Topic 8 of 34

Radical substitution is a chain reaction

Initiation makes radicals; propagation regenerates a radical; termination removes them.

A-Level 9476 (2026-2027)

Radical substitution is a chain reaction

Initiation makes radicals; propagation regenerates a radical; termination removes them.

Ethane reacts with chlorine under ultraviolet light at room temperature to form chloroethane and HCl. The UV light causes homolytic fission of Cl-Cl, not heterolysis to ions. One chlorine radical can sustain many propagation cycles before being removed in a termination event.

Initiation: one bonding electron goes to each chlorine atom

Single-barbed fishhooks track individual electrons. UV supplies energy for Cl₂ → 2Cl•.

The complete ethane/chlorine chain
Stage	Equation	What it achieves
Initiation	Cl₂ → 2Cl•, under UV.	Creates radicals from a non-radical molecule.
Propagation 1	Cl• + CH₃CH₃ → HCl + CH₃CH₂•.	Abstracts H and creates an ethyl radical.
Propagation 2	CH₃CH₂• + Cl₂ → CH₃CH₂Cl + Cl•.	Forms the C-Cl product and regenerates the chain carrier.
Termination example 1	Cl• + Cl• → Cl₂.	Two radicals combine; no new radical.
Termination example 2	CH₃CH₂• + Cl• → CH₃CH₂Cl.	Two radicals combine; no new radical.
Termination example 3	CH₃CH₂• + CH₃CH₂• → CH₃CH₂CH₂CH₃.	Two ethyl radicals form butane.

Adding the propagation steps cancels the ethyl and chlorine radicals, giving CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl. Further substitution can replace more hydrogens, so a mixture forms; using excess ethane reduces the chance that the already-substituted product is attacked. Bromination under UV follows the same stage pattern with bromine radicals.

Check your understandingIs CH₃CH₂• + Cl₂ → CH₃CH₂Cl + Cl• a termination step because it makes the desired product?Think it through, then reveal the answer

No. It regenerates a radical and keeps the chain running, so it is propagation. Stage classification depends on radical creation, regeneration or removal, not on whether an organic product appears.