acid

 

Reducing agent

Zn / acid, Zn - Cu / H₂O or Zn - Cu acid

Zn - Cu / C₂H₅OH, Na - Hg / acid, Al - Hg / H₂O etc.

 

Mechanism

 

(ii) With Metal hydrides

 

(a) TPH (Ph₃SnH) : It reduces 1º, 2º & 3º R - X

 

R - X  R - H

 

(b) NaBH₄ 

 

(c) , 

 

1.2.3 By red P & HI

Red P & HI is strong reducing agent

R - COOH  R - CH₃

 R - CH₃

 R - CH₃

R - X  R - H

R - OH  R - H H₂O

 

1.2.4 By soda lime → Fatty acids are good source of hydrocarbon, correction, heating of sodium salt of carboxylic acid (R - COONa) with soda lime (NaOH - CaO) gives hydrocarbon, which is known as decarboxylation (e.g. replacement of - COOH group by -H) decarboxylation also takes place on heating only, when compound is gem dicarboxylic acid or there is keto group or double bond on b carbon.

 

Ex.2 

What are A and B

 

Ans. A is 

 

Q.2 

Write the structure of A and mention its stereochemistry

 

1.2.5 By Kolbe's electrolysis

2RCOOK 2HOH  RR 2CO₂  H₂  2KOH

 

e.g. 2CH₃ - COOK 2H₂O  CH₃CH₃  2CO₂  H₂  2KOH.

If n is the number of carbon atoms in the salt of carboxylic acid, the alkane formed has 2(n - 1) carbon atoms.

 

1.2.6 Reduction of aldehydes, ketones :

(a) By Clemmensen's reduction : with Zn - Hg / conc. HCl

 

R - CHO  RCH₃  H₂O

 RCH₂R' H₂O

e.g. CH₃ - CHO  CH₃CH₃  H₂O

 CH₃CH₂C₂H₅  H₂O

 

Clemmensen reduction is not used for compound which have acid sensitive group.

 

(b) By Wolff-kishner reduction with NH₂NH₂ / KOH

Wolff-kishner reduction is not used for compounds which have base sensitive groups.

 

1.3 Physical Properties of Alkanes :

3.3.1 Physical State :

The first four members (C₁ to C₄) are gases : the next thirteen members, (C₅ to C₁₇) are liquids while the higher members are waxy solids.

 

1.3.2 Boiling points :

The boiling points of n-alkanes increase regularly with the increase in the number of carbon atoms

 

Among the isomeric alkanes, the branched chain isomers have relatively low boiling points as compared to their corresponding straight chain isomers. Greater the branching of the chain, lower is the boiling point. This is due to the fact that branching of the chain makes the molecules more compact and brings it close to a sphere, so the magnitude of vander wall forces decreases.

 

1.3.3. Melting Points

It is the evident that the increase in melting point is relatively more in moving from an alkane having odd number of carbon atoms to the higher alkane with even no. of `C' while it is relatively less in moving from an alkane with even number of carbon atoms to the higher alkane.

Explanation : The alkanes with even no. of `C' atoms are more closely packed.

 

1.3.4 Solubility

In keeping with the popular rule "like dissolves like" hydrocarbons are insoluble in polar solvent like water because they are predominantly non-polar in nature.

 

1.3.5 Density

The densities of alkanes increase with increasing molecular weight but become constant at about 0.8 g cm^-3. This means that all alkanes are lighter than water so they floats over water.

 

1.4.1 Chemical Reaction of Alkanes :

Characteristic reaction of alkanes are free radical substitution reaction, these reaction are generally chain reactions which are completed in three steps mainely.

(i) chain initiation (ii) chain propagation, (iii) chain termination

 

Examples of free radical substitution reaction →

R - H + X₂  R - X + HX

 

Exp. CH₄ 

When equimolar amount of methane and Cl₂ are taken, a mixture of four possible products are formed, but if we take excess of CH₄ then yield of CH₃Cl will be the major product.

Reactivity of X₂ : F₂ > Cl₂ > Br₂ > I₂

Reactivity of H : 3ºH > 2ºH > 1ºH

with F₂ alkanes reacts so vigorously that, even in the dark and at room temp, reactant diluted with an inert gas.

Iodination is reversible reaction, since HI formed as a by-product is a strong reducing agent and reduces alkyl iodide back to alkane. Hence iodination can be done only in presence of strong oxidizing agent like HIO₃, HNO₃ or HgO

R - H + I₂  R - I + HI

HI + HIO₃  H₂O + I₂

 

Mechanism of halogenation of CH₄ →

(i) Chain initiation  it is a endothermic step

X₂  

 

(ii) Chain propagation 

 

(iii) Chain termination → it is always exothermic

Each photon of light cleaves one chlorine molecule to form two chlorine redicals, each chlorine atom starts a chain and on an average each chain contains 5000 repetitions of the chain propagating cycle so about 10,000 molecules of CH₃Cl are formed by one photon of light.

