Chemistry

Alkynes

Session objectives

1. Preparation of alkynes by

(a) Kolbe’s method

(b) dehydrohalogenation of vicinal dihalide

(c) dehydrohalogenation of geminal dihalide

2. Chemical reactions of alkyne

(a) Addition of halogen, hydrogenhalides.

(b) Ozonolysis

3. Confirmatory test for alkyne

General characteristics of alkynes

•General formula CnH2n–2

•sp hybridization •C–C bond length 1.2 A0

•Shows chain and functional isomerism

Chain isomerism

CH3 CH2 CH2 C CH CH3 CH C CH

CH3

CH3 C CH CH2 C CH2

Functional group isomerism

Preparation

Kolbe electrolysis

At anode At cathode

CH

2CO2 2HNaOH2

CHCOOCH.

Na2

OH2.

CHCOO.CH

.

NaCHCOO CHCOO Na2

NaCOOCH CHCOOC2

CH

Sodium maleate-2e-

e-Current

Dehydrohalogenation of vicinal dihalides

CH3CH CH2

Br Br

CH3 C

2KOH (alc)2H2OCH + 2KBr +

From gem dihalides

CH3

Cl

Cl

KOH.alc

+ 2 KCI + 2 H2O

Miscellaneous methods

Miscellaneous methodsFrom CaC2

CaC H O HC CH Ca(OH) 2 2 2

Lime

Chemical properties

Addition Reactions

(i) Addition of hydrogen

CH3CNi

CH3CH2CH3

Propyne

CH + 2H2

Chemical properties

CH3

H3C

H H

CH3

CH3 C CC CH2 / Lindlar

Catalyst

H

H

CH3

H3CCH3CH3C C CC

Na/NH3 (liq.)

Trans-2-butene

Lindlar catalyst

(H2/Pd/BaSO4) partially reduces a disubstituted alkyne to a cis-alkene.

Chemical properties

Addition of halogens

HC CH Br 22

Br Br

| |

H C C H

| |

Br Br

Addition of hydrogen halides

HC CH 3 22HBr CH CH Br

Mechanism

CHCH3 C CH3CH3 C

+CH3

Br

CH3CH2

BrBr

BrBr

CH3CH3CH2

Br

CH

HBr

(2°) more stable (1°) less stable

C CHBr

Major product

(Anti Markownikoff addition)

CH3 CH CHBrCH3CH +C HBr

Peroxide

Addition of water

HC CH + H2OHgSO4

H2SO4

CH2 CHOH

CH3CHO

Unstable

CH3CH2CH3

CH3CH3

O

C CH2O/H+

HgSO4

C

CH3C CCH2CH3

O

CH3CCH2CH2CH3CH3CH2CCH2CH3

(Major) (Minor)

OH2O/H+

HgSO4

+

Hydroboration oxidation

CH3C CHCH3CH

HC

R2BH

0-10°CBR2

H2O2/OH-

CH3 CH

O

CH

H

CH3CH2CHO

Aldehyde

Tautomerization

: :

Oxidation of alkynes

HCCOOH

COOH

CH CO2 + H2OStrong heatingalk.

KMnO4

CHCH3C CH3COOH + CO2 + H2O 4KMnO.alk

CH3C CCH3 CH3COOH + CH3COOHalk . KMnO4

Ozonolysis of alkynes

RC CR + O3R — C — C — R

O

O — O

RCOOH + RCOOH

R — C — C — R + H2O2

O O

H2O

HC CH + O3H — C — C — H

O

O — O

H — C — C — H + HCOOH

O O

H2O2

H2O

MajorMinor

Formation of alkynides

CH AgNO3

C Ag

+2 RC + 2 NH4OH

(Tollens' reagent)

+2 RC

White ppt

+ 2 NH4CI 2 H2O– +

Formation of alkynides

CH + Cu2Cl2 CCu2 RC +2 RC 2 NH4Cl + 2H2ORed ppt

2 NH4OH+– +

Acidity of Alkynes

H

H

H H

H

H

H H

H

CCC C : :

:

C C- - -

Acetylide anions Vinyl anion Ethyl anion

- - -2 5 2C H > CH = CH > HC C

2 2 3 3HC CH>CH =CH >CH - CH

Acidity of Alkynes

HC CH Na HC C Na H 21

2sodium. acetylide

Acidity of Alkynes

The order of boiling point among hydrocarbons is

alkynes > alkenes > alkanes.

These hydrocarbons also possess low dipole moment.

μ = 0.80 D

CH CH C CH CH CH CH CH CH C C CH 3 2 3 2 2 3 3

μ = 0.30 D μ = 0.0 D

Acetylene having two hydrogen atoms is more acidic than a mono substituted acetylene.

Class exercise

Class exercise 1

Hence, the answer is (c)

Solution:

How will you effect the following conversions?

CH4 CH2 CH2(a)

CH3C CH3COCH3CH(b)

(a) (i) Cl2 + h(ii) Wurtz reaction

(iii) Cl2 + h(iv) alc. KOH, heat

(b) (i) HgSO4, H2SO4 (20%)

Class exercise 2

Write the structural formula of the main organic product.

Hence, the answer is (b)

Solution:

C CH

CH3

(i)

H2O/H+

HgSO4

CH3C CH(ii)2 HOCl

CH3C CCH2CH2CH3(iii)H2/Lindlar's

Catalyst

Solution

O

(i)

CH3 — C— CHCl2

O

(ii)

H H

(iii)

Class exercise 3Give a chemical test to distinguish between 1-butyne and 2-butyne.

Solution:

1 butyne will give white ppt. with ammonical AgNO3.

Class exercise 4

A cylinder contains one of the following gases — propyne, propene, propane. What chemical test would you apply to identify the gases?

Solution:

AgNO3 (ammonical) is used for alkyne, alk. KMnO4, and heat is applied for alkene. No reaction takes place with propane with the above mentioned reagents.

Class exercise 5

Write the product of the following reaction.

Solution:

CH3CH2CHCl2Alkali

Boil

CH3CH2CHO

Class exercise 6

Solution:

What reaction of an appropriate alkyne would lead to the following product?

H3C

Br

Br

CH2CH3

C C

CH3C CCH2CH3

Class exercise 7

An organic compound (A) having molecular formula when treated with NaNH2/ NH3 followed by reactionwith n-propyl bromide yielded (B) . Compound (A) gives a ketone (C)when treated with acidified HgSO4

. (B) on oxidation with hot alk. KMnO4 gives two isomeric acids (D) and (E). Deduce structures from (A) to (E).

Solution:

A: B:O

C COOH

D

COOHE:

Class exercise 8

Write the correct product of the following reaction.

Solution:

HC CH + 2HOCl ? Product

Cl2CHCHO

Class exercise 9

Write the products of the following reaction.

Solution:

H — C C — HNa C2H5Br

KMnO4, HotO3, distillation

(A) (B)

(C) + (D) + (E)

(F) + (G)

H — C C Na–A is +

H — C C — C2H5B is

C is CO2

D is C2H5CO2H

E is H2O

F is HCOOH

G is C2H5COOH

Class exercise 10

Write the intermediate steps of the following reaction

Solution:

C6H5CHC CH C6H5CH CHCHO

OH

H3O+

Solution

C6H5CH — C CH

OH

C6H5CH — C CH

OH2

C6H5CH — C CHC6H5 — CH

C

CH

H2O , –H

C6H5CH C CH — OH

C6H5CH CH — CH O

+

–H2O

+

+

+

Thank you