What is Aryl Halide?
Aryl halide is an organic aromatic compound where the hydrogen atoms are directly bonded to an aromatic ring, which is replaced by a halide. They are also referred to as haloarene or halogenoarene. The haloarene, however, is somewhat different from haloalkanes because of exhibiting various differences in their properties and the method by which they are prepared. This category of compounds is still broad and encompasses a wide variety of applications. Diazonium salts and direct halogenation are the two most important methods which are commonly used in the preparation of aryl halides. There are various types of reactions involved with Aryl Halide and has been discussed further in this article.
So, you are versed with the definition of Aryl Halide. Now, you must be curious to know the other aspects of this reaction for both the purpose of your score and for the enhancement of your knowledge. If you feel any sort of difficulty in terms of understanding the matter, then you are free to connect with the Vedantu team for the same. In the coming paragraphs, you are going to learn about different types of Aryl halide reactions, how they will work with metals, there are some fun facts to refresh your mind, and you will also have a sample question to solve that will further guide you for your preparation. Let’s get started:
Electrophilic Substitution Reaction
Even though aromatic compounds are constituted by multiple bonds, they do not go through an additional reaction. The stability of the ring system in these compounds is the cause of this low reactivity. This stability generates complete π electron delocalization, which is also called resonance. Thus, reactions in aromatic compounds occur through electrophilic substitution reactions where the ring system's aromaticity is preserved. For example, benzene reacts with bromine and results in the formation of bromobenzene.
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Through the process of electrophilic substitution reaction, multiple functional groups can be added to the aromatic compounds. The functional group refers to a substituent that causes chemical reactions in which aromatic compounds do not exhibit.
Aryl Halide Reactions
The following are the different forms of reaction in aryl halides. All of them are examples of electrophilic substitution reactions.
Halogenation
When aryl halide comes close to chlorine with ferric chloride as a solvent, a polarity is developed in the chlorine molecule, and the chlorine molecule which would carry the slight positive charge would act as the electron seeking or electrophile. This electrophile would attack the ortho, which is electron-rich, and the para position of the aryl halide. The para compounds and the ortho together are created in which the major product is para isomer and the minor product is ortho isomer. This is called the halogenation of aryl halide.
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Nitration
In the nitration of aryl halides, NO2 is created from nitric acid which is started off with H2SO4. Nitrogen dioxide’s electrophilic centre is over N because of two electronegative oxygen atoms being present in the molecule. The electron-rich ortho and para positions are attacked by Nitrogen dioxide and we end up with para isomer as the major and ortho isomer as the minor product. This is another form of aryl halide reaction.
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Sulphonation
In the sulphonation of aryl halide, sulfur trioxide is created with the help of sulphuric acid, and this acts as an electrophile. The electron-rich ortho and para positions in an aryl halide are attacked by sulfur trioxide from which para isomers are obtained as the main product. In contrast, the ortho isomer forms the minor product in the reaction. This is again a form of electrophilic substitution reaction.
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Friedel - Crafts Reaction
The alkyl or carbonyl group acts as an electrophile in the Friedel-crafts reaction due to the presence of a positive charge over the carbon atom. These groups attack the electron-rich para and ortho positions of the aryl halides. The isomer of para is the primary product obtained in the Friedel-Crafts reaction while the ortho isomer constitutes the minor product. The reaction which takes place with alkyl chloride is also known as F.C. alkylation, and the reaction that takes place with acyl halide is called F.C. acylation.
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Reaction of Aryl Halide With Metals
The first aryl halide reaction that takes place with metals is called the Wurtz-Fittig reaction. In the Wurtz-Fittig reaction, aryl halides react with alkyl halides, and the metal sodium with dry ether present to yield substituted aromatic compounds. This method wasis discovered by Charles Adolphe Wurtz in 1855 when he formed a new C-C bond using the method of coupling two alkyl halides. This reaction is most appropriate for creating asymmetrical products when the chemical reactivities of the halide reactants are separate. This can be achieved by using halogens of different periods to create reactants. This method is usually used for alkylation of aryl halides, but the reaction can be made even more useful with the help of ultrasound to produce biphenyl compounds.
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Fittig reaction is when haloarenes when treated with dry ether and sodium, cause two aryl groups to get combined. This reaction is known as the fittig reaction.
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Fun Fact
So, what is aryl halide, and what does it mean? The word aryl stands for aromatic or benzene rind, while the word halide refers to types of halogen. An aryl halide is nothing but only four halogen atoms bound together and directly to a benzene ring.
Solved Examples
1. While reacting, although Vinyl chloride does not yield SN reaction, allyl chloride produces the same. Give an explanation.
Ans: In allyl chloride, the reaction is simpler due to the formation of carbonium ion which occurs following stabilization of Cl- due to resonance giving two isomers.
