Question

Arrange the following compounds in increasing order of their rate of hydrolysis by $S_{N}1$ mechanism.

• A. III < II < I < IV
• B. III < I < II < IV
• C. II < I < IV < III
• D. I < II < III < IV

Answer 1

The correct option is B III < I < II < IV

Rate of $S_{N}1$ is directly proportional to the stability of the carbocation formed.
(IV) is most reactive as it involves the most stable carbocation

(III) is least reactive as it is vinylic halide. In vinylic halide, formation of carbocation leads to positive charge on double bonded carbon which is least stable.
(II) is more reactive than (I) because (II) proceeds via benzylic carbocation.
Thus, the increasing order of rate hydrolysis by $S_{N}1$ mechanism is
III < I < II < IV