 

Some reagent affects the rate of halogenation : For example

Q.3 In the given ways which is feasible

 

Q.4 Which of the following reaction has zero activation energy

(A)  

(B) Cl₂  2 Cl

(C)  

(D) 

 

Q.5 If the E_act for a forward reaction is given

 

the E_act for backward reaction will be

(A) 1 kcal

(B) 4 kcal

(C) -4 kcal

(D) 3 kcal

Halogenations of higher alkane :

(i)

 

(ii) 

 

(iii)

 

(iv)

 

(v)

 

 

Relative amounts of the various isomers differ remarkably depending upon the halogen used from the above reaction, it is observed that chlorination gives mixture in which no isomer greatly dominates while, in bromination gives mixture in which one isomer dominates greatly (97% - 99%).

Factors determining the relative yields of the isomeric products.

(i) Probability factor  This factor is based on the number of each kind of H atom in the molecule.

(ii) Reactivity of hydrogen  The order of reactivity is 3º > 2º > 1º

 

1.4.2 Aromatisation:

 

 

1.4.3 Combustion : (i.e. complete oxidation)

 

 O₂  nCO₂  (n 1) H₂O (DH_combustion = -ve)

 O₂  xCO₂    H₂O

C₅H₁₂  8O₂  5CO₂  6H₂O

Heat of combustion : Amount of heat i.e. liberated when 1 mole of hydrocarbon is completely burnt into CO₂ & H₂O.

Heat of combustion as a measure of stability of alkane :

Combustion is used as a measurements of stability.

More branched alkanes are more stable and have lower heat of combustion.

e.g. (I) CH₃ - CH₂ - CH₂ - CH₃ (II) 

stability : II > I

DH_comb. : I > II

More branched alkane has more no. of primary C - H bonds. (therefore it has more bond energy).

Homologues : Higher homologues have higher heat of combustion.

Isomers : Branched isomer has lower heat of combustion.

 

(i) Initiators  they initiate the chain reaction, initiators are R₂O₂, Perester's etc.

R - O - O - R 

 

(ii) Inhibitors  A substance that slow down or stop the reaction are known as inhibitors

For example O₂ is a good inhibitor

all reactive alkyl free radicals are consumed so reaction become stop for a period of time.

Relative reactivity of halogen toward methane 

Order of reactivity is F₂ > Cl₂ > Br₂ > I₂ which can be explained by the value of DH (energy change)

Steps of halogenation, value of DH for each step. (Kcal/mole)

F Cl Br I

(i) X₂  2 X 38 58 46 38

(ii)  -32 1 16 33

(iii)  - 70 - 26 - 24 - 20

 

Ex.3 Explain why the chain initiating step in thermal chlorination of CH₄ is

Cl₂  and not CH₄ 

Ans. Because E_act of Cl₂ is less than E_act of CH₄

 

 

Ex.4 Chlorination of CH₄ involves following steps :

(i) 

(ii) 

(iii) 

Which of the following is rate determining ?

(A) Step (i)

(B) Step (ii)

(C) Step (iii)

(D) Step (ii) and (iii) both

Ans. (B)

Reactivity of hydrogen  3º > 2º > 1º

Because formation of alkyl free radical is Rds so, that H is more reactive which produce more stable free radical (less E_act)

 

order of stability of F.R.

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Alkenes (Properties and Nomenclature) - Hydrocarbons,Class 11, Chemistry

Alkene

1. Introduction

Alkenes are hydrocarbons with carbon-carbon double bonds, Alkenes are sometimes called olefins, a term derived from olefinic gas, meaning "oil forming gas". Alkenes are among the most important industrial compound and many alkenes are also found in plants and animals. Ethylene is the largest - volume industrial organic compound, used to make polyethylene and a variety of other industrial and consumer chemicals.

2. Structure and bonding in Alkenes

(1) Alkenes are unsaturated hydrocarbons having at least one double bond.

(2) They are represented by general Formula (G.F.) C_nH_2n (one double bond)

(3) In Ethene C = C bond length is 1.34 Å

(4) Its bond energy is 146 kcal.mol^-1

(5) The hybridization of (C = C) alkenic carbon is sp²

(6) The πe^- cloud is present above and below the plane of s-bonded skeleton.

(7) They are also known as olefins since ethene, the first member of the homologous series forms oily liquid substance when treated with halogens.

(8) Compounds may exist as conjugated polyenes or as cumulated polyenes or as isolated polyenes

 

Note : That angle a > b since repulsion due to p electrons (double bond - single bond repulsion > single bond single bond repulsion according to VSEPR theory.

 

Ex.1 Write IUPAC names of

 

(a)  (b) 

 

Ans. (a) 2, 3-Dimethylcyclohexene

(b) 1-(2-butenyl) cyclohex -1-ene

 

Ex.2 Give the structure for each of the following

(a) 4-Methyl-1, 3-hexadiene

(b) 1-Isopropenylcyclopentene

 

Ans. (a)  (b) 

 

3. Physical Properties of Alkenes / Hydrocarbons

Table : III

4. Laboratory test of Alkene

Table - IV

 

5. Methods of preparation of alkenes

(I) BY PARTIAL REDUCTION OF ALKYNES

(a) By Catalytic Hydrogenation of Alkynes in presence of poisoned catalyst(A Syn Addition of Hydrogen : Synthesis of cis-Alkenes : This is performed by)

(i) Lindlar's catalyst : Metallic palladium deposited on calcium carbonate with lead acetate and quinoline.

