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  • Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?

Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?

Answers (1)

Aryl halides are less reactive towards nucleophilic substitution reactions for these reasons mentioned as follows:

(i) In case of haloarenes, the benzene ring is attached with the lone pair electron of the halogen in resonance. As a result, C-Cl bond acts as a partial double bond and increases the strength of the bond. This bond is difficult to substitute by nucleophilic substitution method. Hence, they usually react less in the above mentioned reaction.

(ii) In case of haloarenes, the sp^2 hybridized carbon atom is attached to a halogen. An sp^2  hybridized carbon atom is more electronegative and it holds the C-Cl pair strongly and forces the C-Cl bond shorter than the haloalkanes.

Method to increase the reactivity:

The reactivity of the aryl halides can be increased when there is an electron withdrawing group(NO_2) at the ortho and para position. This presence of this electron withdrawing group at the above mentioned position withdraws electron density in the benzene ring. As a result, it will be easier for the nucleophile to attack. As a result carbocation is formed through resonance.

 

So, in case of o- and p-chlorobenzenes, the carbon atom bearing (NO_2) group has a negative charge due to the resonating structure. Therefore, the (NO_2)  group along with the π-electrons of the benzene ring stabilizes the carbon ions. However, none of the resonating structures carries the negative charge along with the carbon atom bearing -NO_2 group, in case of m-nitrochlorobenzene. Therefore, the negative charge does not stabilize by the nitro group at meta position. However, the p-electrons of the benzene ring stabilize the carbanion. In other words, the carbanions formed from o-nitrochlorobenzene and p-nitrochlorobenzene are more stable than those formed from m-nitrochlorobenzene.

Thus, the presence of electron withdrawing groups at o- and p-positions (but not at m-positions) w.r.t. The halogen atom activates the aryl halides towards nucleophilic substitution reaction.

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