Free Radical Reactions

Introduction to Free Radical Reactions

Free radical reactions are a fundamental class of organic reactions involving species with unpaired electrons, known as radicals. These reactions are important in both synthetic organic chemistry and biological processes.

• Free radicals are highly reactive intermediates with an unpaired electron.

• They are typically generated by homolytic bond cleavage, where a bond breaks evenly and each atom retains one electron.

• Commonly initiated by heat () or light ().

Homolytic Bond Cleavage

Homolytic cleavage is the process by which a covalent bond breaks so that each fragment retains one of the shared electrons, forming two radicals.

• General equation:

• This process is the first step in many radical reactions.

Chlorination of Methane: Mechanism

The chlorination of methane is a classic example of a free radical halogenation reaction. The mechanism consists of three main steps: initiation, propagation, and termination.

• Initiation: Formation of chlorine radicals by homolytic cleavage of under light ():

• Propagation: Chlorine radical abstracts a hydrogen atom from methane, forming a methyl radical and HCl:

• Methyl radical reacts with another molecule to form methyl chloride and regenerate a chlorine radical:

• Termination: Two radicals combine to form a stable molecule, ending the chain reaction. Examples:

Multiple Chlorination

With excess and light, all hydrogens in methane can be replaced by chlorine, leading to a series of products:

• Methyl chloride ()

• Methylene chloride ()

• Chloroform ()

• Carbon tetrachloride ()

Radical Halogenation: Selectivity and Stability

The selectivity of radical halogenation depends on the stability of the intermediate radical. The order of stability is:

• Primary < Secondary < Tertiary

This trend is similar to carbocation stability. The most stable radical forms preferentially, so the hydrogen that leads to the most stable radical is abstracted first.

Radical Bromination

Bromination is more selective than chlorination, favoring the formation of the most stable (usually secondary or tertiary) radical. For example, in the bromination of butane, the major product is 2-bromobutane, formed via a secondary radical intermediate.

Radical Initiators

Some reactions require radical initiators, such as peroxides (), which decompose to form radicals and start the chain reaction.

Allylic and Benzylic Bromination

Allylic and benzylic positions are especially reactive in radical halogenation due to resonance stabilization of the resulting radicals.

• Allylic bromination is often performed using N-bromosuccinimide (NBS) and light () to selectively brominate the allylic position.

• Benzylic bromination also uses and light to brominate the benzylic position.

Radical Polymerization

Radical polymerization is a process in which small alkene monomers join together to form long-chain polymers via a radical mechanism.

• Polyethylene: Formed from ethylene monomers ().

• PVC (Polyvinyl chloride): Formed from vinyl chloride monomers ().

• Teflon: Formed from tetrafluoroethylene monomers ().

Summary Table: Key Steps in Radical Halogenation

Key Points

• Free radical reactions proceed via chain mechanisms: initiation, propagation, and termination.

• Stability of radicals determines the major product in halogenation reactions.

• Allylic and benzylic positions are especially reactive due to resonance stabilization.

• Radical polymerization is a major industrial process for producing plastics.

Additional info: The notes also include several practice questions and reaction schemes, which are typical for organic chemistry courses to reinforce understanding of radical mechanisms and selectivity.