Which of the following compounds will undergo $S_N1$ reaction at a fastest rate?

The correct answer is Option (3) →

SN1 reaction proceeds via carbocation intermediate.

Rate of SN1 reaction depends on stability of carbocation formed.

Order of carbocation stability: Tertiary > Secondary > Primary

Tertiary carbocation is most stable due to:

Greater hyperconjugation

Strong +I effect from alkyl groups

Thus, tertiary alkyl chloride undergoes SN1 reaction fastest.

Option 1: Secondary alkyl chloride Forms secondary carbocation. Moderate stability. Slower than tertiary. Incorrect.

Option 2: Primary alkyl chloride Forms primary carbocation. Very unstable. Very slow SN1. Incorrect.

Option 3: Tertiary alkyl chloride --- Forms highly stable tertiary carbocation. Fastest SN1 reaction. Correct.

Option 4: Secondary alkyl chloride Forms secondary carbocation. Slower than tertiary. Incorrect.