CAIE A-Level Chemistry 14.1.3 Free-Radical Substitution Mechanism

Practise constructing initiation, propagation and termination equations for alkane halogenation.

Exam points

show homolytic X–X fission under UV light in the initiation step to form two halogen radicals
write two propagation steps that consume a radical, form product and regenerate another radical
write a termination step in which two radicals combine, with no radical remaining in the product

[Maximum number: 1]

Allyl chloride is an important chemical used in the manufacture of plastics, pharmaceuticals and pesticides.

allyl chloride

Allyl chloride can be produced by many different methods. The most common method is chlorination of propene which proceeds via a free-radical substitution mechanism.

Figure for Question 14.1.3—Mechanism: free-radical substitution question 1 — CAIE A-Level Chemistry AS

The propenyl radical, $\mathrm{CH}_{2}=\mathrm{CHCH}_{2}$ , is formed in the first propagation step of the reaction. Write an equation to show the formation of $\mathrm{CH}_{2}=\mathrm{CHCH}_{2}{ }^{\bullet}$ in this propagation step.

$\begin{aligned} \mathrm{CH}_{2}=\mathrm{CHCH}_{3}+\mathrm{C} l^{\bullet} \rightarrow \mathrm{CH}_{2}=\mathrm{CHCH}_{2}^{\bullet}+\mathrm{HCl} \mathrm{OR} \mathrm{C}_{3} \mathrm{H}_{6}+\mathrm{C} l^{\bullet} \rightarrow \rightarrow \mathrm{C}_{3} \mathrm{H}_{5}{ }^{\bullet}+\mathrm{HCl} \end{aligned}$

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14.1.3—Mechanism: free-radical substitution question 1