(ii) P-2 catalyst (Ni₂B nickel boride)

General Reaction R - C º C - R 

 

Mechanism of hydrogenation :

 

Steps : The reactant alkyne molecules and hydrogen molecules get adsorbed at the surface of metal catalyst. It is chemical adsorption (chemisorption).

In this state, the reactants lie very close to each other and so the hydrogen atoms start forming bond with carbon. Two hydrogen atoms are added to two triply bonded carbon atom from the same side of p bond and a cis or syn addition product is formed. The product alkene now escapes away from the surface of the catalyst. Quinoline occupies the metal surface inhibiting further reduction to alkanes Quinoline therefore is called catalyst poison and palladium is called deactivated catalyst or poisoned catalyst.

 

e.g. 

 

(b) Birch Reduction : (Anti Addition of Hydrogen : Synthesis of trans-Alkenes)

General Reaction 

 

 

e.g. CH₃ - CH₂ - C = C - CH₂ - CH₃

 

Note : This process of reduction is not eligible when terminal alkynes are taken. (R - C º CH) because terminal alkynes form sodium salt with Na metal.

CH₃ - C = CH Na / NH₃  CH₃ - CH = C- Na   [H]

 

Ex.3 Identify the reagent for following synthesis.

 

 

Ans. H₂ / Lindiar's catalyst.

 

Ex.4 Identify the products in the following reaction :

 

 

Ans. 

(II) BY DEHAL OGENATION OF VICINAL DIHALIDES

There are two types of dihalides namely gem (or geminal) dihalides in which the two halogen atoms are attached to the same carbon atom and vicinal dihalides in which the two halogen atoms are attached to the adjacent carbon atoms.

Dehalogenation of vicinal dihalides can be effect either by NaI in acetone or zinc in presence of acetic acid or ethanol.

General Reaction

 

(i) 

 

(ii) CH₃ - CHBr - CH₂Br  CH₃ - CH = CH₂

 

Remarks

(1) Both are E2 elimination.

(2) Both are stereospecific anti elimination.

 

(III) DEHYDRO HALOGENATION OF ALKYL HALIDES

Dehydro halogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene.

Dehydro halogenation can take place by E1 and E2 mechanism.

(i) Hot alcoholic solution of KOH EtO^- / EtOH (ii) NaNH₂

(iii) t-BuO-K  in t-BuOH

 

(i) Dehydrohalogenation by the E2 mechanism : Second-order elimination is a reliable synthetic reaction, especially if the alkyl halide is a poor S_N2 substrate. E2 dehydrohalogenation takes place in one step, in which a strong base abstracts a proton from one carbon atoms as the leaving group leaves the adjacent carbon.

General reaction :

 

 

e.g. 

Here b - H is eliminated by base hence called b elimination following Saytzeff rule.

i.e, (Highly substituted alkene is major product). It also involves an anti elimination of HX.

 

e.g. 

 

e.g. 

 

e.g. 

 

e.g. 

 

(ii) Formation of the Hoffmann product

Bulky bases can also accomplish dehydro halogenation that do not follow the saytzeff rule. Due to steric hindrance, a bulky base abstracts the proton that leads to the most highly substituted alkene. In these cases, it abstracts a less hindered proton, often the one that leads to formation of the least highly substituted product, called the Hoffmann product.

 

 

Stereospecific E2 reactions

The E2 is stereospecific because it normally goes through an anti and coplanar transition state. The products are alkene, and different diastereomers of starting materials commonly give different diastereomers of alkenes.

 

 

Ex.5 What alkyl halide would yield each of the following pure alkene on reaction with alcoholic KOH ? 

(i)  

(ii) CH₃ - CH₂ - CH₂ - CH = CH₂ 

 

Ans. (i)

  

(ii) CH₃CH₂CH₂CH₂ CH₂Cl

(iii)

 

 

Ex.6 What are the various product due to loss of HBr from

 

Ans.   

 

(IV) DEHYDRATION OF ALCOHOLS

Alcohols when heated in presence of following reagents undergo loss of water molecule and form alkenes. The elimination is b elimination.

(i) H₂SO₄ / 160ºC

(ii) H₃PO₄ / Δ

(iii) P₂O₅ / Δ

(iv) Al₂O₃ / 350ºC undergo loss of water molecule and form alkenes

 

General Reaction RCHCH₂OH  R - CH = CH₂  H₂O

e.g. 

(V) BY PYROLYSIS OF ESTERS

Thermal cleavage of an ester involves formations of a six membered ring in the transition state leading to the elimination of an acid leaving behind an alkene.

As a direct consequence of cyclic transition state, both the leaving groups namely proton and carboxylate ion are eliminated from the cis position. This is an example of syn elimination.

 

(VI) BY HOFMANN ELIMINATION METHOD

Alkenes can be prepared by heating quaternary ammonium hydroxide under reduced pressure at a temperature between 100ºC and 200ºC.

Less substituted alkenes are formed as major product in this case, which are defined as Hofmann alkenes.

