How does free radical substitution work?
(This explanation would make use of diagrams on a whiteboard)It is useful to break this down into three steps: 1- the initiation step. Let's take the reaction between Cl2 and CH4. the Cl2 molecule has a relatively weak bond, this can be quite easily broken by incoming UV light. The breaking of this bond does not happen normally however, it splits evenly, and one electron from the bonded pair is returned to each chlorine atom to form 2Cl. radicals, each of which has an unpaired electron. These radicals are very reactive.Step 2- propagation. This very reactive Cl radical can now 'rip' a hydrogen atom from the CH4 molecule. As the chlorine bonding electron is unpaired, it only requires one electron from the H-C bond it tears apart, and the other bonded electron returns to the C atom, creating a .CH3 radical. This newly created radical can now go on to react with other Cl2 and CH4 molecules, creating new radicals and causing a chain reaction which we call propagation.Step 3- termination. Two free radicals react with one another and each donate their unpaired electron to the created bond (eg. Cl. + .CH3 -> Cl-CH3). This causes free radicals to be 'removed' from the reaction, hence terminating it.
