IB Chemistry HL 3.3.3 Radical Substitution Questions

Use real HL exam questions to build complete alkane-halogen radical substitution mechanisms and explain product mixtures and yields.

Exam points

Write the halogen initiation step under UV light or heat, showing homolytic fission and correctly marked radicals.
Write propagation steps for alkane halogenation, consuming one radical and regenerating the chain carrier.
Write termination by combining radicals and explain the mixture of substitution products formed from an alkane.

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Study 3.3.3—Radical substitution conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 4]

This question is about carbon and chlorine compounds.

(a)

Ethane, $\mathrm{C}_{2} \mathrm{H}_{6}$ , reacts with chlorine in sunlight. State the type of this reaction and the name of the mechanism by which it occurs.

Type of reaction:

Mechanism:

[ 1 ]

(b)

Formulate equations for the two propagation steps and one termination step in the formation of chloroethane from ethane.

Two propagation steps:

One termination step:

[ 3 ]

Two propagation steps:
$\mathrm{C}_{2}\mathrm{H}_{6}+\bullet\mathrm{Cl} \rightarrow \mathrm{C}_{2}\mathrm{H}_{5}\bullet+\mathrm{HCl}$ $\mathrm{C}_{2}\mathrm{H}_{5}\bullet+\mathrm{Cl}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Cl}+\bullet\mathrm{Cl}$

One termination step, for example:
$\mathrm{C}_{2}\mathrm{H}_{5}\bullet+\mathrm{C}_{2}\mathrm{H}_{5}\bullet \rightarrow \mathrm{C}_{4}\mathrm{H}_{10}$

or:
$\mathrm{C}_{2}\mathrm{H}_{5}\bullet+\bullet\mathrm{Cl} \rightarrow \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Cl}$

or:
$\bullet\mathrm{Cl}+\bullet\mathrm{Cl} \rightarrow \mathrm{Cl}_{2}$

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IB Chemistry HL 3.3.3 Radical Substitution Questions question 1

Question (a)

Question (b)