(VII) BY WITTIG REACTION

The aldehydes and ketones are converted into alkenes by using a special class of compounds called phosphorus ylides, also called Wittig reagents.

The Triphenyl group of phosphorane has a strong tendency to pull oxygen atom of the aldehyde or ketone via a cyclic transition state forming an alkene.

(R, R', R" and R"' may be hydrogen or any alkyl group)

e.g. 

 

Ex.7 Complete the following reaction :

 

 

Ans. 

 

Ex.8 Identify the (X), (Y), and (Z) in the following reactions

(i) PhCH₂Br CH₃ - - CH₃  (X)

(ii) CH₃I PhCOCH₃  (Y)

(iii) PhCH₂Br PhCH = CHCHO  (Z)

Ans. (X) = Ph - CH = C(CH₃)₂

(Y) = Ph - C(CH₃) = CH₂

(Z) = Ph - CH = CH - CH = CH - Ph

 

 

6. Chemical reactions of alkenes

 

(I) CATALYTIC HYDROGENATION OF ALKENES : (HETEROGENEOUS HYDROGENATION)

Hydrogenation : The function of catalyst

Hydrogenation of a alkene is exothermic reaction (DHº = - 120 kJ mol^-1)

R - CH = CH - R H₂  R - CH₂ - CH₂ - R heat

As a consequence ,both hydrogen atoms usually add from the same side of the molecule. This mode of addition is called a syn addition.

Hydrogenation of an alkene is formally a reduction, with H₂ adding across the double bond to give an alkane.

The process usually requires a catalyst containing Pt, Pd or Ni.

 

e.g. CH₃ - CH - = CH - CH₃  H₂  CH₃ - CH₂ - CH₂ - CH₃

 

e.g. 

 

Ex.9 Complete the following reactions :

CH₃CH = CH₂  H₂  ?

Sol. CH₃CH₂CH₃

 

(II) ELECTROPHILIC ADDITION REACTIONS :

Mechanism

Step 1 : Attack of the electrophile on p bond forms a carbocation.

 

 

Step 2 : Attack by a nucleophile gives the product of addition

 

 

(i) Acid-Catalyzed Hydration of Alkenes

Alkenes add water in the presence of an acid catalyst to yield alcohols. The addition takes place with Markovnikov regioselectivity. The reaction is reversible, and the mechanism for the acid-catalyzed hydration of an alkene is simply the reverse of that for the dehydration of an alcohol.

The carbocation intermediate may rearrange if a more stable carbocation is possible by hydride or alkanide migration. Thus, a mixture of isomeric alcohol products may result.
 

General Reaction 

 

 

e.g. 

 

e.g. 

 

Ex.10 Identify the product in following reaction

 

Ans. 

 

(ii) (a) Oxymercuration - Demercuration

Alkenes react with mercuric acetate in a mixture of water and tetrahydrofuran (THF) to produce (hydroxyalkyl) mercury compounds. These can be reduced to alcohols with sodium borohydride and water :

Oxymercuration

 

General Reaction 

 

 

In the oxymercuration step, water and mercuric acetate add to the double bond; in the demercuration step, sodium borohydride reduces the acetoxymercury group and replaces in with hydrogen. Then net addition of H -and -OH takes place with Markovnikov regioselectivity and generally takes place without the complication of rearrangements.

 

e.g.

 

 

(b) Alkoxymercuration - demercuration

 

General reaction 

 

e.g. 

 

Ex.11 Supply the structures for (X) and (Y) in the following two - step reaction :

 

C₃H₇CH = CH₂ 

 

Sol. (X) = C₃H₇CH(OH)CH₂-HgOAC (Y) = C₃H₇CH(OH)CH₃

(An organomercurial alcohol) 

Ex.12 Identify final product in the following :

 

(a) 

 

(b) 

 

Ans. (a)  (b) 

 

 

(iii) Hydroboration-oxidation (SYN ADDITION)

 

General Reaction 

 

An alkene reacts with BH₃ : THF of diborane to produce an alkylborane. Oxidation and hydrolysis of the alkylborane with hydrogen peroxide and base yields an alcohol.

 

e.g. 

 

Oxidation

 

 

In the first step, boron and hydrogen undergo syn addition to the alkene in the second step, treatment with hydrogen peroxide and base replaces the boron with -OH with retention of configuration. The net addition of -H and -OH occurs with anti Markovnikov regioselectivity and syn stereoselectivity. Hydrogboration -oxidation therefore, serves as a useful regiochemical complement to oxymercuration demercuration
 

e.g. 

 

e.g. 

(i) Hydration with dil. H₂SO₄ proceeds via carbocation rearrangement

(ii) Hydration with Hg(OAc)₂, H₂O, following by NaBH₄ proceeds via Markonikov's rule

(ii) Hydration with (BH₃)₂ followed by H₂O₂ / OH^- proceeds via Anti Markonikov's rule

 

(iv) Addition of hydrogen halides

 

General Reaction 

 

 

e.g. 

 

 

e.g. 

 

 

Ex.13 Predict the major products of the following reactions and propose mechanism to support your predictions.

