Question

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) ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CH}}_{\mathbf{3}}\mathbf{,}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{NH}}_{\mathbf{2}}\mathbf{,}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{NH}}_{\mathbf{2}}$.

Short answer

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.

Step-by-step solution

Electrophilic aromatic substitution

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.

Effect of substituents on electrophilic 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.

Order of reactivity of the given compounds

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.