Reactions of Haloarenes: Nucleophilic & Electrophilic Substitution Reaction

Can you believe that marine organisms can produce haloarenes? Haloarenes can be produced by marine creatures that can utilise the chloride and bromide found in ocean waters. They’ve been found to have a variety of medicinal effects. As a result, haloarenes undergo a variety of reactions both intentionally and naturally.

In this article, let’s learn everything about the reaction of haloarenes in detail. 

Reactions of Haloarenes

Haloarenes or aryl halides are the organic compounds in which the halogen atom is bonded to the \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised carbon atom of an aromatic ring. These compounds are generally represented as \(\mathrm{Ar}-\mathrm{X}\) where \({\rm{Ar}}\) represents the aryl group, and \(\mathrm{X}\) denotes halogen atom, which can be fluorine \(\left( {\rm{F}} \right)\) chlorine \(\left( {{\rm{Cl}}} \right)\) bromine \(\left( {{\rm{Br}}} \right)\) or iodine \(\left( {\rm{I}} \right)\)

Haloarenes undergo a particular set of reactions based on the nature of the C-X bond. Some of these reactions are discussed below.

Haloarenes or aryl halides undergo reactions that can be broadly classified as follows:

1. Nucleophilic Substitution Reactions
2. Electrophilic Substitution Reactions
3. Reaction with Metals

Nucleophilic Substitution Reactions

A nucleophile (nucleus-seeking species) reacts with the substrate (the haloarene) in this type of reaction. The haloarene substrate carries a partial positive charge present on the carbon atom bonded to halogen. The nucleophile substitutes the halogen atom that departs as the halide ion. The halogen atom is known as the leaving group. A nucleophile initiates this substitution reaction, hence is known as the nucleophilic substitution reaction. 

Haloarenes do not undergo nucleophilic substitution reactions readily. However, under specific reaction conditions, they do undergo these reactions. 

The primary reasons for the decreased reactivity nature of haloarenes towards nucleophilic substitution reactions are as below:

i. Resonance Effect

In haloarenes, the \({\rm{\pi }}\) – electrons present in the benzene ring undergo conjugation with the halogen atom. This results in resonance between the benzene ring and halogen atom, resulting in a partial double bond character in the C-X bond. 

Hence, the C-X bond cleavage becomes more difficult as compared to haloalkanes. Therefore, the C-X bond in haloarenes cannot be cleaved by a nucleophile easily; hence haloarenes are less reactive towards the nucleophilic substitution reactions.

ii. The Difference in Hybridisation of Carbon Atom in C-X Bond

In haloarenes, the halogen group is attached to an \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised carbon atom of the benzene ring. However, in haloalkanes, the halogen is attached to the \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised carbon atom. 

Hence, the \({\rm{s\% }}\) character in \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) carbon of haloarenes is stronger in comparison to \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) carbon of haloalkanes. This results in increased electronegativity of the \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) carbon atom than that of the \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) carbon atom.

Thus, the \({\rm{s}}{{\rm{p}}^{\rm{2}}}\)  hybridised carbon atom of the haloarene has a greater electron-withdrawing capacity and can hold the electron pair of \({\rm{C – X}}\) bond more tightly than \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised carbon in haloalkane. As a consequence, the bond length becomes shorter in haloarenes than that of haloalkanes. A shorter bond length results in a stronger bond. For instance, the \({\rm{C – Cl}}\) bond length in haloalkane is \(177 \mathrm{pm}\), whereas, in haloarene, it is \(169 \mathrm{pm}\)

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

iii. Phenyl Cation Instability

The cleavage of the \({\rm{C – X}}\) bond in haloarenes results in the formation of phenyl cation and a halide ion \({{\rm{X}}^ – }\). The resultant phenyl cation is highly unstable due to the presence of a positive charge on the electronegative \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) C atom. Additionally, the phenyl cation cannot be stabilised by resonance. Hence, the \(\mathrm{S}_{\mathrm{N}} 1\) reaction mechanism is ruled out.