(A) 

(B)  CH₃ - CH₂ - O - O - CH₂ - CH₃ 

 

(C) 

 

Sol. (A)  (B)  (C) 

 

Ex.14 Identify the products in the following reactions :

(a) F₃C - CH = CH₂  HCl  (b) O₂N - CH = CH₂  HCl 

(c) CH₃O - CH = CH₂  HCl  (d) PhCH = CHCH₃  HCl 

(e) 

 

Q.6 Give the products of the following reactions : -

 

Q.7 Give the reactant (alkene) of the following products.

 

(v) Addition of halogen

Halogen add to alkenes to form vicinal dihalides.

General Reaction 

(X₂ = Cl₂, Br₂)

The nucleophile attacks the electrophilic nucleus of one halogen atom, and the other halogen serves as the leaving group, departing as halide ion. Many reactions fit this general pattern.

 

Note : (i) F₂ is not added because F  is never generated. Morever reaction is explosive giving CO₂ & H₂O

(ii) I₂ is not added because reaction is reversible with equilibrium in backward direction.

(iii) Reaction with bromine is basis for test of alkenes.

(iv) Halogen addition is stereospecific anti addition

(v) Halogens can also be added in presence of sun light and give free radical addition.

(Reactivity of halogen addition in sunlight is F₂ (explosive) > Cl₂ > Br₂ > I₂)

 

Mech.

Step-1 Formation of a halonium ion

 

 

Step-2 Opening of the halonium ion

 

X attacks from the back side of halonium ion.

 

e.g. 

 

e.g. 

 

Ex.15 Give the product of the following reaction.

 

Me₂C = CH₂  ICl  ?

 

Sol. Cl is more electronegative than I making I the E  that, according to the Markovnikov rule, adds to the C with the greater number of H's. The product is 2-chloro-1-iodo-2-methylpropane, (Me₂CClCH₂I).

 

Ex.16 What are the products and (type of isomers) when Br₂ adds to : -

 

(a)  (b) 
 

 

Ans. (a)  (b) 

 

(vi) Hydroxylation of Alkenes

(a) Syn Hydroxylation : (Reaction with Bayer's reagent, (cold dilute alkaline KMnO₄ solution).

Both OH groups are added from same stereochemical side. This addition is example of syn addition

General Reaction 

e.g. 

The same function of syn addition of 2 - OH groups is performed by OsO₄ / H₂O₂

e.g. 

 

(b) Anti hydroxylation

General Reaction 

 

e.g. 

 

Ex.17 Identify the product in the following reaction :

 

 

Ans. 

 

Ex.18 Identify the product (X) in the following reaction :

 

 

Ans. (x) : 

Since C = C bond in ring is more substituted than that in open chain.

 

(vii) Addition of carbenes to Alkenes :

 

Methylene is the simplest of the carbenes : uncharged, reactive intermediates that have a carbon atom with two bonds and two nonbonding electrons. Like borane (BH₂), methylene is a potent electrophile because it has an unfilled octet. It adds to the electrons rich pi-bond of an alkene to form a cyclopropane.

 

General Reactions 

 

Heating or photolysis of diazomethane (CH₂N₂) gives nitrogen gas and methylene.

There are two difficulties with using CH₂N₂ to cyclopropene double bonds. First, it is extremely toxic and explosive. A safer reagent would be more convenient for routine use. Second, methylene generated from CH₂N₂ is so reactive that it inserts into C - H bonds as well as C = C bonds.

 

e.g. 

 

Ex.19 Identify the product in the following reactions

(a) 

(b) 

 

Ans. (a) (b) 

 

 

(III) EPOXIDATION OF ALKENES :

An alkene is converted to an epoxide by a peroxyacid, a carboxylic acid that has an extra oxygen atom in a -O - O - (peroxy) linkage.

 

General Reaction 

 

The epoxidation of an alkene is clearly an oxidation, since an oxidation, since an oxygen atom is added Peroxyacids are highly selective oxidizing agents. Some simple peroxyacids (sometimes called per acids) and their corresponding carboxylic acids are shown below :

 

 

 

Mech.

e.g. 

 

Ex.20 Complete the following reaction

 

Ans. 

 

Ex.21 Predict the product, including stereochemistry where appropriate, for the m-chloroperoxy-benzoic acid expoxidations of the following alkenes.

(a)  (b) 

(c) Cis-cyclodecene  (D) Trans-cyclodecene 

 

(IV) HALOHYDRIN FORMATION

General Reaction 

X' and H₂O are generated as attacking species from X₂  H₂O

 

e.g. 

 

Ex.22 Predict the product in the following reactions

 

(a) 

 

(b) 

 

Ans. (a)  (b) 

 

(V) OXIDATIVE CLEAVAGE OF ALKENES

 

(i) Cleavage by permanganate

In a KMnO₄ hydroxylation, if the solution is warm or acidic or too concentrated, oxidative cleavage of the glycol may occur. Mixtures of Ketones and carboxylic acids are formed, depending on whether there are any oxidizable aldehydes in the initial fragments. A terminal = CH₂ group is oxidized to CO₂ and water.

General Reaction 

e.g.

 

e.g

. 

