Aryl Halide Reactions

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.


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 are 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 reaction.


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 being 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 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 was 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 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, however allyl chloride produces the same. Give an explanation.


Ans. In allyl chloride the reaction is simpler due to formation of carbonium ion which occurs following stabilisation of Cl- due to resonance giving two isomers.

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FAQ (Frequently Asked Questions)

1. What are Aryl Halides, and What are the Methods of Preparation?

Ans. Haloarenes or aryl halides are different names for the same chemical compounds which contain multiple or one hydrogen atoms that are bonded to one aromatic ring, which is replaced with halogens. In notations, they are indicated using 'Ar-X' where 'X' stands for halogen atom, which is attached, and 'Ar' is the aryl group. These compounds are prepared using other organic compounds employing several methods, but the most important of them are electrophilic reaction and Sandmeyer's reaction. Lewis acid is used in the former manner. The latter applies diazonium salt from the main aromatic amine with cuprous chloride and cuprous bromide.

2. Which are the Important Reactions of Aryl Halide?

Ans. The haloarenes have C-X bonds, which are much stronger compared to the C-X bond in haloalkanes. Therefore, it is not reactive when we talk about nucleophile reactions. Fidel crafts reactions is an essential reaction of aryl halides where the halide is treated using Lewis acid. This process is called electrophilic substitution reaction. Wutz-Fittig reaction is a combination of alkyl halide and aryl halide, which involves treatment using sodium in dry ether. The fittig reaction occurs when sodium in dry ether is used to treat haloarenes, causing the two aryl groups to join together. The aryl halides undergo various other reactions.