Question

What alkyl halides might the following alkenes have been made from ?

(a)

(b)

Short answer

1. The alkyl halide formed from which the given alkene is made,
   
2. The alkyl halide formed from a given alkene is,
   

Step-by-step solution

Formation of alkenes from alkyl halides

The alkenes are formed from alkyl halides when treated with alcoholic KOH.Here, alcoholic KOH is used for dehydrohalogenation, where hydrogen and halogen are cleaved and form a double bond.

The alkyl halide from which the alkene is formed

1. When 1-bromo-3,6-dimethylheptane (alkyl halide) is treated with KOH and ethanol, it forms 4,7-dimethyloctene (alkene). It does not give a mixture of products on elimination.
   Formation of an alkene from alkyl halide

Synthesis of alkenes

For the synthesis of alkenes, when alkyl halides are heated with alcoholic KOH, one molecule of halogen acid is eliminated.The work of alcoholic KOH is dehydrohalogenation.

The alkyl halide from which the alkene is formed

(b) When 1-bromo-3,4-dimethylcyclopentane is treated with alcoholic KOH, it forms 3,4-dimethylcyclopent-1-ene. It does not give a mixture of products on elimination.

Formation of an alkene from alkyl halide