 

Ex.23 What is the main utility of this reaction and why is it superior to KMnO₄cleavage for this purpose

 

Sol. It locates the position of C = C's in molecules. KMnO₄ cleavage is more vigorous and can oxidiz other groups, i.e., OH.

 

Ex.24 Give the products of the following reactions : -

 

(i)  (ii) 

 

(iii) 

 

Sol. X = Cis-1, 2-Cyclopentanediol Y = meso - CH₃ - CHOH - CHOH - CH₃

Z = rac - CH₃CHOHCHOHCH₃

 

 

(ii) Ozonolysls : Like permanganate ozone cleaves double bonds to give ketones and aldehydes. However, ozonolysis is milder, and both ketones and aldehydes can be recovered without further oxidation.

 

General Reaction 

Mech.

 

 

 

e.g. 

 

e.g.  CH₃CH₂CHO CH₃(CH₂)₄CHO (65%)

 

e.g. 

 

(VI) HALOGENATION, ALLYLIC SUBSTITUTION

 

General Reaction 

 

NBS = N-Bromosuccinimide 

 

NCS = N-Chlorosuccinimide 

 

 

e.g. 

 

Ex.25 CH₂ = CHCH₂CH = CH₂  (X), (X) is

(A)  (B) CH₂ = CHC = CHCH₂Br

(C) CH₂ = CHCH₂CH = CHBr (D) 

 

Ans. A

 

Ex.26 Assertion (A) : Propene (CH₃CH = CH₂) undergoes allylic substitution.

Reason (R) : CH₂ = CHCH₂ (allylic) free radical is stabilised by resonance.

Ans. (A)

 

Ex.27 Identify the product in the following reactions

(a)  (b) 

 

Ans. (a)  (b) 

 

Ex.28 Identify the products (x, y) of following reaction : -

 

Ans. (X) :  (Y) : 

 

(VII) ADDITION OF FREE RADICALS

 

General Reaction 

 

e.g. 

 

 

Ex.29 Which of the following reactions are correct ?

 

(a) 

 

(b) RCH = CH₂  BrCCl₃ 

 

(A) only (a)     (B) only (b)    (C) both are correct     (D) None of these

Ans. (C)

Ex.30 Isobutylene  product is :

(A) Tertiary butyl bromide (B) Isobutyl bromide

(C) Tertiary butyl alcohol (D) Isobutyl alcohol

Ans. (B)

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Alkynes (Properties and Nomenclature) - Hydrocarbons, CBSE, Class 11, Chemistry

Alkynes

1. Introduction

A triple bond gives an alkyne four fewer hydrogen atoms than the corresponding alkane. There fore the triple bond contributes two degree of unsaturation (DU).

Alkynes are not as common in nature as alkenes, but some plants do use alkynes to protect themselves against disease or predators. Acetylene is by far the most important commercial alkyne. Acetylene is an important industrial feedstock but its largest use is as the fuel for the oxyacetylene welding torch.

 

2. Structure and Bonding in Alkynes

(1) Alkynes are hydrocarbons that contain carbon -carbon triple bond.

(2) Alkynes are also called acetylenes because they are derivatives of acetylene.

(3) The general formula is : C_nH_2n-2. (one triple bond)

(4) In alkyne C º C bond length is 1.20 Å.

(5) Its bond energy is 192 kcal. mol^-1

(6) The hybridization of carbon atoms having triple bond (C º C) in alkynes is sp

(7) Overlapping of these sp hybrid orbitals with each other and with the hydrogen orbitals gives the sigma bond framework which is linear (180º) structure.

(8) Two p bonds result from overlap of the two remaining unhybridized p orbitals on each carbon atom. These orbitals overlap at right angles (90º) to each other, forming one p bond with electron density above and below the C - C sigma bond, and the other with electron density in front and in back of the sigma bond. This result in a cylindrical p electron cloud around s bonded structure

Note : Any type of stereoisomerism does not arise in acetylenic bond due to linearity of C º C bond.

 

Ex.1 Cis-trans isomerism is not possible in alkynes because of :

Ans. 180º bond-angle at the carbon-carbon triple bond.

 

Ex.2 Draw the geometrical isomers of hept -2-en-5-yne?

Ans. 

 

 

3. Physical Properties of Alkynes

(1) Alkynes are relatively nonpolar (w.r.t. alkyl halides and alcohols) and are nearly insoluble in water (but they are more polar than alkenes and alkanes). They are quite soluble in most organic solvents, (acetone, ether, methylene chloride, chloroform and alcohols).

(2) Acetylene, propyne, and the butynes are gases at room temperature, just like the corresponding alkanes and alkenes. In fact, the boiling point of alkynes are nearly the same as those of alkanes and alkenes with same number of carbon atoms.

 

4. Table

 

5. TABLE - COMPARATIVE STUDY OF ALKANES, ALKENES, ALKYNES

Ex.3 Which has a longer carbon-methyl bond, 1-butyne or 2-butyne. Explain?

Ans. The bond from the methyl group in 1-butyne is to an sp³-hybridised carbon and so is longer than the bond from the methyl group in 2-butyne, which is to an sp-hybridised carbon.

