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

Aryl halides are more stable and less reactive due to resonance over the aryl structure part where the lone pair of electrons are in conjugation with a pi bond whereas in alkyl halides the carbon halogen bond is a sigma bond .Click to see full answer. People also ask, why is aryl halide less reactive than alkyl halide?Aryl halides are less reactive than alkyl halides towards nucleophilic substitution reactions because-(i) Resonance effect-The electron pairs on halogen atom are in conjugation with pi-electrons of the ring. This causes delocalisation of electrons on C-X bond (X= halogen) which acquires partical double bond character.Furthermore, why are alkyl halides more reactive than alkyl halides towards nucleophilic substitution reaction? Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to resonance stabilization. In alkyl halides, the carbon atom is s p 3 displaystyle sp^3 sp3 hybridized whereas in aryl halides, it is s p 2 displaystyle sp^2 sp2 hybridized and hence, more electronegative. Furthermore, why are aryl halides less reactive to nucleophiles? Aryl halides are relatively unreactive toward nucleophilic substitution reactions. This lack of reactivity is due to several factors. Steric hindrance caused by the benzene ring of the aryl halide prevents S N2 reactions. Likewise, phenyl cations are unstable, thus making S N1 reactions impossible.How can we enhance the reactivity of aryl halides?The reactivity of aryl halides can be increased by the presence of an electron withdrawing group (-(NO2) at ortho and para positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. Mechanism of the reaction is as depicted with -OH ion.

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Solution : Aryl halides are less reactive towards nucleophilic substitution because C-X bond in aryl halides has partial double bond character due to resosnance. Therefore, its cleavage is very difficult as compared to alkyl halides where C-X bond is not involved in any resosnace. for more details, <br> (a). <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/SKM_COMP_CHM_V02_XII_C11_S01_127_S01.png" width="80%"> <br> (b). By deactivating the ring: Benzene ring can be deactivated with the help of electron withdrawing groups such as nitro `(-NO_(2))` groups and as a result, nucleophilic substitution can be carried under milder conditions. for Example <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/SKM_COMP_CHM_V02_XII_C11_S01_127_S02.png" width="80%">.

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 SN1 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.