Therefore, the instability of phenyl cation decreases the reactivity of the haloarene towards nucleophilic substitution reaction. 

iv. Possible Electronic Repulsion

Haloarenes consist of a bulky and electron-rich aryl group. When an electron-rich nucleophile approaches the electron-rich phenyl ring (arenes) in haloarenes can experience possible electronic repulsion.

Conditions when Haloarenes are Reactive

Though haloarenes are unreactive or less reactive towards nucleophilic substitution reactions, there are conditions when haloarenes are reactive towards nucleophilic substitution reactions. Some of these reactions are discussed below.

Replacement by Hydroxyl Members

Chlorobenzene undergoes nucleophilic substitution to form phenol when heated with aqueous sodium hydroxide at a temperature of \(623 \mathrm{~K}\) and a pressure of \(300\) atmospheres.

However, the presence of an electron-withdrawing group enhances the reactivity of haloarenes towards nucleophilic substitution reactions. This means if an electron-withdrawing group such as \(-\mathrm{NO}_{2}\) is present in the ortho, para, and both ortho and para positions. The reactivity of haloarenes towards nucleophilic substitution increases and the reaction requires much less drastic conditions to proceed.

Mechanism:

However, the presence of the electron-withdrawing group at meta-position does not affect the reactivity of haloarenes significantly. The mechanism of the reaction is as depicted:

The presence of a nitro group at ortho- and para-positions withdraws the electron density from the benzene ring and results in the formation of a stable carbanion during the attack of the nucleophile on haloarene. The carbanion has an electron density at ortho- and para- positions with respect to the halogen substituent. While in the case of meta-nitrobenzene, none of the resonating structures bears the negative charge on carbon atom bearing the -\(-\mathrm{NO}_{2}\) group. Therefore, the presence of a nitro group at meta-position does not affect haloarenes’ reactivity towards the nucleophilic substitution reaction. It does not stabilise the negative charge as it does in the case of ortho and para positions.

Electrophilic Substitution Reactions

An electrophile is an electron-seeking species, and an electrophilic substitution reaction is a chemical reaction in which an electrophile replaces a group attached to the compound. The displaced group is typically a hydrogen atom. Haloarenes undergo electrophilic substitution

reactions with benzene rings such as nitration, halogenation, sulphonation, and Friedel-Crafts reactions.

Due to the –I effect (electron-withdrawing nature) of the Halogen atom, the benzene ring is slightly deactivated towards electrophilic substitution reaction. The halogen atom is ortho, para-directing; hence, electrophilic substitution occurs at ortho- and para-positions with respect to the halogen atom. 

The electrons of the halogen atom are delocalised in the benzene ring, making the ortho- and para- positions of the ring electron-rich,  than the meta- position. Thus, haloarenes are ortho- and para- directive towards electrophilic substitution reaction.

Further, due to the –I effect, the halogen atom tends to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated, and the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions than those in benzene.

Halogenation

The halogenation of haloarenes occurs when haloarene reacts with chlorine in the presence of a lewis acid (Ferric chloride). The chlorine molecule acts as an electrophile and will attack by the compound’s electron-rich ortho and para position.

The reaction results in the formation of both ortho and para compounds. However, para isomer will be the major product, and ortho isomer will be the minor product of the reaction. 

Nitration

In nitration of haloarenes, the \(-\mathrm{NO}_{2}^{+}\) ion acts as the electrophile. The reaction between nitric acid and sulphuric acid results in the formation of \(-\mathrm{NO}_{2}^{+}\) electrophile. The electron-rich centres attack the electrophile at ortho and para positions—the reaction results in the formation of both ortho and para compounds. However, para isomer will be the major product, and ortho isomer will be the minor product of the reaction.

Sulphonation

In sulphonation of haloarenes, \(\mathrm{SO}_{3}\) acts as the electrophile. The electron-rich haloarene attacks it at ortho and para positions. The reaction results in the formation of para and ortho Chlorobenzenesulphonic acids, where para isomer forms the major product and ortho isomer forms the minor product.

