Question

Write the mechanism of bimolecular nucleophilic substitution reaction ($S_N2$) of alkyl halide.

Answer 1

The $S_N2$ Mechanism (Substitution Nucleophilic Bimolecular):
This reaction proceeds in a single concerted step (one-step mechanism) without the formation of any intermediate. Key Features:

• Attack of Nucleophile: The incoming nucleophile ($Nu^-$) attacks the carbon atom bonded to the halogen (leaving group) from the side opposite to the leaving group (back-side attack).
• Transition State: A transition state is formed where the bond between the carbon and nucleophile starts forming, and the bond between carbon and halogen starts breaking simultaneously. In this state, the carbon atom is partially bonded to five atoms.
• Stereochemistry: The reaction results in the inversion of configuration (Walden inversion), similar to an umbrella turning inside out in a gale.

Mechanism Diagram: \[ \text{Nu}^- + \text{C} - \text{X} \longrightarrow [\text{Nu} \dots \text{C} \dots \text{X}]^{\ddagger} \longrightarrow \text{Nu} - \text{C} + \text{X}^- \] Example: Reaction of Chloromethane ($CH_3Cl$) with Hydroxide ion ($OH^-$). \[ OH^- + CH_3Cl \longrightarrow CH_3OH + Cl^- \]