Preparation of Alkenes

METHODS OF PREPARATION

1. Dehydrohalgoenation

It is possible to form alkenes by base – induced elimination from alkyl halides.

Alcoholic KOH converts alkyl halide into alkene by a reaction called dehydrohalogenation which involves removal of the halogen atom together with a hydrogen atom from a carbon adjacent to the one having the halogen.

Note:

(i) Ease of dehydrohalogenation of alkyl halides is in order 3⁰ > 2⁰ > 1⁰.

(ii) Ease of formation of alkenes is in order

R₂C = CR₂ > R₂C = CHR > R₂C = CH₂ or RCH = CHR > RCH = CH₂ > CH₂= CH₂ and same is the stability order of alkenes.

Also

Thus by general rule: More stable the alkene, the more easily it is formed

Increasing rate of dehydrohalogenation is in the order.

RF < RCl < RBr < RI

Also greater the conjugation, greater is the stability (due to resonance) hence easier is the dehydrohalogenation.

As II is more stable than IV hence dehydrohalogenation of I is easier than that of (III)

In case of elimination of HBr from 1 – bromo – 1 – methyl cyclohexane, loss of bromide provides a tertiary cation. This species is symmetrical and loss of proton from either of the adjacent methylene groups leads to the same product, 1 – methyl – 1 cyclohexene in which the double bond is in the ring (endocyclic) on the other hand loss of proton from the methyl group produces methylene cyclohexane (Y) in which the double bond is outside the ring (exocyclic). By saytzeff rule, the more highly substituted an alkene, the more stable it is hence formation of 1 – methyl – 1 – cyclohexene is favoured.

Formation of less substituted alkene in an elimination reaction is referred to as a Hofmann elimination.

Note:

Hindered base gives Hofmann product as major isomer.

2. Dehydration of Alcohols

Alcohols undergo dehydration to give alkenes.

3. Dehalogenation

(i) Vicinal dihalides undergo dehalogenation in the presence of Zn, Ag or Mg.

(ii) Geminal dihalides undergo coupling reaction Via dehalogenation to give alkenes.

4. Thermal elimination reaction

The product formation takes place by Hofmann rule. Following compounds give thermal elimination reactions.

(i) Acetates

(ii) Amine oxide

Thermal elimination of amine oxides is known as copper elimination.

(iii) Quaternary ammonium hydroxide

Thermal elimination of this compound is known as Hofmann elimination.

5. By partial reduction of alkynes:

Alkynes undergo partial reduction to give alkenes in the presence of catalyst

6. Wittig Reaction

Carbonyl compounds react with 1⁰ and 2⁰ alkyl halides in the presence of triphenyl phosphine and strong base (RLi, NaH etc) to give alkenes. This reaction is known as Wittig reaction.

Note:

For writing product remove H and X from the a - carbon of alkyl halide and oxygen from- carbonyl carbon and join these two carbons (a - carbon and carbonyl carbon) by double bond.

7. Kolbe hydrocarbon synthesis

Electrolysis of potassium salt of succinic acid gives alkene at the anode

Illustration 1. Explain which of the following reactions would provide a better synthesis of

2 – pentene.

Solution:

Hence (a) is better