Which of the following compounds will produce a racemic mixture upon reaction with concentrated $\text{HBr}$?

Solution

1. The reaction of ${\text{CH}}_3\text{CH}(\text{OH}){\text{CH}}_2{\text{CH}}_3$ with $\text{HBr}$ involves the formation of a secondary carbocation ${\text{CH}}_3{\text{CH}}^{+}{\text{CH}}_2{\text{CH}}_3$.
2. The carbocation is planar and achiral.
3. Nucleophilic attack by ${\text{Br}}^{-}$ can occur from either side of the planar carbocation.
4. This results in the formation of both enantiomers of 2-bromobutane.
5. Hence the answer is (B).