Question

What is the effect of substituents on the reactivity of haloarenes? Explain with suitable examples.

Answer 1

The reactivity of haloarenes (aromatic halides) is significantly influenced by the nature of substituents present on the aromatic ring. Substituents can be either electron-donating or electron-withdrawing, and they affect the electron density on the ring and the carbon bearing the halogen, thereby influencing the reactivity towards substitution reactions, particularly nucleophilic substitution.

Effect of Substituents on Reactivity of Haloarenes:

1. Electron-Withdrawing Groups (EWG): Substituents such as -NO₂, -CN, -CHO, -COOH withdraw electron density from the aromatic ring by the inductive and/or resonance effect. This increases the partial positive charge on the carbon attached to the halogen, making the carbon more electrophilic. Consequently, the C–X bond (especially in chlorobenzene or bromobenzene) becomes more susceptible to nucleophilic attack because the ring carbon can better stabilize the negative charge on the intermediate Meisenheimer complex formed during nucleophilic aromatic substitution (NAS).
   For example:
   4-nitrochlorobenzene undergoes nucleophilic substitution much faster than chlorobenzene due to the strong electron-withdrawing effect of the nitro group positioned para to the halogen.
2. Electron-Donating Groups (EDG): Substituents such as -OH, -OCH₃, -NH₂, and alkyl groups donate electron density towards the ring through resonance or inductive effects, increasing the electron density at the carbon bearing the halogen. This makes the carbon less electrophilic and decreases the rate of nucleophilic substitution. However, EDGs generally activate the ring towards electrophilic substitution by increasing the electron density.
   For example:
   4-methoxychlorobenzene is less reactive towards nucleophilic substitution compared to chlorobenzene because the electron-donating methoxy group reduces the partial positive charge on the halogen-bearing carbon.