Question

Question:What reagents are needed to convert cyclopentene into

(a)bromocyclopentane;

(b) trans-1,2-dibromocyclopentane;

(c) 3-bromocyclopentene?

Short answer

Answer

(a) HBr: HBr addition to alkene

(b) Br₂ : Anti-addition

(c) NBS in presence of light: Allylic bromination

Step-by-step solution

HBr addition to an alkene

There are two ways to convert alkenes to alkyl bromides. One is to treat it with HBr, and another is to treat it with HBr in the presence of light, heat, or peroxides.

Here, Br gets added to the more substituted carbon center.

HBr addition to alkene

In the presence of light, heat or peroxide, Br gets added to the less substituted carbon center, as shown below:

HBr addition is presence of light, heat, or peroxide

  Allylic bromination of alkene

You can convert alkene to allyl bromide by treating it with NBS in the presence of light. One such example is shown below:

Allylic bromination of alkene

   Reactions of cyclopentene

(a) Bromo-cyclopentane

One can convert cyclopentene to bromo-cyclopentane using HBr as the reagent. The reaction is shown below:

Conversion of cyclopentene to bromo-cyclopentene

(b) trans-1,2-dibromocyclopentane

You can convert cyclopentene to trans-1,2 dibromocyclopentane using as the reagent. The reaction is shown below:

Conversion of cyclopentene to trans-1,2-dibromocyclopentene

(c) 3-bromocyclopentene

Keeping the double bond unchanged, cyclopentene can be treated with NBS in presence of light to form 3-bromocyclopentene. The reaction is shown below:

Conversion of cyclopentene to 3-bromocyclopentene