Recall the requirement for an $S_N2$ reaction

In an $S_N2$ reaction, the nucleophile attacks the carbon atom from the back side of the carbon-halogen bond in a single-step process.

$\text{R-X} + \text{Nu}^- \rightarrow \text{R-Nu} + \text{X}^-$

This mechanism needs easy access to the electrophilic carbon.

Consider tertiary alkyl halides

A tertiary alkyl halide has the halogen-bearing carbon attached to three alkyl groups.

$\text{R}_3\text{C-X}$

These three bulky alkyl groups surround the reaction center.

Effect on $S_N2$ attack

Since $S_N2$ requires backside attack, the nucleophile must approach the carbon atom directly opposite to the leaving group. In tertiary alkyl halides, the bulky groups block this approach.

Therefore, the nucleophile cannot easily attack, and the $S_N2$ reaction becomes practically impossible.

Evaluate the options

• A: insolubility — Not the fundamental reason.
• B: instability — Tertiary alkyl halides are not inert to $S_N2$ because of instability.
• C: inductive effect — Not the main factor preventing $S_N2$.
• D: steric hindrance — Correct. Bulky alkyl groups hinder backside attack.

Final answer

The correct reason is steric hindrance.