Question

In S\(_N\)1 reaction, the alkyl halide that on hydrolysis produces racemic mixture is

• A. Tertiary butyl bromide
• B. 2-bromobutane
• C. Isopropyl bromide
• D. Methyl bromide

Hint:

To predict racemization in a substitution reaction, check two conditions: 1. The reaction must proceed via an S\(_N\)1 mechanism (favored by 3\(^\circ\)>2\(^\circ\) halides, polar protic solvents). 2. The carbon atom bearing the leaving group in the starting material must be a chiral center. If both conditions are met, racemization is the expected outcome.

Answer 1

The Correct Option is B

Step 1: Understanding Racemization in S\(_N\)1 Reactions.
The S\(_N\)1 reaction mechanism proceeds through a carbocation intermediate. If the carbocation is formed at a chiral center, the intermediate will be planar (sp\(^2\) hybridized) and achiral. The incoming nucleophile can then attack this planar carbocation from either the front side or the back side with equal probability. This leads to the formation of both possible enantiomers in equal amounts. A 50:50 mixture of two enantiomers is called a racemic mixture. Therefore, for a racemic mixture to be produced, the starting alkyl halide must be chiral.
Step 2: Analyzing the Chirality of the given Alkyl Halides.
A molecule is chiral if it has a carbon atom bonded to four different groups. Let's examine each option.
(A) Tertiary butyl bromide ((CH\(_3\))\(_3\)C-Br): The carbon atom bonded to bromine is also bonded to three identical methyl groups. It is not a chiral center. The product, tert-butanol, is also achiral.
(B) 2-bromobutane (CH\(_3\)-CH(Br)-CH\(_2\)-CH\(_3\)): The second carbon atom is bonded to four different groups: a hydrogen atom (-H), a bromine atom (-Br), a methyl group (-CH\(_3\)), and an ethyl group (-CH\(_2\)CH\(_3\)). Therefore, 2-bromobutane is chiral. Its hydrolysis via the S\(_N\)1 mechanism will form a planar secondary carbocation, which upon attack by H\(_2\)O will produce a racemic mixture of (R)-butan-2-ol and (S)-butan-2-ol.
(C) Isopropyl bromide ((CH\(_3\))\(_2\)CH-Br): The carbon atom bonded to bromine is also bonded to two identical methyl groups. It is not a chiral center.
(D) Methyl bromide (CH\(_3\)Br): The carbon atom is bonded to three identical hydrogen atoms. It is not a chiral center. (Also, methyl halides primarily undergo S\(_N\)2 reactions).
Step 3: Final Answer.
Only 2-bromobutane is a chiral alkyl halide among the options. Therefore, its hydrolysis via an S\(_N\)1 mechanism will produce a racemic mixture.