Free radical substitution

Question 1

Initiation

Answer 1

The chlorine molecule absorbs the energy of a single quantum of ultraviolet (UV) light.

The bond will break as the energy of one quantum of UV light is greater than the Cl-Cl bond energy.

The bond will break homolytically and one electron goes to each chlorine atom, forming two chlorine free radicals. Cl○ ○Cl

Free radicals are highly reactive

Question 2

Propagation: Stage 1

Answer 2

Stage 1: The chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, a stable compound. This leaves a methyl free radical ○CH3

Question 3

Propagation: Stage 2

Answer 3

Stage 2: The methyl free radical is also very reactive and reacts with a chlorine molecule. This produces another chlorine free radical and a molecule of chloromethane, a stable compound.

Question 4

Termination

Answer 4

Free radicals are removed in any of the following three ways:

Two chlorine free radicals react together to give chlorine

Two methyl free radicals react together to give ethane.

A chlorine free radical and a methyl free radical react together to give chloromethane

Question 5

Other products formed by chain reaction

Answer 5

Ethane produced at termination

Dichloromethane produced at propagation, if a chlorine radical reacts with chloromethane

Isomers of longer chain alkanes

Question 6

Chlorofluorocarbons (CFC’s)

Answer 6

Destroying the ozone layer

Chlorine free radicals are formed from CFC’s as the Cl-Cl bond breaks homolytically, forming chlorine free radicals (Cl○).

Ozone molecules are then broken down into oxygen by the free radicals

2O3 -> 3O2