Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance wherein the halogen atom ends up as a part of that substance or a compound. In general, during the reaction, there is usually an addition of one or more halogens to the substance.

Halogens are a group of elements such as iodine, chlorine, fluorine and bromine. Most of the time these elements exhibit similar behaviour and are therefore categorized under the same group.  Halogenation can be further described as the process of replacing any number of the hydrogen atom with these given elements in the group. The product that is formed after halogenation will possess new and unique properties than the initial substance.

Types Of Halogenation

Halogenation can occur in several ways for organic compounds as well as inorganic compounds. The types of halogenation are given below.

Halogen Addition

This type of reaction is common in unsaturated carbons. Typically, alkynes and alkenes follow this reaction where they add halogens. Example, the addition of bromine to ethane.

Halogen Substitution

Here, we consider saturated hydrocarbons where the hydrogen atoms are usually replaced by halogens. The hydrocarbons basically undergo free radical halogenation. We will take the example of halogens reacting with alkanes in the influence of heat to form alkyl halides.

As you can see, the halogen atom replaces hydrogen atom into alkane hence called a substitution reaction.

Meanwhile, aromatic compounds also go through substitution reactions or electrophilic halogenation in the presence of Lewis acids.

Electrophilic Substitution Reaction

Benzene reacts with bromine or chlorine in an electrophilic substitution reaction only in the presence of a catalyst which is either chloride or iron.

Iron is not a catalyst because it changes permanently during the reaction. It reacts with some bromine to form iron 3 chloride, FeCl3 or iron 3 bromide, FeBr3.

Mechanism Of Halogenation Of Benzene

Step 1: The bromine reacts with Lewis acid to create a complex that makes bromine more electrophilic.

Step 2: The π electrons of aromatic C=C behave as a nucleophile which attacks the electrophilic Br and displaces iron tetrabromide.

Step 3: The proton is removed from sp3 C and bears the Bromo group that reforms C=C and aromatic system generates HBr and regulates active catalyst.

The compounds act as a catalyst and behave similarly to aluminium chloride in the following chemical reactions.

We will further look at how halogenation is influenced by each halogen or the halogenating agents.

Reaction with Chlorine

The reaction between chlorine and benzene gives chlorobenzene in the presence of either iron or aluminium chloride.

Reaction with Bromine

The reaction between bromine and benzene gives bromobenzene in the presence of either iron or aluminium bromide. Iron is used normally because it is readily available and cheaper.

Reaction with Fluorine

If we take any organic compounds it readily reacts with fluorine (usually explosive in nature). However, in the case of elemental fluorine (F2) we will need some proper apparatus and fulfil certain conditions. A variety of fluorinating reagents such as xenon difluoride and cobalt (III) fluoride are also used sometimes for the reaction.

Reaction with Iodine

In comparison to the above three halogens, iodine is the least reactive halogen. It does not readily react with organic substances.

Simultaneously, when we study inorganic chemistry, almost every element excluding helium,  argon, and neon form fluorides when it is reacted with fluorine.

Know more about applications of halogenation of benzene along with chemical reactions involved in every application at BYJU’S.

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