Question

In which of the following molecules, the substituent does not exert its resonance effect.
A. $C_6{H}_{5}N{H}_2$
B. $C_6{H}_5\stackrel{+}{N}{H}_3$
C. $C_6{H}_{5}OH$
D. $C_6{H}_{5}Cl$

Answer 1

Hint: We can define resonance effect as the withdrawal effect or releasing effect of electrons attributed to a particular substituent through the delocalization of pi-electrons, which could be seen by drawing various canonical structures. Resonance effect is also called mesomeric effect.

Complete step by step answer: We can define resonance effect as a chemical phenomenon that is seen in the characteristic compounds containing double bonds in the organic compounds. The organic compounds show the presence of these double bonds in the structures and generally contain the overlapping of the p-orbitals on the two adjacent sides of carbon atoms.
The resonance effect is not seen in anilinium ions (protonated aniline). The lone pair of electrons present in nitrogen is donated to a proton. So, this lone pair of electrons is not in resonance with the benzene ring.
Negative inductive effect is exerted by $\stackrel{+}{N}{H}_3$. The negative inductive effect accounts for the higher electronegativity of nitrogen atom than the carbon atom present in benzene rings and this is because of the positive charge. As there is no lone pair of electrons, it would not be able to exert a resonance effect. $C_6{H}_5\stackrel{+}{N}{H}_3$ does not contain a lone pair of electrons to participate in resonance effect but it contains positive charge.
In all other options, the lone pair of electrons on the heteroatom participates in resonance with the benzene ring. Therefore, the options A, C and D are incorrect.
Therefore, Option (B) is correct.

Note: We can say there are two types of resonance effects. Positive resonance effect and negative resonance effect are the types of resonance effects.
Positive resonance effect takes place when the groups give electrons to the other molecules by delocalization. We can represent these groups as $+R$ (or) $+M$. $-OH$, $-OR$, $-SR$ are some of the groups that exhibit positive resonance effects.
Negative resonance effect takes place when the groups extract the electrons from other molecules by delocalization. We can represent these groups as $-R$ (or) $-M$. $-COOH$, $-C\equiv N$, $C=O$ are some of the groups that exhibit negative resonance effects.