Bromination

What is Bromination?

Bromination is a chemical reaction involving the reaction of a compound, and bromine results in bromine being added to the compound. The product formed after bromination will exhibit new properties from the initial reactant.

Table of Content

• Types of Bromination
• Bromination via a Free Radical Mechanism
• Bromination via an Addition Reaction
• Bromination via an Electrophilic Substitution Mechanism
• Importance of Bromination
• Frequently Asked Questions – FAQs

Types of Bromination

Bromination can occur in different ways, depending on the reactant.

• A saturated compound undergoes bromination via a free radical mechanism.
• An unsaturated hydrocarbon undergoes bromination via an addition reaction.
• An aromatic compound undergoes bromination via an electrophilic substitution mechanism.

We will now briefly discuss types of bromination in detail.

Bromination via a Free Radical Mechanism

This type of reaction occurs in the saturated compound.

The mechanism for the reaction takes place in the following steps.

Initiation Step

The Br-Br bond of elemental bromine undergoes homolytic fission when irradiated under UV rays, and this process yields two bromine radicals.

Propagation Step

• A bromine radical abstracts a hydrogen atom from ethane to produce the ethyl radical. In turn, the ethyl radical abstracts a bromine atom from a bromine molecule, and bromoethane is formed.
• The second step also regenerates a bromine atom. These steps repeat many times until termination occurs.

Termination Step

Termination takes place when

• A bromine atom reacts with another bromine atom to generate Br₂.
• A bromine atom reacts with ethyl radical to form bromoethane.
• Two ethyl radicals can also combine to produce butane.

Bromination via an Addition Reaction

• This type of reaction occurs in the unsaturated compound.
• Bromination in unsaturated compounds proceeds through bromonium ion intermediate.
• This reaction is also used to check unsaturation.

Bromination via an Electrophilic Substitution Mechanism

• Benzene reacts with bromine in lewis acid-like FeBr₃ to form aryl bromide.
• This reaction is known as the bromination of benzene.

Activation of Bromine by Lewis Acid

Bromine reacts with lewis acid FeBr₃ to form a complex that makes the bromine more electrophilic.

Attack of Benzene

• Benzene, being electron-rich, attacks electrophilic bromine.
• This step produces cyclohexadienyl cation intermediate, destroying the aromaticity of benzene.

Deprotonation to generate Aromatic Ring

• Proton is removed from sp³ Carbon bearing a Bromo group.
• Aromaticity is retained.

Importance of Bromination

• Bromination is used in many chemical industries as a suitable building block for numerous syntheses.
• Bromination is used in agricultural, pharmaceuticals, and chemical intermediates.