Aryl Halide

Aryl Halide: meaning

Before we talk about aryl halides, also known as halogenoarenes, let's talk about arenes.

So, benzene forms the base for all arenes. Benzene is a cyclic hydrocarbon of 6 carbon atoms, with alternating double and single bonds between carbon atoms.

The structure can also be drawn as a hexagon with a circle inside to represent the alternating double bonds. The structural formula of benzene is C₆H₆.

Benzene has a very large role in industrial manufacturing. It is used to synthesise chemicals that make dyes, resins, plastics, synthetic fibres, rubber, detergents. It is also used in the manufacture of drugs and pesticides.

Now that you know what arenes are, you can probably guess what halogenoarenes are.

Aryl Halide: Structure and formula

The general formula of aryl halide is C₆H₅X, where X is a halide atom.

Fig. 2: General Formula of Aryl Halides | StudySmarter Originals

Aryl Halide: Example

For example, structural formula of aryl chloride is C₆H₅Cl. Another name for aryl chloride is chlorobenzene. The names of other aryl halides are similar.

From Left: Aryl Fluoride, Aryl Chloride, Aryl Bromide, and Aryl Iodide | StudySmarter Originals

Aryl Halide: functional groups

How are halogenoarenes formed?

Aryl halides are most commonly prepared using these two methods -

• Halogenation
• Sandmeyer reaction

Let us go through them in detail.

Halogenation

This is the simplest and the oldest method of synthesising aryl halides. In this process, benzene is reacted with a halogen gas in the presence of the aluminium salt of the halogen. The halogen substitutes a hydrogen atom on the benzene ring.

Fig. 3: Synthesis of Aryl Chloride Through Halogenation | StudySmarter Originals

Sandmeyer Reaction

The sandmeyer reaction is used to synthesise halogenoarenes from diazonium salts. A diazonium salt is reacted with a copper salt or potassium salt to get the aryl halide with the required halogen.

The first step involves the formation of a diazonium salt by reacting aniline with nitrous acid at a low temperature.

Fig. 4: Formation of Benzenediazonium chloride by the Reaction of Aniline with Nitrous Acid | StudySmarter Originals

Now, the diazonium salt is reacted with copper (I) chloride, or copper (I) bromide, or potassium iodide. The copper or potassium salt is chosen depending on which aryl halide is to be made. Nitrogen gas is released as a byproduct of this reaction.

Reactions of Aryl Halides

Let us see some typical reactions of aryl halides.

Grignard Reaction

When reacted with magnesium in the presence of ether, halogenoarenes form Grignard reagents.

Fig. 6: Grignard Reaction of Aryl Halides | StudySmarter Originals

In the example above, the Grignard reagent formed is phenylmagnesium bromide.

Substitution Reaction

In terms of nucleophilic substitution reactions, aryl halides are rather unreactive. There are various causes for this lack of reaction.

1. S_N2 reactions are prevented by steric hindrance brought on by the halogenarene's benzene ring.
2. Also, because phenyl cations are unstable, S_N1 reactions are not conceivable.
3. Halogenoarenes have a shorter and therefore stronger carbon-halogen bond than alkyl halides. In aryl halides, the C-X bond is shortened for two reasons -First, resonance causes the C-X bond to exhibit some double bond properties.Second, As opposed to alkyl halides, which have sp³ hybridised carbon atoms, aryl halides have sp² hybridised carbon atoms.

S_NAR Reactions

In organic chemistry, S_NAR reactions are nucleophilic aromatic substitution reactions. In these reactions, a nucleophile displaces a leaving group (ex - a halide) on an aromatic ring. When strong electron-withdrawing groups (deactivating groups) are ortho or para to the carbon atom that is linked to the halogen in aryl halides, they can undergo a nucleophilic substitution process. The carbon atom is vulnerable to a nucleophilic attack due to this configuration.

The following two examples show nucleophilic substitution on deactivated halogenoarenes.

Fig. 7: Nucleophilic Substitution on o-nitroiodobenzene | Cliffs Notes

Fig. 8: Nucleophilic Substitution on 2,4,6-trinitrobromobenzene | Cliffs Notes

Addition-Elimination Reactions

Addition-elimination reactions are two step reactions in which the first step is an addition reaction, and the second step is elimination, which gives an overall effect of substitution. In general, elimination-addition reactions do not occur in aryl halides. However, high pressure and temperature can be used to drive substitution reactions in halogenoarenes.

When chlorobenzene reacts with sodium hydroxide under high temperature and pressure, it undergoes addition-elimination reaction and converts to sodium phenoxide.

Fig. 9: Chlorobenzene Undergoes Addition-Elimination at High Temperature and Pressure to Form Sodium Phenoxide | StudySmarter Originals