Question

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

Answer 1

Aryl halides are less reactive towards nucleophilic substitution reaction due to the following reasons.

(i) In haloarenes, the lone pair of electron on halogen are in resonance with benzene ring. So, C – Cl bond acquires partial double bond character which strengthen C – Cl bond and difficult to be substituted by nucleophile.

Therefore, they are less reactive towards nucleophilic substitution reaction.

(ii) In haloarenes, the carbon atom attached to halogen is sp² hybridised. The sp² hybridised carbon is more electronegative than sp³ hybridised carbon. This sp²-hybridised carbon in haloarenes can hold the electron pair of \[\ce{C - X}\] bond more tightly and make this \[\ce{C - Cl}\] bond shorter than \[\ce{C Cl}\] bond of haloalkanes.

Since, it is difficult to break a shorter bond than a longer bond, therefore, halorenes are less reactive than haloalkanes.

(iii) In haloarenesm the phenyl cation is not stabilised by resonance therefore S_N1 mechanism cannot be followed.

(iv) Because of the repulsion between the nucleophile and electron-rich arenas, aryl halides are less reactive than alkyl halides.

How to increase reactivity

The reactivity of aryl halides can be increased by the presence of an electron withdrawing group (-NO₂) at ortho and para positions. No effect is observed by the presence of electron withdrawing group at weta-position Mechanism of the reaction is as depicted with OH ion:

The presence of NO₂ groups at ortho and para positions withdraws electrons density from the benzene ring and therefore, facilitates the attack of the nucleophile on haloarenes. The carbanion, thus formed is stabilized through resonance as shown below:

It is clear from above structure that in case of o- and p-chlorobenzenes, on the resonating structures (II in case of p-nitro chlorobenzene and V in case of o-chlorobenzene) bear a negative charge on the carbon atom bearing the \[\ce{NO2}\] group.

Therefore, these carbanions are stabilized by the -NO₂ groups as well as π-electrons of the benzene ring. However, in case of m-nitrochlorobenzene, none of the resonating structures bear the negative charge on carbon atom bearing the -NO₂ group. Therefore, the nitro group at meta position does not stabilize the negative charge but the carbanion is stabilized only by the p-electrons of the benzene ring. In other words, the carbanions formed from o-nitrochlorobenzene and p-nitrochlorobenzene are most stable than that formed from m-nitrochlorobenzcne.

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