Free Radical Substitution

Define the term Free Radical

An atom or molecule which has an unpaired electron.

Define the term Homolytic fission

A bond breaks evenly in the presence of UV light so that each atom has one unpaired electron.

How are free radicals formed

• Free radicals are formed when a covalent bond breaks by homolytic fission.

• In homolytic fission, each atom takes one electron from the shared pair.

• This leaves two free radicals, each with an unpaired electron.

Free radicals are formed by homolytic fission of a covalent bond. Each atom takes one electron from the bond, producing two highly reactive species with unpaired electrons.

Explain how Haloalkanes are formed from alkanes by Free radical substitution 

Haloalkanes are formed by free radical substitution. In the initiation step, UV light causes homolytic fission of a halogen molecule, forming radicals. In propagation, radicals attack alkane C–H bonds to form haloalkanes and regenerate radicals. In termination, radicals combine to end the reaction.

Using Methane and Bromine to form Bromomethane, outline the process of free Radical substitution

Overall reaction: CH₄ + Br₂  →  CH₃Br + HBr

Step 1: Initiation

• UV light provides the energy for homolytic fission of Br₂.

Br2  →  Br⋅ + Br⋅

• Two bromine radicals are formed.

Step 2 Propagation: (chain reactions)

• Stage 1: A bromine radical attacks methane, breaking a C–H bond:

Br⋅ + CH₄  →  HBr + CH₃⋅

• Stage 2: The methyl radical reacts with another Br₂ molecule:

CH₃⋅ + Br₂  →  CH₃Br+Br⋅

• This regenerates a bromine radical, so the chain reaction continues.

Step 3: Termination

• Two radicals combine to end the chain:

Br⋅ + Br⋅  →  Br₂

CH3⋅ + Br⋅  →  CH₃Br

CH₃⋅ + CH₃⋅  →  C₂H₆

Explain the difference between Homolytic and heterolytic bond fission 

Homolytic bond fission

• The covalent bond breaks evenly.

• Each atom takes one electron from the shared pair.

• This produces two free radicals, each with an unpaired electron.

Example:

• Cl₂ →  Cl⋅ + Cl⋅

Heterolytic bond fission

• The covalent bond breaks unevenly.

• One atom takes both electrons from the shared pair.

• This produces a positive ion (cation) and a negative ion (anion).

Example:

H−Br  →  H⁺ + Br⁻

What conditions are needed for homolytic fission to occur?

• High energy input is needed to break the covalent bond:

  • Often provided by UV light (photons) or high temperatures.

  • This energy overcomes the bond dissociation energy.

• Absence of a solvent or reagent that would cause heterolytic fission:

  • Homolytic fission occurs more readily in gases or in non-polar environments.

Explain why free radical substitution is said to be a chain reaction

Free radical substitution is a chain reaction because the radicals produced in the propagation steps are regenerated and continue reacting with more molecules, allowing the reaction to proceed without the continuous addition of initiators.

Explain how free radical substitution is terminated

Free radical substitution is terminated when two radicals combine to form a stable molecule, removing radicals and stopping the chain reaction.