Radical substitution

Question 1

Initiation

Answer 1

• One (or more) molecules react to form radicals.
• A bond breaks by homolytic fission so the bond is exothermic.
• Happens in the presence of UV light.

Question 2

Propagation

Answer 2

• A molecule and a radical react to form a new molecule/radical pair.
• A bond breaks and a new bond forms.
• Propagation reactions occur in pairs. One propagation step produces an intermediate that then reacts in the second step (making a chain reaction).

Question 3

Termination

Answer 3

• To stop the chain reaction.
• Two radicals react to form one molecule.
• A bond forms, so the step is exothermic.

Question 4

Radical substitution

Answer 4

• You can identify intermediates by adding up the two reactions.
• The intermediates appear on both sides so you cancel them out.
• The equation now should be identical to overall equation for the completed radical substitution.

Question 5

Limitation: Further substitution

Answer 5

• During the propagation process further substitutions can occur until all hydrogen atoms have been substituted.
• The result is a mixture of compounds which is undesirable because you have to separate the molecule needed from the unwanted ones.
• Best way of reducing this is to have an excess of the alkane which makes it more likely for the halogen to react with the alkane that the other molecules that have formed.

Question 6

Limitation: Substitution at different positions

Answer 6

• Another problem of radical sub is that it can take place at any point along the carbon chain.
• So a mixture of isomers can be formed which is undesirable.