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FAQs on Aryl Halide Reactions
1. What are aryl halides, and how does their structure influence their reactivity?
Aryl halides are organic compounds where a halogen atom (F, Cl, Br, I) is directly bonded to an sp²-hybridised carbon atom of an aromatic ring. This unique structure, particularly the C-X bond, has partial double bond character due to resonance. This makes the bond shorter, stronger, and less polar than the C-X bond in alkyl halides, significantly reducing its reactivity towards nucleophilic substitution.
2. What are the two primary methods for preparing aryl halides as per the CBSE 2025-26 syllabus?
According to the CBSE Class 12 syllabus, the two main methods for preparing aryl halides are:
- Direct Halogenation: This is an electrophilic substitution reaction where benzene or its derivatives react with a halogen (like Cl₂ or Br₂) in the presence of a Lewis acid catalyst (such as FeCl₃ or AlCl₃) to form the corresponding aryl halide.
- Sandmeyer's Reaction: This method involves the conversion of a primary aromatic amine into a diazonium salt, which is then treated with a cuprous halide (CuCl/HCl or CuBr/HBr) to yield the desired aryl halide. This is a very important method for introducing halogens to the benzene ring.
3. What are the major types of reactions that aryl halides undergo?
Aryl halides primarily undergo three major types of chemical reactions:
- Nucleophilic Aromatic Substitution: These reactions are difficult and require drastic conditions (high temperature and pressure) or the presence of strong electron-withdrawing groups on the ring.
- Electrophilic Substitution: Since the halogen is deactivating but ortho, para-directing, aryl halides undergo reactions like halogenation, nitration, sulphonation, and Friedel-Crafts reactions at the ortho and para positions.
- Reactions with Metals: These include important name reactions like the Wurtz-Fittig reaction (coupling with an alkyl halide) and the Fittig reaction (coupling of two aryl halides).
4. Why are aryl halides extremely unreactive towards nucleophilic substitution reactions compared to alkyl halides?
Aryl halides are significantly less reactive than alkyl halides in nucleophilic substitution reactions due to several key factors:
- Resonance Effect: The lone pair of electrons on the halogen atom participates in resonance with the benzene ring, creating a partial double bond character in the C-X bond. This makes the bond stronger and harder to break.
- Hybridisation of Carbon: The carbon atom attached to the halogen in aryl halides is sp²-hybridised, which is more electronegative than the sp³-hybridised carbon in alkyl halides. It holds the C-X bond more tightly.
- Instability of Phenyl Cation: A SN1 mechanism would require the formation of a highly unstable phenyl cation, which does not happen.
- Electronic Repulsion: The electron-rich nucleophile is repelled by the electron-rich benzene ring, making the attack difficult.
5. How does adding a nitro group (-NO₂) to the benzene ring change the reactivity of an aryl halide in nucleophilic substitutions?
Adding a strong electron-withdrawing group, such as a nitro group (–NO₂), at the ortho or para positions drastically increases the reactivity of an aryl halide towards nucleophilic substitution. The –NO₂ group stabilises the negatively charged intermediate (Meisenheimer complex) formed during the nucleophilic attack through resonance. This stabilisation lowers the activation energy for the reaction, allowing it to proceed under much milder conditions. A nitro group at the meta position has no such effect, as its resonance effect cannot delocalise the negative charge.
6. Why is a halogen atom considered deactivating yet ortho, para-directing in the electrophilic substitution reactions of aryl halides?
This is a classic example of competing electronic effects. The halogen atom exhibits two opposing effects:
- Inductive Effect (-I): Being highly electronegative, the halogen atom pulls electron density from the benzene ring, making the ring less reactive towards electrophilic attack than benzene itself. This is why it is a deactivating group.
- Resonance Effect (+R): The lone pair of electrons on the halogen can be delocalised into the benzene ring, increasing electron density specifically at the ortho and para positions.
Although the inductive effect is stronger and deactivates the entire ring, the resonance effect directs the incoming electrophile to the electron-rich ortho and para positions.
7. What is the difference between the Wurtz-Fittig reaction and the Fittig reaction for aryl halides?
The primary difference between the Wurtz-Fittig and Fittig reactions lies in the types of halides used:
- Wurtz-Fittig Reaction: This reaction involves the coupling of an aryl halide and an alkyl halide in the presence of sodium metal in dry ether. The product is an alkyl-substituted aromatic compound, for example, Toluene from chlorobenzene and methyl chloride.
- Fittig Reaction: This reaction involves the coupling of two molecules of an aryl halide with sodium metal in dry ether. The product is a biphenyl or its derivative, for example, Biphenyl from two molecules of chlorobenzene.
