a
The ease of nucleophilic substitution depends upon the nature of leaving group. When the leaving tendency of a group in a compound is high, then the compound is more reactive towards nucleophilic substitution. The nucleophilic acyl substitution is completed in two steps as shown below.

The order of leaving tendency is

\(C l^{-}>R C O O^{-}>R O^{-}>N H_{2}^{-}\) and therefore, the order of reactivity of acyl compound is as

\(\mathop {ACOCl}\limits_{(Acyl\,chloride)}  > \mathop {\begin{array}{*{20}{c}}
  {^{RCO}\backslash } \\ 
  {_{RCO}/} 
\end{array}O}\limits_{(Acid\,anhydride)}  > \mathop {RCOOR}\limits_{(Ester)}  > \mathop {RCON{H_2}}\limits_{(Amide)} \)