Have you ever wondered what the chemical structure of coolant is, or even of some anaesthetics that are used when you have to have a wisdom tooth removed, or have an operation on your kneecap? Well, these chemical compounds that we use every day are halogenoarenes. Read on to find out more about them.
- In this article, you will learn what halogenarenes are, thei general formula and examples.
- You will learn about the reactions that are used to prepare halogenarenes.
- To finish, you will learn some basic reactions of halogenarenes.
Aryl Halide: meaning
Before we talk about aryl halides, also known as halogenoarenes, let's talk about arenes.
Arenes are hydrocarbons which have benzene as a structural unit. Arenes are also called aromatic hydrocarbons.
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.
Fig. 1: Benzene | Vaia Originals
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.
Halogenoarenes are formed when one hydrogen atom in a benzene ring is substituted with a halogen atom. Halogenoarenes are also called aryl halides.
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 | Vaia 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 | Vaia 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 | Vaia 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 | Vaia 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.
Fig. 5: Sandmeyer Reaction to form a Halogenarene by reacting a Diazonium Salt with CuCl, CuBr, or KI | Vaia Originals
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 | Vaia 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.
- SN2 reactions are prevented by steric hindrance brought on by the halogenarene's benzene ring.
- Also, because phenyl cations are unstable, SN1 reactions are not conceivable.
- 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.
SNAR Reactions
In organic chemistry, SNAR 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 | Vaia Originals
Relative Ease of Hydrolysis of Acyl Chlorides, Alkyl Chlorides, and Aryl Chlorides
Hydrolysis is the chemical reaction of a compound with water, in which breakdown of both the compound and water takes place. The ease of hydrolysis differs in different compounds. When we compare alkyl chlorides, acyl chlorides, and aryl chlorides, the order of ease of hydrolysis is - Acyl chloride > alkyl chloride > aryl chloride
Aryl chlorides do not undergo hydrolysis. This can be explained by the strength of carbon-chlorine bond.
Acyl Chlorides
Fig. 10: Acyl Halide | Vaia Originals
Hydrolysis of acyl halides is easiest compared to alkyl halides and aryl halides, and can be done at
room temperature. This is because the carbon that the halide is attached to, is also attached to an oxygen atom. The oxygen atom and the halide atom are both very electronegative and pull electron away from the carbon atom. As a result, the
carbon atom is left very δ+. This makes the C-X bond weak, making it an easy site for a nucleophile to attack.
Alkyl Halides
Fig. 11: Alkyl Halide | Vaia Originals
In alkyl halides, there is only one electronegative atom, the halogen, pulling electrons away from the carbon atom which it is attached to. Therefore, the carbon atom is not as δ+, and the C-X bond is stronger than that in acyl halides. Therefore, hydrolysis of alkyl halides needs to be heated, and also to be refluxed with OH-, which is a stronger nucleophile than H2O.
Aryl Halides
The C-X bond in aryl halides is the strongest. This is because a lone pair of electrons on the halide atom is part of the delocalized 𝜋-bonding. This results in the C-X bond in developing some double-bond characteristics. Therefore, hydrolysis of aryl halides does not occur.
Let us look at the resonance structures that aryl halides exhibit in the example of aryl chloride.
Fig. 12: Resonance Structures of Aryl Chloride | Cliffs Notes
As you can see in the resonance structures diagram, 3 out of 4 resonance structures have a double bond between the carbon atom and the chlorine atom. Therefore, in the hybrid structure, there are some double bond characteristics in the C-X bond.
Aryl Halide - Key takeaways
- Arenes or aromatic hydrocarbons are compounds which have benzene as a structural unit.
- Halogenoarenes or aryl halides are formed when one hydrogen atom in a benzene ring is substituted with a halogen atom.
- The general formula of aryl halide is C6H5X, where X is the halide atom. structural formula of aryl chloride is C6H5Cl
- Halogenation and Sandmeyer reaction are two most common methods of synthesising aryl halides.
- In halogenation, benzene is reacted with a halogen gas in the presence of the aluminium salt of the halogen.
- In sandmeyer reaction, A diazonium salt is reacted with a copper salt or potassium salt to get the aryl halide with the required halogen. The diazonium salt is prepared by reacting aniline with nitrous acid at low temperatures.
- Aryl halides form Grignard reagents when they react with magnesium in the presence of ether.
- Aryl halides are relatively unreactive in substitution reactions.
- Aryl halides can undergo SNAR reactions when strong electron-withdrawing groups (deactivators) are ortho- or para- to the carbon atom attached to the halogen atom.
- Aryl halides need high temperature and pressure conditions to undergo addition-elimination reaction.
- Order of ease of hydrolysis - Acyl halide > alkyl halide > aryl halide.
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