Question

Which of the following alkyl halides is hydrolyzed most rapidly by the \(\text{SN}^2\) mechanism ?

• A. \(\text{CH}_3\text{-Br}\)
• B. \((\text{CH}_3)_3\text{C-Br}\)
• C. \(\text{CH}_3\text{-CH}_2\text{-Br}\)
• D. \((\text{CH}_3)_2\text{CH-Br}\)

Hint:

SN2 is fastest where the carbon is least crowded: methyl halide.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
In an \(\text{SN}^2\) reaction the nucleophile attacks from the back side of the carbon in a single step. Bulky groups near that carbon block the attack.

Step 2: Detailed Explanation:
Order of reactivity in \(\text{SN}^2\): methyl > primary > secondary > tertiary.
\(\text{CH}_3\text{-Br}\) is methyl with three small H atoms, so the carbon is easily approached.
\(\text{CH}_3\text{CH}_2\text{Br}\) is primary, slower. \((\text{CH}_3)_2\text{CHBr}\) is secondary, slower still. \((\text{CH}_3)_3\text{CBr}\) is tertiary, so crowded that \(\text{SN}^2\) is almost blocked.

Step 3: Final Answer:
Option (A), \(\text{CH}_3\text{-Br}\).

Final Answer:
Least hindered carbon reacts fastest in SN2. \[ \boxed{\text{(A) }\text{CH}_3\text{-Br}} \]