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Aryl Halide

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Aryl halides commonly known as haloarenes are organic compounds obtained by the replacement of a hydrogen in aromatic compounds with halogen. In aryl halides the aromatic sp2 hybriised carbon atom is bonded to the halogen by a sigma bond.

An aryl halide is a compound formed by the substitution of a halogen atom for a hydrogen atom on benzene. Another name for an aryl halide is halobenzene. All monosubstituted aryl halides possess a dipole moment because of the more electronegative halogen atom. Chlorobenzene for instance has $\mu$ = 1.76D; the value is lower than that of methyl chloride.

Definition

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Aryl halides are compounds in which the halogen atom (F, Cl, Br, I) is attached directly to the carbon atom of an aromatic ring. The aryl group in such compounds is denoted by the symbol -Ar to distinguish it from the alkyl group -R. They differ from alkyl halides in many of their properties. The compounds do not occur naturally but are prepared industrially in large quantities.

Aryl halides are insoluble in water but soluble in most organic solvents. The boiling points of chlorobenzene and bromobenzene are similar to their open chain counterparts, n-hexyl chloride and n-hexyl bromide respectively. But in the aryl halide series the boiling points increase with the increase in the atomic weight of the halogens.
Aryl halides are named by prefixing the name of the halogen to benzene. For example,

Aryl Halide
The term meta, ortho and para are used to indicate the positions of substituents on a disubstituted benzene ring. Numbers are also used to indicate the positions of the substituents on a benzene ring. Numbering however is needed when more than two halo atoms or other substituents are present on the ring. The halo atoms in the later case are considered as substituents.
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Synthesis 

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1. Halogenation

Chlorination or bromination of benzene can be affected in the presence of a halogen carrier or a catalyst. The function of the catalyst is to make the helogen a better electrophile.

C6H6 + Br2 $\overset{FeBr_{3}}{\rightarrow}$ C6H6Br + HBr

Mesitylene is a highly activated ring and needs no catalyst, it gives bromomsitylene.

Bromination of Mesitylene
2. From Diazonium salts

This is the most general method used to prepare all types of aryl halides. An aromatic amine is first diazotised (NaNO2.HCl) which then can be reacted with a metal halide to obtain an aryl halide.

Aryl Halide From Diazonium Salts
3. Hunsdiecker reaction

Treatment of silver salt of benzoic acid with bromine yields beomobenzene.

C6H5COO-Ag+ $\xrightarrow[\Delta ]{Br_{2}}$ C6H5Br + CO2 + AgBr

Properties 

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Aryl halides are insoluble in water and soluble in organic solvents. The aryl halides are denser than water and most of these have a pleasent smell. The aryl halides are colorless oily liquids having characteristic odour of aromatic compounds. Inspite of the fact that aryl halides are polar molecules they are insoluble in water. This is because aryl halides cannot form hydrogen bond with water. However aryl halides are soluble in organic solvents like alcohol, ether etc. The aryl halides in general are heavier than water.

Aryl halides like vinyl halides do not react readily with nucleophiles unlike alkyl halides. However they undergo most of the common electrophilic substitution reactions of the benzene ring with a decreased activity to give mainly o-p-substituted products.

Aryl Halide Reactions

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Aryl halides behave different chemically than alkyl halides in many respects.
  • Aryl halides undergo nucleophilic displacement with typical nucleophiles with great ease. The aryl halides on the other hand react with great difficulty. Chlorobenzene like vinyl chloride under normal conditions does not hydrolyse to give phenol because of unreactivity of $\pi$-orbital of the benzene ring overlaps with the p-orbitalsof the chlorine atom to for a delocalised cloud of $\pi$-electrons.

Nucleophilic Substitution
  • Chloro or bromo benzene can react with a strong base like potassium amide in liq.ammonia to give aniline.

Aryl Hallide Amination

This reaction occurs through an elimination addition mechanism as well that involves the formation of an intermediate called benzyne, and consists of the following steps.

Elimination Addition Mechanism
  • Aryl halides form grignard reagents with magnesium in dry ether. The reagent on treatment with solid carbon dioxide and subsequent acid hydrolysis forms benzoic acid.
Aryl Halides with Grignard Reagents