Friedel-Crafts Reaction

There are generally two types of Friedel-Crafts reaction

Friedel-Crafts Alkylation Reaction

In Friedel-Crafts Alkylation, the alkyl group acts as the electrophile and is attacked by the ortho and para positions of the haloarene. The reaction results in the formation of both ortho and para compounds. However, para isomer will be the major product, and ortho isomer will be the minor product of the reaction. 

Friedel-Crafts Acylation Reactions

In Friedel-Crafts Acylation, the acyl group acts as the electrophile and is attacked by the ortho and para positions of the haloarene. The reaction results in the formation of both ortho and para compounds. However, para isomer will be the major product, and ortho isomer will be the minor product of the reaction. 

Reactions with Metals

Haloarenes undergo few reactions with metals. Two primary reactions are:

1. Wurtz-Fittig Reaction
2. Fitting Reaction

Wurtz-Fittig Reaction

In this reaction, an alkyl arene is produced when a mixture of alkyl halide and an aryl halide reacts with sodium in the presence of dry ether and sodium. 

Fitting Reaction

In this reaction, a diphenyl (Diarene) is formed when a mixture of haloarenes reacts with sodium in the presence of dry ether. 

Reduction of Haloarenes 

Haloarenes are converted into their corresponding arenes by reduction with Ni-Al alloy in the presence of alkali.

Reaction with aq. \({\rm{N}}{{\rm{H}}_3}\) 

Aniline is formed when chlorobenzene is heated with aq. ammonia in the presence of cuprous oxide \(\left(\mathrm{Cu}_{2} \mathrm{O}\right)\) at \(200\,^\circ {\rm{C}}\) under a pressure of \(60\) atm.

Action with Cuprous Cyanide

Benzonitrile is formed when chlorobenzene is heated with cuprous cyanide \((\mathrm{CuCN})\) at \(200\,^{\circ} \mathrm{C}\) in the presence of pyridine.

Phenyl cyanide can be used to prepare different important compounds such as:

Summary

Haloarenes are the halogen derivatives of benzene. These compounds resemble benzene as well as haloalkanes. The presence of the benzene ring contributes a lot to the reactivity of the haloarenes. Their reactivity also resembles that of the haloalkanes. Due to the difference in the C-X bond in haloalkanes and haloarenes, they also differ with respect to chemical reactivity. In this article, we learned electrophilic, nucleophilic substitution of haloarenes. We also learned two new name reactions – The fitting reaction and the Wurtz-Fittig reaction.

FAQs on Reactions of Haloarenes

Q.1. Why is nucleophilic substitution reaction difficult in haloarene?
Ans: In haloarenes, due to \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridisation and partial double bond character of the \({\rm{C – X}}\) bond, cleaving the bond and replacing it with the nucleophile becomes difficult. Arenes are electron-rich molecules; hence, they repel Nucleophiles attacking them.

Q.2. Why do electrophilic substitution reactions in Haloarenes occur slowly?
Ans: Electrophilic substitution reaction in haloarenes occur slowly because halogen has a negative inductive effect; it decreases the electron density on the benzene ring due to resonance.

Q.3. At what position electrophilic substitution reaction occurs in Haloarenes?
Ans: Due to the \(+\mathrm{M}\) effect of the halogens, the concentration of electron density will be high at the ortho and para positions.

Q.4. Why do Haloarenes undergo an electrophilic substitution reaction at ortho and para positions?
Ans: Halogens are electron-rich species and can donate their lone pair of electrons inside the rings for resonance. Due to resonance, the electron density increases at ortho and para positions.

Q.5. What is nitration of Haloarenes?
Ans: In nitration of haloarenes, the \(-\mathrm{NO}_{2}^{+}\) ion acts as the electrophile. The reaction between nitric acid and sulphuric acid results in the formation of \(-\mathrm{NO}_{2}^{+}\) electrophile. The electrophile attacks the electron-rich centres at ortho and para positions—the reaction results in the formation of both ortho and para compounds. However, para isomer will be the major product, and ortho isomer will be the minor product of the reaction. 

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