(4) Addition of HBr to Alkenes in The(4) Addition of HBr to Alkenes in The

Presence of Peroxides Presence of Peroxides

(Anti Markovnikov’s rule)(Anti Markovnikov’s rule) When alkenes are treated with HBr in the presence

of peroxides, ROOR (eg: H2O2) the addition occurs

in an anti-Markovnikov manner

The hydrogen atom of HBr attached to the doubly bonded carbon with fewer hydrogen atoms.

Example:

CH2CH2CH3Br

CH2 CH CH3 HBr ROOR+

CH2C CH2CH3

CH3

+ HBrH2O2

Complete the following reactions:Complete the following reactions:

(1)

(2)

CH2HBr

H2O2+

(5) Hydration of Alkene(5) Hydration of Alkene The acid-catalyzed addition of water to the

double bond of an alkene (hydration of an alkene) is a method for the preparation of low molecular weight alcohols.

The acid most commonly used to catalyze the hydration of alkenes are dilute solution of sulphuric acid and phosphoric acid.

• The addition of waterThe addition of water to the double bond follows Markovnikov’s rule.

C C + H2OH3O

+

C C

H OH

Example:

CH3 C CH2

CH3

H2OH3O

+

+

C CH3CH3

OH

CH3

Complete the following reactions:Complete the following reactions:

(1)(1)

CH2H2O

H3O+

+

(2)

CH2 CH CH3 H2OH3O+

+

(6) (6) Addition of Sulphuric Acid Addition of Sulphuric Acid to to AlkenesAlkenes

Alkenes dissolve in concentrated sulphuric acid to form alkyl hydrogen sulphatesalkyl hydrogen sulphates.

Alkyl hydrogen sulfates can be easily hydrolyzed to alcohols by heating them with water.

The overall result of the addition of sulphuric acid to alkenes followed by hydrolysis is the Markovnikov addition of -H and -OH.

C C

CH3

CH3 CH3

CH3conc. H2SO4 C C CH3CH3

CH3

H OSO3H

CH3

alkyl hydrogen sulphate

H2O

heatC C CH3CH3

CH3

H OH

CH3

• Example:

CH2

CH CH3

conc.H2SO

4 CH3

CH CH2

OSO3H

H

H2O

heatCH

3CH CH

2

OH

H

Alkenes undergo a number of reactions in which the carbon-carbon double bond is oxidized

6 ) Oxidation of Alkenes6 ) Oxidation of Alkenes

KMnO4 ozonolysis

cold & dilute KMnOcold & dilute KMnO44

hot , OHhot , OH-- or H or H++

i)i) With With cold cold and dilute potassium and dilute potassium permanganate, KMnOpermanganate, KMnO4 4

Potassium permanganate in base can be used to oxidize alkenes to 1,2-diols (glycols).

C C + KMnO4(purple)

OH-,cold

C C

OH OH

+ MnO2

(brown precipitate)

This reaction is called Baeyer’s test.

It can serve as a test for the presence of test for the presence of carbon-carbon double bondscarbon-carbon double bonds where the purple colour of the KMnO4 decolourised,

and brown precipitate of MnO2 is formed.

Example:

CH2 CH2 + KMnO4 cold

OH-,H2O

C C HH

OH

HH

OH

+ MnO2

ii) With hot hot potassium permanganate solutions to alkenes (oxidative

cleavage of alkenes)

When oxidation of the alkene is carried out with KMnO4, in acidic rather than basic acidic rather than basic

solutionsolution, cleavage of the double bond occurs and carbonyl-containing products are obtained.

If the double bond is tetrasubstitutedtetrasubstituted, the two carbonyl-containing products are ketonesketones;

If a hydrogen is present on the double a hydrogen is present on the double bondbond, one of the carbonyl-containing products is a carboxylic acidcarboxylic acid;

If two hydrogens are present on one carbon, COCO22 and H2O are formed.

Example:1)

C

O

CH3CH3+ C

O

CH3CH3

C C CH3CH3

CH3

CH3

+ KMnO4 (ii) H3O+

(i) OH-,heat

2)

CH2 CH CH3 + KMnO4(i) OH

-,heat

(ii) H3O+

C

O

CH3HO+ CO2 + H2O

• The oxidative cleavage of alkenes can be used to establish the location of the establish the location of the double bonddouble bond in an unknown alkene.

Example:

• An unknown alkene with the formula CC77HH1414

was found on oxidation with hot basic permanganate, to form a three-carbon three-carbon carboxylic acid (propanoic acid)carboxylic acid (propanoic acid) and four-four-carbon carboxylic acid (butanoic acid).carbon carboxylic acid (butanoic acid). What is the structure of this alkene?

C7H14 + KMnO4

(i) OH-,heat

(ii) H3O+

C

O

OHCH2

CH3 + C

O

OHCH2

CH2CH3

Answer:

C

O

OHOHCH2

CH3 C

O

OHOHCH2

CH2CH3

propanoic acid butanoic acid

CH2CH2CH3CCCH2CH3

H H

3-heptene

ExampleExample

• An unknown alkene undergoes oxidation by hot basic KMnO4, after acidification, gives the following product:

CH3CCH2CH2CH2CH2C OH

Deduce the structural formula for the unknown alkene.

O O

iii) Ozonolysis of Alkenes

(Oxidative Cleavage of Alkenes)

A more widely used method for locating the double bond of an alkene involves the use of ozone (O3).

Ozone reacts vigorously with alkenes to form unstable compounds called molozonidesmolozonides, which rearrange spontaneously to form compounds as ozonidesozonides..

C C + O3

C C

O

ozonide

O

O

Ozonides:Ozonides: very unstable compounds

can easily explode violently

they are not usually isolated but are reduced directly by treatment with water and in the presence of zinc and acid (normally acetic acid) to give carbonyl compounds (either aldehydes or ketones).

ozonide

C C

O O

O

+ ZnH2O,H

+

H

R

C = O

+ O = C

R

H

Example:

C CHCH3 CH3

CH3 (i)O3

(ii)Zn,H2O/H

+

CO

CH3

CH3

CO

CH3

H+

Example Example • Deduce the structural formula of an

alkene that gives the following compound when it reacts with ozone in the presence of Zn / H+.

O=CH-CH2-CH2-CH(CH3)CH=O

1. Write the structure of alkene that would produce the following products when treated with ozone followed by water, zinc and acid

CH3COCH3 and CH3CH(CH3)CHO

ExerciseExercise

2. Acid-catalyzed dehydration of neopentyl alcohol, (CH3)3CCH2OH,

yields 2-methyl-2-butene as the major product. Outline a mechanism showing all steps in its formation.

ExerciseExercise

• Compounds A, B, C and D are isomers with the molecular formula C4H8. AA and B B give a positive Baeyer testpositive Baeyer test, while C and D do not. AA exist as cis- and trans- isomerscis- and trans- isomers, while B does not have geometrical isomers. C has only secondary hydrogenC has only secondary hydrogen, while D has primary, secondary and D has primary, secondary and tertiary hydrogentertiary hydrogen. Give the IUPAC names for A, B, C, and D.