Question

The correct order of reactivity of the halides, ethyl chloride $(I)$, isopropyl chloride $(II)$ and benzyl chloride $(III)$ in $S_N1$ reaction is

• A. $I > II > III$
• B. $III > II > I$
• C. $II > I > III$
• D. $I > III > II$

Answer 1

Answer: (B)

$III > II > I$

Answer 2

For $S _{ N } 1$ reactions, the greater the stability of carbocations formed, faster will be the rate of reaction. Thus, the order of reactivity of halides is benzyl chloride $>$ isopropyl chloride $>$ ethyl chloride.
(Due to greater stabilisation of benzylic carbocation intermediate by resonance, it shows higher reactivity in $S_{N} 1$ reactions than other aliphatic halides).