3.3.2—Homolytic fission

$Cl2(g) → 2Cl·(g) under UV light or heat$

Homolytic cleavage gives one electron to each fragment. Use single-electron arrows to show radical movement in the chain mechanism.

Draw two single-barbed arrows from the breaking X–X bond, one toward each atom, to account for both electrons. The UV or heat step creates radicals and is initiation; a step that consumes one radical and forms another belongs to propagation.

Representative question

[Maximum number: 1]

Write an equation for the initiation reaction.

$\mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{Cl} \cdot(\mathrm{g}) \downarrow$

Marking guidance:

Accept $\frac{1}{2} \mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow \mathrm{Cl} \cdot(\mathrm{g})$ .

Do not accept equations without a dot indicating the radical.

Retrieve the route: locate the unpaired electron, split the bond homolytically, initiate with UV or heat, propagate by single-electron steps and terminate by radical combination.

Every propagation step must regenerate a radical, and every radical symbol and single-electron movement must be shown where required.

3.3.2—Homolytic fission

Homolytic Fission and Initiation

Writing the Initiation Step

Assessment in practice

Radical Chain Summary

Question 1