Free Radical Reactions

Introduction to Free Radical Reactions

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

• Free radicals are atoms or molecules with an unpaired electron, making them highly reactive.

• Radical reactions often occur via homolytic bond cleavage, where a covalent bond breaks evenly, and each atom retains one electron.

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

Homolytic Bond Cleavage

Homolytic cleavage is the process by which a covalent bond splits evenly, producing two radicals.

• General equation:

• Requires energy input, such as heat or light.

• Example: Chlorine molecule under UV light forms two chlorine radicals.

Mechanism of Radical Halogenation of Methane

Radical halogenation is a key reaction for introducing halogens into alkanes. The mechanism involves three main steps: initiation, propagation, and termination.

1. Initiation: Formation of radicals by homolytic cleavage. (by hv)

2. Propagation: Radicals react with substrate to form new radicals and products.

3. Termination: Radicals combine to form stable molecules, ending the chain reaction.

Multiple Chlorination of Methane

With excess chlorine and light, all hydrogens in methane can be replaced by chlorine, forming methyl chloride, methylene chloride, and chloroform.

• Stepwise chlorination:

Radical Bromination of Alkanes

Bromination follows a similar mechanism to chlorination but is more selective due to the lower reactivity of bromine radicals.

• Major product: The most stable radical intermediate leads to the major product.

• Example: Mono-bromination of butane yields 2-bromobutane as the major product due to the formation of a secondary radical.

Stability of Alkyl Radicals

The stability of alkyl radicals increases with the number of alkyl substituents attached to the radical center.

• Stability trend is similar to carbocations.

• The hydrogen that leads to the most stable radical is most reactive.

Radical Initiators

Radical reactions can be initiated by compounds that easily form radicals, such as peroxides.

• Peroxides (ROOR): Under heat, peroxides decompose to form alkoxy radicals, which can initiate radical reactions.

Allylic and Benzylic Bromination

Radical bromination can selectively occur at allylic and benzylic positions due to the resonance stabilization of the resulting radicals.

• Allylic bromination: Bromine reacts at the carbon adjacent to a double bond.

• Benzylic bromination: Bromine reacts at the carbon adjacent to an aromatic ring.

• NBS (N-bromosuccinimide): Used for selective bromination at allylic and benzylic positions.

Radical Polymerization

Radical polymerization is a process where monomers with double bonds form long-chain polymers via radical intermediates.

• 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

Example: Major Product in Radical Bromination

For the reaction: with 2-methylbutane, the major product is 2-bromo-2-methylbutane, due to the formation of the most stable tertiary radical.

Practice Questions

• Predict the major product for the radical bromination of cyclohexene using NBS and light.

• Explain why tertiary radicals are more stable than secondary or primary radicals.

• Describe the role of peroxides in radical reactions.

Additional info: The notes also briefly mention the use of NBS for selective bromination and the importance of radical stability in determining product distribution.