36. Hydrocarbon 4

8/13/2019 36. Hydrocarbon 4

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Alkynes


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Session objectives

1. Preparation of alkynes by

(a) Kolbes method

(b) dehydrohalogenation of vicinal dihalide

(c) dehydrohalogenation of geminal dihalide2. Chemical reactions of alkyne

(a) Addition of halogen, hydrogenhalides.

(b) Ozonolysis

3. Confirmatory test for alkyne


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General characteristics ofalkynes

General formula CnH2n2sp hybridizationCC bond length 1.2 A0

Shows chain and functionalisomerism


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Chain isomerism

CH3

CH2 CH2 C CH CH3 CH C CH

CH3

CH3

C CH CH2 C CH2

Functional group isomerism


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Preparation

Kolbe electrolysis

At anode At cathode

CH

2CO22HNaOH2

CHCOOCH

.

Na2

OH2.

CHCOO.CH.

NaCHCOO CHCOO Na2

NaCOOCH CHCOO C2

CH

Sodium maleate-2e-

e-

Current


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Dehydrohalogenation of vicinaldihalides

CH3

CH CH2

Br Br

CH3

C

2KOH (alc)2H2OCH + 2KBr +

From gem dihalides

CH3

Cl

Cl

KOH.alc

+ 2 KCI + 2 H2O


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Miscellaneous methods

Miscellaneous methodsFrom CaC2

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

Lime


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Chemical properties

Addition Reactions

(i) Addition of hydrogen

CH3C

NiCH

3CH

2CH

3

Propyne

CH + 2H2


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Chemical properties

CH3

H3C

H H

CH3

CH3 C CC C

H2/ Lindlar

Catalyst

H

H

CH3

H3C

CH3CH3C C CC

Na/NH3(liq.)

Trans-2-butene

Lindlar catalyst

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


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Chemical properties

Addition of halogens

HC CHBr

22

Br Br| |

H C C H

| |

Br Br

Addition of hydrogen halides

HC CH 3 22HBr CH CH Br


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Mechanism

CHCH3 C CH3

CH3 C

+CH

3

Br

CH3

CH2

BrBr

Br

Br

CH3CH3

CH2

Br

CH

HBr

(2) more stable (1) less stable

C CHBr

Major product

(Anti Markownikoff addition)

CH3 CH CHBrCH

3CH +C HBr

Peroxide


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Addition of water

HC CH + H2O

HgSO4

H2SO

4

CH2 CHOH

CH

3CHO

Unstable

CH3

CH2CH3

CH3

CH3

O

C C

H2O/H+

HgSO4

C

CH3C CCH2CH3

O

CH3CCH2CH2CH3CH3CH2CCH2CH3

(Major) (Minor)

O

H2O/H+

HgSO4+


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Hydroboration oxidation

CH3C CHCH

3CH

HC

R2BH

0-10CBR2

H2O2/OH-

CH3

CH

O

CH

H

CH3CH

2CHO

Aldehyde

Tautomerization

: :


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Oxidation of alkynes

HCCOOH

COOH

CH CO2+ H2OStrong heatingalk.

CHCH3C CH3COOH + CO2+ H2O

4KMnO.alk

CH3

C CCH3

CH3COOH + CH

3COOH

alk . KMnO4


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Ozonolysis of alkynes

RC CR + O3

RC C R

O

O O

RCOOH + RCOOH

RC C R + H2O

2

O O

H2O

HC CH + O3 H C C H

O

O O

H C C H + HCOOH

O O

H2O

2

H2

O

MajorMinor


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Formation of alkynides

CH AgNO3

C Ag

+2 RC + 2 NH4OH

(Tollens' reagent)

+2 RC

White ppt

+ 2 NH4CI 2 H2O +


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Formation of alkynides

CH + Cu2Cl

2 CCu2 RC +2 RC 2 NH4Cl + 2H2O

Red ppt

2 NH4OH+ +


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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


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Acidity of Alkynes

HC CH Na HC C Na H 21

2sodium. acetylide


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Acidity of Alkynes

The order of boiling point among hydrocarbons is

alkynes > alkenes > alkanes.

These hydrocarbons also possess low dipole moment.

= 0.80 D

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

= 0.30 D = 0.0 D

Acetylene having two hydrogen atoms is more acidic

than a mono substituted acetylene.


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Class exercise


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Class exercise 1

Hence, the answer is (c)

Solution:

How will you effect the followingconversions?

CH4 CH2 CH2(a)

CH3C CH3COCH3CH(b)

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

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

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


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Class exercise 2

Write the structural formula of themain organic product.

Hence, the answer is (b)

Solution:

C CH

CH3

(i)

H2O/H+

HgSO4

CH3C CH(ii)2 HOCl

CH3C CCH

2CH

2CH

3(iii)

H2/Lindlar's

Catalyst


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Solution

O

(i)

CH3CCHCl

2

O

(ii)

H H

(iii)


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Class exercise 3

Give a chemical test to distinguishbetween 1-butyne and 2-butyne.

Solution:

1 butyne will give white ppt. with ammonical AgNO3.


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Class exercise 4

A cylinder contains one of thefollowing gases propyne,

propene, propane. What chemicaltest would you apply to identifythe gases?

Solution:

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


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Class exercise 5

Write the product of thefollowing reaction.

Solution:

CH3CH

2CHCl

2

Alkali

Boil

CH3CH2CHO


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Class exercise 6

Solution:

What reaction of an appropriatealkyne would lead to the following

product?

H3C

Br

Br

CH2CH3

C C

CH3

C CCH2CH

3


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Class exercise 7

An organic compound (A) havingmolecular formula when treated

with NaNH2/ NH3followed by reactionwith n-propyl bromide yielded (B) .Compound (A) gives a ketone (C)when treated with acidified HgSO4

.(B) on oxidation with hot alk. KMnO

4

gives two isomeric acids (D) and (E).Deduce structures from (A) to (E).

Solution:

A: B:O

CCOOH

D

COOHE:


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Class exercise 8

Write the correct product of thefollowing reaction.

Solution:

HC CH+2HOCl ? Product

Cl2CHCHO


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Class exercise 9

Write the products of the followingreaction.

Solution:

H C C HNa C2H5Br

KMnO4, Hot

O3, distillation

(A) (B)

(C) + (D) + (E)

(F) + (G)

HC C NaA is +

H C C C2H5B is

C is CO2

D is C2H5CO2H

E is H2O

F is HCOOH

G is C2H5COOH


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Class exercise 10

Write the intermediate steps of thefollowing reaction

Solution:

C6H

5CHC CH C6H5CH CHCHO

OH

H3O+


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Solution

C6H

5CH C CH

OH

C6H

5CH C CH

OH2

C6H

5CH C CHC6H5CH

C

CH

H2O , H

C6H

5CH C CH OH

C6H

5CH CH CH O

+

H2O

+

+

+


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Thank you

36. Hydrocarbon 4

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