 

Ex.4 Arrange the following bond-lengths in increasing order.

Ans. (d) < (b) < (c) < (e) < (a)

 

Q.3 Arrange C - H bond -lengths (a,b,g) in increasing order as shown : -

 

 

6. Laboratory test of Alkyne

 

7. Laboratory test of terminal alkynes

 

When triple bond comes at the end of a carbon chain. The alkyne is called a terminal alkyne.

 

1-Butyne, terminal alkyne

 

 

8. Acidity of Terminal Alkynes

Terminal alkynes are much acidic than other hydrocarbons due to more electronegative sp hybridised carbon. The polarity (acidity) of a C - H bond varies with its hydridization, increasing with the increase in precentage's character of the orbitals.

sp³ < sp² < sp

The hydrogen bonded to the carbon of a terminal alkyne is considerably more acidic than those bonded to carbons of an alkene and alkane (see section). The pK_a values for ethyne, ethene & ethane illustrate this point

pK_a = 25 pK_a=44 pK_a=50

The order of basicity of their anions is opposite to that of their relative acidity:

Relative Basicity

CH₃CH^!₂: > H₂C = CH:^- > HC º C:^-

 

Relative acidity

pK_a 15.7 16-17 25 38 44 50

 

Relative Basicity

 

9. General methods of preparation :

(I) By dehydro halogenation of gem and vic dihalide:

General Reaction: RCH = CHR + Br₂ →  

A vic - dibromide

The dehydrohalogenations occur in two steps, the first yielding a bromoalkene and the second alkyne.

Mechanism :

Step 1

Amide ion Vic-Dibromide Bromoalkene Ammonia Bromide

(The strongly ion

basic amide ion

brings about an

E2 reaction.)

Step 2

e.g. CH₃CH₂CH = CH₂   CH₃CH₂C º CH

e.g.   [CH₃CH₂C º CH]  CH₃CH₂C º C^!Na^Å

 

 

CH₃CH₂C º C:^-Na   CH₃CH₂C º CH NH₃  NaCl

 

General Reaction 

 

Ex.5 Give the structure of three isomeric dibromides that could be used as starting materials for the preparation of 3,3-dimethyl-1-butyne.

 

Sol. (I)  (II)  (III) 

 

Ex.6 Show the product in the following reaction

 

 ? Sol. 

 

Q.4 1,1-dibromopentane on reaction with fused KOH at 470 K gives 2-pentyne

 

 

1,1-dibromo pentane 2-pentyne

 

Give the mechanism of this rearrangement.

 

(II) By Dehalogenation of Tetrahaloalkane:

 

General Reaction 

 

(III) Replacement of The Acetylenic Hydrogen atom of terminal Alkynes.

 

General Reaction 

Sodium ethynide and other sodium alkynides can be prepared by treating terminal alkynes with sodium amide in liquid ammonia.

 

(R or R' or both may be hydrogen)

The following example illustrates this synthesis of higher alkyne homologues.

(R'-X must be an unhindered primary halide or tosylate)

The unshared electron pair of the alkynide ion attacks the back side of the carbon atom that bears the halogen atom and forms a bond to it. The halogen atom departs as a halide ion.

 

 

e.g. 

 

Addition of acetylide ions to carbonyl groups

 

e.g. 

Sodium Propanal 1-Pentyn-3-ol

acetylide

e.g. 

3-Methyl-1-butyne 4-Methyl-1-Phenyl pent-2-yne-1-ol

 

e.g. 

Cyclohexanone 1-Ethynylcyclohexanol(3º)

 

Ex.7 Show how to synthesize 3-decyne from acetylene along with necessary alkyl halides.

Sol. H - C º C - H  H₃C - (CH₂)₅ - C º C - H

1-Octyne

H₃C - (CH₂)₅ - C º C - H  CH₃ - (CH₂)₅ - C º C - CH₂CH₃

1-Octyne 3-Decyne

Q.5 Show how you would synthesize the following compound, beginning with acetylene and any necessary additional reagents.

 

(IV) By Kolble's Electrolytic synthesis.

 

(V) By Hydrolysis of carbides

CaC₂  2HOH → C₂H₂  Ca(OH)₂

MgC₂  2HOH → C₂H₂  Mg(OH)₂

Mg₂C₃  4HOH→ CH₃ - C º CH 2Mg(OH)₂

 

10. Chemical reactions of Alkyne

(I) Reduction to alkenes

(a) By Lindlar's reagent

General Reaction 

 

(b) By Brich reduction

 

General Reaction R - C º C - R'  (anti addition)

 

e.g. 

 

 

(c) By hydroboration reduction

 

General Reaction R - C º C - R' 

 

Ex.8 Identify (X) and (Y) in the following reaction

 

CH₃-CH₂ - C º CH

 

Ans. (X) :  (Y) : 

 

 

(II) Addition of Halogen (X₂=Cl₂, Br₂)

General Reaction R - C º C - R'  R - CX = CX - R'  (Anti-addition)

 

Ex.9 Explain why alkynes are less reactive than alkenes toward addition of Br₂.

