Chemistry Reactions of Haloarenes

Topics Covered :

● Reactions of Haloarenes
● Nucleophilic Substitution
● Replacement by Hydroxyl Group
● Electrophilic Substitution Reactions
● Reaction with Metals

Reactions of Haloarenes :

Following reactions are given by haloarenes :

Nucleophilic substitution :

`=>` Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons :

(i) `color{green}("Resonance effect ")` : In haloarenes, the electron pairs on halogen atom are in conjugation with `color{red}(π)`-electrons of the ring and the resonating structures as shown in fig.1 are possible.

`=>` `color{red}(C—Cl)` bond acquires a partial double bond character due to resonance.

● Therefore, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.

(ii) `color{green}("Difference in hybridisation of carbon atom in C—X bond")` : In haloalkane, the carbon atom attached to halogen is `color{red}(sp^3)` hybridised while in case of haloarene, the carbon atom attached to halogen is `color{red}(sp^2)`-hybridised. See fig.2.

`=>` The `color{red}(sp^2)` hybridised carbon with a greater `color{red}(s)`-character is more electronegative and can hold the electron pair of `color{red}(C—X)` bond more tightly than `color{red}(sp^3)`-hybridised carbon in haloalkane with less `color{red}(s)`-chararcter.

● `color{red}(C—Cl)` bond length in haloalkane is `177` pm while in haloarene is `169` pm.

● Because it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.

(iii) `color{green}("Instability of phenyl cation ")` : In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, `color{red}(S_N 1)` mechanism is ruled out.

(iv) Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes.

Some important reactions of haloarenes :

`=>` Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of `623K` and a pressure of `300` atmospheres. See fig.1.

`=>`The presence of an electron withdrawing group `color{red}((-NO_2))` at ortho- and para-positions increases the reactivity of haloarenes. See fig.2.

`=>` The effect is pronounced when `color{red}((-NO_2))` group is introduced at ortho- and para- positions.

`=>` 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 in fig.3.

Electrophilic substitution reactions :

`=>` Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions.

`=>` Halogen atom besides being slightly deactivating is `color{red}(o, p)`-directing; therefore, further substitution occurs at ortho- and para-positions with respect to the halogen atom.

`=>` The `color{red}(o, p)`-directing influence of halogen atom can be easily understood if we consider the resonating structures of halobenzene as shown in fig.1.

`=>` Due to resonance, the electron density increases more at ortho- and para-positions than at meta-positions.

`=>` Further, the halogen atom because of its `color{red}(–I)` effect has some tendency to withdraw electrons from the benzene ring.

`=>` Therefore, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.

`=>` Electrophilic reactions given by haloarenes is shown in fig.2.
Q 2616678570

Although chlorine is an electron withdrawing group, yet it is ortho-, para- directing in electrophilic aromatic substitution reactions. Why?


Chlorine withdraws electrons through inductive effect and releases electrons through resonance. Through inductive effect, chlorine destabilises the intermediate carbocation formed during the electrophilic substitution.Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the inductive effect for the attack at ortho- and parapositions and hence makes the deactivation less for ortho- and paraattack. Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by resonance effect.

Reaction of haloarenes with metals :

`color{green}("Wurtz-Fittig Reaction ")`: A mixture of an alkyl halide and aryl halide gives an alkylarene when treated with sodium in dry ether and is called Wurtz-Fittig reaction. See fig.1.

`color{green}("Fittig Reaction ")` : Aryl halides also give analogous compounds when treated with sodium in dry ether, in which two aryl groups are joined together. It is called Fittig reaction. See fig.2.