Question

Write salient features of SN\(^2\) mechanism. What is the action of following reagents on bromomethane : (i) bromobenzene (ii) mercurous fluoride

Hint:

For substitution reactions, SN\(^2\) means "substitution, nucleophilic, bimolecular." Think "backside attack" and "inversion." For preparing specific alkyl halides, remember the Swarts reaction (using metal fluorides) for fluoroalkanes and the Finkelstein reaction (using NaI/acetone) for iodoalkanes.

Answer 1

Salient Features of SN\(^2\) Mechanism

It is a single-step, concerted mechanism where bond breaking and bond formation occur simultaneously.
It follows second-order kinetics, i.e., the rate is proportional to the concentration of both the substrate and the nucleophile: Rate = k[Alkyl Halide][Nucleophile].
The attack of the nucleophile occurs from the side opposite to the leaving group (backside attack).
It results in a complete inversion of the stereochemical configuration, known as Walden inversion.
The reactivity order for alkyl halides is governed by steric hindrance: CH\(_3\)X>1\(^\circ\)>2\(^\circ\)>3\(^\circ\).
Reactions of Bromomethane

[(i)] Bromobenzene (in presence of Na/dry ether): This is the Wurtz-Fittig reaction. Bromomethane reacts with bromobenzene in the presence of sodium metal in dry ether to form toluene. \[ \underset{\text{Bromobenzene}}{\text{C}_6\text{H}_5\text{Br}} + \underset{\text{Bromomethane}}{\text{CH}_3\text{Br}} + 2\text{Na} \xrightarrow{\text{dry ether}} \underset{\text{Toluene}}{\text{C}_6\text{H}_5\text{CH}_3} + 2\text{NaBr} \]
[(ii)] Mercurous fluoride (Hg\(_2\)F\(_2\)): This is a halogen exchange reaction known as the Swarts reaction, used for the synthesis of alkyl fluorides. Bromomethane reacts with mercurous fluoride to form fluoromethane. \[ 2\text{CH}_3\text{Br} + \text{Hg}_2\text{F}_2 \rightarrow 2\text{CH}_3\text{F} + \text{Hg}_2\text{Br}_2 \]