Question

Arrange the following compounds in decreasing order of reactivity for hydrolysis reaction.
I. $PhC{H}_{2}Br$
II. $Ph-\stackrel{\stackrel{Br}{|}}{C}H-C{H}_{2}C{H}_3$
III.
IV.

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

Answer 1

The correct option is D IV > II > I > III

Hydrolysis will proceed by $S_{N}1$ mechanism.
Rate of the $S_{N}1$ reaction depends on the stability of carbocation formed. More stable carbocation will undergo $S_{N}1$ reaction fastly.
Comparing (I) and (II)
Benzyl cation is generally stable but in (II) the carbocation is also stabilised by +I group of $C{H}_{2}C{H}_3$. This make (II) more stable than (I)
Comparing (III) and (IV)
III will be least reactive as it involves the least stable, anti aromatic carbocation. IV will be most reactive as its carbocation is the most stable.
Thus, the decreasing order of reactivity for hydrolysis is,
IV > II > I > III