Question

Explain the mechanism of Substitution Nucleophilic Bimolecular (SN\(_2\)) reaction of haloalkanes with a suitable example.

Hint:

SN\(_2\) reactions are favored by primary haloalkanes, strong nucleophiles, and polar aprotic solvents.

Answer 1

Step 1: Nature of SN\(_2\) reaction SN\(_2\) stands for Substitution Nucleophilic Bimolecular. The rate of reaction depends on the concentration of both the haloalkane and the nucleophile: \[ \text{Rate} = k[\text{RX}][\text{Nu}^-] \]
Step 2: Mechanism The nucleophile attacks the carbon atom bonded to the halogen from the backside, opposite to the leaving group. Bond formation and bond breaking occur simultaneously through a single transition state.
Step 3: Transition state A pentacoordinate transition state is formed in which the carbon atom is partially bonded to both the nucleophile and the leaving group.
Step 4: Stereochemical outcome The reaction results in inversion of configuration (Walden inversion). Example: Reaction of methyl bromide with hydroxide ion: \[ \text{CH}_3\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{OH} + \text{Br}^- \]