Alkanes undergo radical substitution when exposed to halogens under UV light, forming haloalkanes through a chain mechanism. This topic explores the steps and limitations of this reaction.

Radical Substitution: Core Principles

Radical substitution is a multi-step reaction pathway in which an alkane reacts with a halogen (e.g., chlorine or bromine) through a chain mechanism involving initiation, propagation, and termination. The process occurs because UV light provides the energy required to break halogen bonds and generate reactive species.

When first introduced, a radical must be defined.

Radicals are highly reactive because they seek electron pairing, allowing the chain mechanism to proceed rapidly once initiated.

Homolytic Fission and Radical Formation

The mechanism begins with homolytic fission, where a covalent bond splits evenly, producing two radicals. In radical substitution, the halogen molecule absorbs UV light, causing this symmetrical cleavage.

This image shows the homolytic fission of Cl2 under ultraviolet light, producing two Cl• radicals. It reinforces how UV initiates radical formation in the mechanism. Source

Because homolytic fission requires energy input, radical substitution is photochemical and will not proceed in the dark. The high reactivity of radicals makes the steps that follow both fast and difficult to control.

A covalent bond’s energy is crucial to this process.

Strong C–H and C–C σ-bonds mean alkanes do not undergo many reactions, but once radicals form, the mechanism becomes self-sustaining.

Stages of Radical Substitution

Initiation

UV light breaks the halogen molecule (e.g., Cl₂ → 2Cl•) via homolytic fission. The formation of halogen radicals launches the chain mechanism.

Propagation

These steps allow the reaction to continue, generating products and regenerating radicals.

• A halogen radical removes a hydrogen atom from the alkane:
  Cl• + CH₄ → CH₃• + HCl

• The alkyl radical then reacts with another halogen molecule:
  CH₃• + Cl₂ → CH₃Cl + Cl•

The regeneration of a halogen radical means propagation can repeat many times. This chain-like nature explains why even a small amount of UV light produces substantial substitution.

Termination

Termination occurs when radicals combine, removing reactive species from the mixture. Common termination steps include:

• Cl• + Cl• → Cl₂

• CH₃• + Cl• → CH₃Cl

• CH₃• + CH₃• → C₂H₆

Since radicals are short-lived and highly reactive, any combination of two radicals is a termination event.

This figure shows the complete mechanism for free radical chlorination, illustrating initiation, propagation, and termination with clear structures and curly arrows consistent with A-Level expectations. Source

Limitations of Radical Substitution

Although often presented as a straightforward route to haloalkanes, radical substitution has significant limitations that restrict its synthetic usefulness.

1. Further Substitution

Once the first haloalkane (e.g., chloromethane) forms, the process does not stop. The product itself may undergo additional substitution, especially under prolonged UV exposure.

Possible sequential products include:

• CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄

Because each step also proceeds via radical pathways, mixtures of mono-, di-, tri-, and tetra-substituted compounds accumulate.

This image shows how chloromethane undergoes successive radical substitutions to form multiple chlorinated products, demonstrating why mixtures are inevitable in these reactions. Source

2. Positional Isomers

With alkanes longer than methane, substitution can occur at multiple positions along the carbon chain. Radicals do not significantly discriminate between equivalent hydrogen sites unless steric or stability factors intervene.

For example, substituting a hydrogen on propane may produce:

• 1-chloropropane (primary substitution)

• 2-chloropropane (secondary substitution)

The formation of positional isomers is a major limitation, as it leads to complex product mixtures requiring extensive separation.

After discussing positional variation, it is appropriate to define structural isomer.

Because the mechanism generates radicals indiscriminately, both primary and secondary carbon sites may be substituted, depending on radical stability and reaction conditions.

Factors Affecting Selectivity and Product Distribution

Although radical substitution lacks precision, several factors influence the relative amounts of substituted products:

Radical Stability

More stable radicals (tertiary > secondary > primary) form more readily, increasing substitution at positions that generate the most stable intermediates.

Halogen Reactivity

• Fluorination is explosive and largely uncontrollable.

• Chlorination is moderately reactive and often yields mixtures.

• Bromination is more selective because its propagation steps differ in activation energies.

Reaction Conditions

• Higher temperatures or stronger UV sources increase further substitution.

• Diluting the halogen or limiting exposure time can reduce over-substitution.

• Using excess alkane limits the chances of a haloalkane being attacked again.

Why Radical Substitution Fits the Syllabus

The reaction demonstrates radical behaviour, chain mechanisms, and the challenges posed by uncontrolled reactivity. For OCR A-Level Chemistry, understanding the three mechanistic stages and appreciating the limitations of further substitution and positional isomer formation are essential for explaining why this reaction is more suitable for illustrating mechanisms than for precise organic synthesis.