. Consider the following compounds

The order of reactivity of these towards nucleophilic substitution by S_N1 mechanism is

A. I > III > II > IV

B. IV > II > III > I

C. IV > III > II > I

D.IV > II > I > III

Best Answer

Ans: B

Sol: The carbocations formed from these halides are

Since the stability of these carbocations decrease in the order IV > II > III > I (because IV is stabilized by two phenyl groups (resonance) and one —CH₃ group (+I effect and hyperconjugation), II is stabilized by two phenyl groups, III is stabilized by one phenyl and one methyl group while I is stabilized by only phenyl group and therefore, the reactivity (in S_N1) also decreases in the same order.

i.e. IV > II > III > I

You should also note that reactivity towards S_N2 reaction is in the reverse order i.e.,

I > III > II > IV