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:
Alkaline hydrolysis of primary alkyl halides like bromomethane (\( CH_3Br \)) involves the attack of a hydroxide ion (\( OH^- \)) as a nucleophile.
Step 2: Detailed Explanation:
The reaction occurs in a single concerted step.
1. Backside Attack: The nucleophile (\( OH^- \)) attacks the carbon atom from the side opposite to the leaving group (\( Br^- \)).
2. Transition State: A pentacoordinate transition state is formed where the bond to \( OH \) is partially formed and the bond to \( Br \) is partially broken. The carbon is \( sp^2 \) hybridized in this state.
3. Product Formation: The \( C-Br \) bond breaks completely, and the \( C-OH \) bond is fully established.
4. Inversion of Configuration: If the substrate were chiral, we would see "Walden Inversion", much like an umbrella turning inside out in a strong wind.
Rate Law: \( \text{Rate} = k [CH_3Br] [OH^-] \). It is a bimolecular, second-order reaction.
Step 3: Final Answer:
Alkaline hydrolysis of bromomethane is an \( S_N2 \) reaction involving a single-step transition state and inversion of configuration.