Question

Why is chlorobenzene resistant to nucleophilic substitution reactions?

Hint:

The electron-withdrawing effect of the chlorine atom and the resonance stabilization in chlorobenzene make it resistant to nucleophilic substitution reactions, unlike alkyl halides where such substitution is easier.

Answer 1

Chlorobenzene is resistant to nucleophilic substitution reactions due to the following reasons:

1. Resonance Stabilization:
The lone pair of electrons on the chlorine atom participates in resonance with the π-electrons of the benzene ring, leading to delocalization of the electron density. This resonance stabilization strengthens the C-Cl bond and makes it less susceptible to nucleophilic attack.

2. Partial Double Bond Character:
Due to resonance, the C-Cl bond in chlorobenzene acquires partial double bond character, which increases its bond strength and reduces its reactivity towards nucleophiles.

3. sp² Hybridization of the Carbon Atom:
The carbon atom bonded to chlorine in chlorobenzene is sp² hybridized (part of the aromatic ring), which makes it more electronegative and less accessible for nucleophilic substitution compared to sp³ hybridized carbons in alkyl halides.

4. High Electron Density on the Ring:
The benzene ring has a high electron density, which repels nucleophiles and further hinders the substitution reaction.

5. Stability of the Aromatic System:
Nucleophilic substitution would disrupt the aromaticity of the benzene ring, which is energetically unfavorable. The aromatic system is highly stable, and reactions that compromise this stability are less likely to occur.

Conclusion:
The combined effects of resonance stabilization, partial double bond character, sp² hybridization, high electron density, and aromatic stability make chlorobenzene resistant to nucleophilic substitution reactions.