Consider the reaction of $(2S,3S)\text{-}3\text{-bromo-}2\text{-butanol}$ with concentrated $\text{HBr}$. What is the major product formed?

Solution

1. The protonation of the hydroxyl group creates a good leaving group, ${\text{-OH}}_2^{+}$.
2. Loss of water generates a secondary carbocation at ${\text{C}}_2$.
3. The neighboring bromine atom participates in anchimeric assistance, forming a cyclic bromonium ion intermediate.
4. Nucleophilic attack by ${\text{Br}}^{-}$ occurs with inversion at one center, resulting in the formation of the meso-2,3-dibromobutane.
5. Hence the answer is (A).