Rank the compounds in each group in order of increasing reactivity in electrophilic aromatic substitution: (a) ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{6}}\mathbf{,}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{Cl}\mathbf{,}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{CHO}$; (b) ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CH}}_{\mathbf{3}}\mathbf{,}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{NH}}_{\mathbf{2}}\mathbf{,}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{NH}}_{\mathbf{2}}$.

The reactions that involve the replacement of a substituent (or hydrogen) on an aromatic ring (usually benzene) with an electron-deficient species are referred to as electrophilic aromatic substitution reactions.

The nature of substituents on the aromatic ring influences its reactivity towards electrophilic substitution reactions.

The presence of an electron-donating group on the aromatic ring decreases its reactivity towards the electrophile, while an electron-donating group on the ring increases the reactivity.

a.

Among the given compounds, $C_6{H}_{5}Cl$ and $C_6{H}_{5}CHO$contain electron-withdrawing groups (Cl and CHO) and hence will be less reactive towards electrophilic aromatic substitution than $C_6{H}_6$.

Since the CHO group is more electron-withdrawing than Cl, $C_6{H}_{5}CHO$ is the least reactive.

(b)

The order of electron-donating ability of the substituents on the benzene ring is as follows:

$N{H}_2>C{H}_3>C{H}_{2}N{H}_2$

The higher the electron-donating ability of the substituents on the ring, the higher is its reactivity toward electrophilic substitution reactions.

Therefore, $C_6{H}_{5}N{H}_2$ is the most reactive and $C_6{H}_{5}C{H}_{2}N{H}_2$is the least reactive towards electrophilic substitution reactions.