The reaction of cyclopentene with N-bromosuccinimide (NBS) and light produces 3-bromocyclopentene. Which statement accurately describes the stereochemistry of the product?
- The product is achiral and does not have any stereocenters.
- A single enantiomer, (R)-3-bromocyclopentene, is formed selectively.
- A racemic mixture of (R)- and (S)-3-bromocyclopentene is formed. (correct answer)
- A pair of diastereomers, cis- and trans-3-bromocyclopentene, is formed.
Explanation: The reaction is an allylic bromination that proceeds through a radical mechanism. The starting material, cyclopentene, is achiral. The mechanism involves the abstraction of an allylic hydrogen to form a cyclopentenyl radical. This radical intermediate is planar (or very nearly planar) at the radical center due to sp2 hybridization. The bromine atom can then add to this planar intermediate from either the top face or the bottom face with equal probability. This non-selective addition creates a new stereocenter at C3. Since attack from both faces is equally likely, both the (R) and (S) enantiomers are produced in equal amounts, resulting in a racemic mixture.