A three-step synthesis of 3-bromocyclopentene from cyclopentane is as follows:
(1) Halogenation of an alkane
(2) Dehydrohalogenation
(3) Allylic bromination
What is a halogenation reaction?
Generally, halogenation is the reaction of a halogen with an alkane in which the introduction of halogen atoms occurs into the organic molecule by an addition reaction or by a substitution reaction.
(1) Halogenation of an alkane
- Halogenation of an alkane produces a hydrocarbon derivative in which one or more halogen atoms have been substituted for hydrogen atoms.
- Reaction with Br₂ in the presence of UV light to give bromocyclopentane.
(2) Dehydrohalogenation
- Dehydrohalogenation is an elimination reaction which removes a hydrogen halide from a substrate.
- Reaction with hot alcoholic KOH to give cyclopentene.
(3) Allylic bromination
- NBS reacts with trace amounts of HBr to produce a low enough concentration of bromine to facilitate the allylic bromination reaction.
- Reaction with N-bromosuccinimide and a drop of HBr(aq) gives 3-bromocyclopentene.
Hence, the three steps are halogenation, dehydrohalogenation, and allylic bromination.
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