A radical reaction is a chemical reaction that involves radicals, which are atoms or groups of atoms with unpaired electrons. Radicals are often formed by homolytic bond cleavage, especially during the initiation step, where a covalent bond breaks evenly and each atom receives one electron. ^[2,4,7]

Common Types

Radical reactions are classified according to what the radical does during the reaction. It may replace an atom, add to a multiple bond, start polymer formation, or form at a resonance-stabilized position such as an allylic or benzylic carbon.

1. Radical Substitution

In radical substitution, one atom or group in a molecule is replaced by another through a radical chain mechanism. This type of reaction commonly occurs in alkanes, where a hydrogen atom attached to carbon can be replaced by a halogen atom. ^[4,5,8]

Example: Chlorination of methane

CH₄ + Cl₂  → CH₃Cl + HCl

(in the presence of UV radiation)

Here, one hydrogen atom in methane is replaced by a chlorine atom to form chloromethane.

2. Radical Addition

In radical addition, a radical adds to a double bond or triple bond. This type of reaction commonly occurs in alkenes because the carbon–carbon double bond can open up and form new bonds. ^[2,3,4]

Example: Addition of HBr to propene

CH₃CH=CH₂ + HBr → CH₃CH₂CH₂Br

(in the presence of peroxide)

In this reaction, HBr adds to propene by a radical mechanism and forms the anti-Markovnikov product, 1-bromopropane. This means the bromine atom attaches to the less substituted carbon of the double bond.

3. Radical Polymerization

In radical polymerization, a radical intermediate repeatedly adds to alkene monomers, forming a long-chain polymer. This type of reaction commonly occurs with unsaturated monomers such as ethene, styrene, and vinyl chloride. ^[2,8]

Example: Polymerization of ethene

nCH₂=CH₂ → (–CH₂–CH₂–)_n

Many ethene molecules join together to form polyethene.

4. Allylic and Benzylic Radical Reactions

In allylic and benzylic radical reactions, the radical forms at a special position next to a double bond or benzene ring. An allylic radical forms next to a carbon–carbon double bond, while a benzylic radical forms next to a benzene ring. These reactions are often discussed separately because allylic and benzylic radicals are relatively stable due to resonance. ^[3,7,8]

Example: Allylic bromination of propene

CH₂=CHCH₃ + NBS → CH₂=CHCH₂Br

(under hν or heat, or in the presence of a radical initiator)

NBS is commonly used for allylic bromination because it promotes bromination at the allylic position rather than directly across the double bond.

Main Steps

Radical reactions usually occur in three stages: initiation, propagation, and termination. ^[2,5,8]

To understand each stage, take a simple example: chlorination of methane.

1. Initiation is the first stage, where radicals are formed from stable molecules, usually by heat or UV light. Here, a covalent bond breaks evenly, and each atom receives one electron. ^[2,7,8]

In methane chlorination, chlorine molecules absorb UV light and split evenly to form chlorine radicals.

Cl₂ – UV light → 2 Cl·

The dot, ·, represents an unpaired electron.

2. Propagation is the chain-carrying stage, where one radical reacts and produces another radical. ^[2,7,8]

In this example, a chlorine radical reacts with methane and removes a hydrogen atom, forming hydrogen chloride and a methyl radical.

Cl· + CH₄ → HCl + CH₃·

The methyl radical then reacts with another chlorine molecule to form chloromethane and a new chlorine radical.

CH₃· + Cl₂ → CH₃Cl + Cl·

The new chlorine radical continues the reaction chain.

3. Termination is the final stage, where two radicals combine and no new radical is produced. ^[2,7,8]

For example, two chlorine radicals may combine to reform chlorine gas.

Cl· + Cl· → Cl₂

Other possible termination reactions are:

CH₃· + Cl· → CH₃Cl

CH₃· + CH₃· → C₂H₆ 

Note that methane chlorination may continue beyond monochlorination to give dichloro-, trichloro-, and tetrachloro-products.

Applications

• Polymerization: Radical reactions are used to make plastics such as polyethene, polystyrene, and polyvinyl chloride by joining alkene monomers into long polymer chains. ^[1,4,8]
• Pharmaceutical and drug synthesis: Radical reactions help form carbon–carbon and carbon–heteroatom bonds, making them useful in preparing some complex organic molecules and drug intermediates.
• Petrochemical and industrial processing: They occur in cracking, halogenation, combustion, and oxidation of hydrocarbons.
• Materials and energy: They are used in making resins, coatings, adhesives, and some advanced materials. They also play a role in fuel combustion and energy-related oxidation processes.

Radical reactions explain how highly reactive intermediates can drive powerful transformations in organic chemistry. Their ability to form new bonds, build polymers, and support industrial and biological processes makes them important in both laboratory synthesis and large-scale chemical production.