 

Sol. The three memebered ring bromonium ion fromed from the alkyne (A) has a full double bond causing it to be more strained and less stable than the one from the alkene (B).

 

(A)  (B) 

(A) less stable than (B)

Also, the C's of A that are part of the bormonium ion have more s-character than those of B, further making A less stable than B.

 

(III) Addition of Hydrogen halides (Were HX = HCl, HBr, HI)

 

General Reaction R - C º C - R' 

e.g. 
 

e.g. CH₃-C º C - CH₂CH₃  HBr → 

e.g. H - C º C - CH₂CH₂CH₃ 

 

Ex.10 Identify the product when one equivalent of HBr reacts with 1-pentene-4-yne

Ex.11   (X)  + (Y)

Idntify (X) and (Y) in the above reaction.

Ans. After first HBr molecule is added, product is ₂ : Second addition ₃ and . Since 2º carbocation ion is more stable than 1º, hence final product is  Y is CH₃CH₂CHBr₂

 

(IV) Addition of water

 

(a) Mercuric ion catalyzed hydration:

General Reaction  + H₂O    

(Martkovnikov rule) Vinyl alcohol Ketone

(unstable) (stable)

Electrophilic addition of mercuric ion gives a vinyl cation, which reacts with water and loses a proton to give an organomercurical alcohol. Under the acidic reaction condition, Hg is replaced by hydrogen to give a vinyl alcohol, callled an enol.

 

Mech. 

e.g.  + H₂O  

 

Ex.12  + H₂O  X

 

Identify the (X) in the above reaction

 

Sol. (X) =  Acetone (a ketone) stable

 

Ex.13 When 2-heptyne was treated with aq. H₂SO₄ containing some HgSO₄, two products, each having the moleuclar formual C₇H₁₄O, were obtained approximately in equal amounts. What are these two compounds?

 

Ans.   

 

Q.9 From which alkyne could each of the following compound be prepared by acid-catalysed hydration?

(a)  (b)  (c) 

 

(b) Hydroboration-oxidation

In alkyne, except that a hindered dialkylborane must be used to prevent addition of two molecules of borane across the triple bond.

 

General Reaction

 

Ex.14 Compare the results of hydroboration oxidation and mercuric ion-catalysed hydration for

(a) 2-butyne (b) Cyclohexyl-actylene

 

Ans.

 

(V) Formation of Alkylide anions (Alkynides)

 

(VII) Reactions with Carbonyl Groups

General Reaction

 

Ex.15 Give the products of the following reactions.

Sol. (a) 
 

(b) 

(c) 

 

Q.10 What are the products of the following reactions:

(a)  (b)

(c)

 

Q.11 Identify 'X' in the following reaction

 

(VIII) Oxidation of a-Dlketones

If an alkyne is treated with aqueous KMnO₄ under nearly neutral conditions, an a-diketone results.

General Reaction 

e.g. 

 

Ex.16 Give the product of the following reactions.

 

Sol. X = CH₃CH₂C - CCH₂CH₂CH₃                                          Y = CH₃CH₂COOH HOOCCH₂CH₂CH₃                                               ||   ||

                           O  O

                      (α diketone)

 

(IX) Oxidative Cleavage

If the reaction mixture becomes warm or too basic the diketone undergoes oxidative cleavage. The products are the salts of carboxylic acids, which can be converted to the free acids by adding dilute acid.

General Reaction 

 

e.g. 

e.g. 

 

Ex.17 Give the products of the following reactions

 

Sol. X: (CH₃)₂CHCOOH HOOCCH₂CH₂CH₃

Y: 2CH₃CH₂COOH, symmetrical internal alkynes give one acid

Z: CH₃COOH HOOCCH₂CH₂COOH HOOCCH₂CH₃

 

 

Ex.18 C₈H₁₀ (A)  Acid (B) Identify (A) and (B) in the above reaction

Sol. (A)  (B) COOH

 

Ex.19 A certain hydrocarbon has the formula C₁₆H₂₆. Ozonation followed by hydrolysis gave CH₃(CH₂)₄CO₂ H and succinic acid as the only product. What is hydrocarbon

 

Sol. DU = 4

Solution Unsolved problems

1. It means there is chiral carbon, hence structure is

 

2. Alkyne  has unsaturation hence C₅H₈ has also one ring of three or four carbon atoms.

(a)  (b) 

 

(a) Exists as cis-and trans-isomer

 

(cis) (trans)

3 

4. As the S-character of the orbital that binds carbon to another atom increases, the pair of electrons in that orbital is more strongly held and it requires more energy for homolytic cleavage of both the C - H and C - C bonds.

5. Mech.

6. We need to add two groups to acetylene and ethyl group and a six-carbon aldehyde (to form the secondary alcohol). If we formed the alcohol group first, the weakly acidic - OH group would interfere with the alkylation by the ethyl group. Therefore, we should add the less reactive ethyl group first, and add the alcohol group later in the synthesis.

The ethyl group is not acidic and it does not interefere with the addition of the second group

Reason : Electron donating groups such as R's make the p-bond more electron - rich and more reactive. Conversely, electron - withdrawing groups such as halogens make the p-bond more electron-poor and less reactive.

7.