Question

The correct order of reactivity of the halides, ethyl chloride $(I)$, isopropyl chloride $(II)$ and benzyl chloride $(III)$ in $S_{N}1$ reaction is

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

Answer 1

Answer: (B)

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).