Chemistry

Properties of alkenes

Session objectives Chemical reactions of alkene Addition of hydrogen, halogen, hydrogen halides Markownikoff’s rule Anti-Markownikoff’s rule Hydroboration-oxidation Oxymercuration-demercuration Oxidation of alkene Ozonolysis Conjugated dienes

The ease of hydrogenation Addition of Hydrogen The ease of hydrogenation Addition takes predominantly cis manner.

Addition of Halogens Brown Colourless Test of unsaturation

Mechanism Step I Step II stereospecific reaction

Types of reaction Stereospecific reaction Stereoselective reaction Cis-isomer (major)

Addition of hydrogen halides HI > HBr > HCI > HF

Markownikoff’s rule Polar addition to an unsymmetrical alkene takes place in such a manner that the negative part of the addendum attaches to that carbon atom which is more highly substituted (or contains the least number of H atoms)

Markownikoff’s rule

Markownikoff’s rule If HBr is added to a symmetrical alkene, only one product is obtained. Markownikoff’s addition product depends on the stability of intermediate carbocation.

Markownikoff’s rule

Markownikoff’s rule Mechanism

Markownikoff’s rule

Peroxide effect (Anti Markownikoff’s addition) Mechanism:

Addition of water (Hydration) Acid catalyzed addition of water

Hydroboration–oxidation The addition is anti–Markownikoff

Hydroboration–oxidation In a substituted cyclic alkene, a trans alcohol is formed.

Oxymercuration–demercuration

Oxidation of alkenes Test for unsaturation Baeyer’s test This is also called Hydroxylation cis-diol

Oxidation of alkenes Cis- hydroxylation

Oxidative cleavage of alkenes

Oxidation with peroxide

Ozonolysis of alkenes

Do you know Write IUPAC name of alkene which on ozonolysis gives one molecule of propanal and one molecule of propanone

Addition of methylenes

Conjugated dienes (alkadienes)

Conjugated dienes (alkadienes)

Class exercise

Class exercise 1 If singlet carbene is added to the ethylene, the product formed will be (a) propane (b) butane (c) cyclopropane (d) cyclobutane Solution: Cyclopropane Hence the answer is (c)

Class exercise 2 Hydroboration-oxidation of 1-methyl cyclopentane will give (a) trans-1-methyl cyclopentan-2-ol (b) cis-1-methyl cyclopentan-2-ol (c) Mixture of (a) and (b) (d) 1-methylcyclopentan-1-ol Solution: cis-1-methyl cyclopentan-2-ol Hence the answer is (b)

Class exercise 3 Anti-Markownikoff’s addition of HBr is not observed in (a) But-2-ene (b) Propene (c) But-1-ene (d) Pent-2-ene Solution: As but-2-ene is a symmetrical structure, we get same product while anti-Markownikoff’s rule applies. Hence the answer is (a)

Class exercise 4 Electrophilic addition of Br2 to ethylene proceeds through (a) a transition state (b) a cyclic bromonium ion Solution: A three-centred cyclic bromonium ion is formed. Hence the answer is (a)

Class exercise 5 Predict the major product of each of the following reaction. Solution:

Class exercise 6 Predict the major product of each of the following reaction. Solution:

Class exercise 7 Predict the major product of each of the following reaction. Solution:

Class exercise 8 Write a mechanism for each of the following reaction. Solution:

Class exercise 9 A hydrocarbon (R) adds one mole of hydrogen to form n-hexane . When R is oxidized vigorously with hot alk. KMnO4 followed by acid treatment it yields a single carboxylic acid containing 3 carbons. Give the structure of R Solution: R: CH3CH2CH = CHCH2CH3

Class exercise 10 An organic compound (A) having molecular formula decolourizes bromine water.On hydrogenation (A) yields 2-methylbutane and reductive ozonolysis forms ethanal and propanone. What is A ? Solution:

Thank you