Question: Rank the compounds in each group in order of increasing reactivity in nucleophilic acyl substitution.

$$\begin{gathered} \mathbf{}\mathbf{a}\mathbf{.}\mathbf{}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CO}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{3}\mathbf{,}\mathbf{}}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{COCl}\mathbf{}\mathbf{,}\mathbf{}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CONH}}_{\mathbf{2}} \\ \mathbf{}\mathbf{b}\mathbf{.}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}\mathbf{}\mathbf{,}\mathbf{}{\mathbf{\left(}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{CO}\mathbf{\right)}}_{\mathbf{2}}\mathbf{O}\mathbf{}\mathbf{,}\mathbf{}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CONHCH}}_{\mathbf{3}} \end{gathered}$$

The reactivity of a compound towards nucleophilic substitution reactions depends on the electron deficiency of the compound and the stability of the leaving group.

The more stable the leaving group, the higher is the reactivity of the acyl chloride

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COCl}$ is the most reactive among the given compounds as the chlorine atom is highly electronegative (is the least basic) and will not easily donate its electrons.

Thus, the carbonyl carbon of the given compound is highly electron-deficient.

${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CONH}}_2$ is the least reactive as the nitrogen atom readily donates its electrons, decreasing the carbonyl atom’s electron deficiency.

Reactivity of compounds in a.

The anhydride ${\left({\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CO}\right)}_2\mathrm{O}$ is the most reactive as it contains two electron-deficient centers (two carbonyl compounds).

The amide$\left({\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CONHCH}}_3\right)$ is the least reactive as the nitrogen atom can easily donate its electrons, decreasing the carbonyl carbon’s electron deficiency.

Reactivity of compounds in b.