Question

Explain the mechanism of alkaline hydrolysis of bromomethane.

Hint:

Reactivity for \( S_N2 \) follows the order: Methyl \(>1^\circ>2^\circ>3^\circ \). This is due to steric hindrance; the nucleophile needs space to attack from the back.

Answer 1

Step 1: Understanding the Concept:
The hydrolysis of primary alkyl halides like bromomethane (\(CH_3Br\)) follows a second-order nucleophilic substitution (\(S_N2\)) mechanism.
Step 3: Detailed Explanation:
1. Reaction: \(CH_3Br + OH^- \rightarrow CH_3OH + Br^-\).
2. Mechanism: This is a single-step concerted process.
3. Attack: The nucleophile (\(OH^-\)) attacks the carbon atom from the side opposite to the leaving group (\(Br^-\)) (back-side attack).
4. Transition State: A pentacoordinate transition state is formed where the \(C-OH\) bond is partially formed and the \(C-Br\) bond is partially broken.
5. Result: As the \(Br^-\) leaves, the configuration of the carbon atom is inverted (Walden Inversion), much like an umbrella turning inside out in a strong wind.
Step 4: Final Answer:
The mechanism is a one-step \(S_N2\) process involving a transition state and Walden inversion.