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Reaction mechanism

Most chemical reactions take place in a sequence of small steps. The reaction mechanism shows exactly how an overall reaction takes place, identifying these individual stages.

In the reaction of an alkane with a halogen in ultra-violet light, the first stage involves the energy from the uv light breaking the halogen-halogen bond. This is called the initiation step. It produces two halogen atoms. These atoms will have an unpaired electron in their outer shell, and as such are extremely reactive species called free radicals. I shall use the reaction of bromine with hexane as an example. The overall reaction is

First, we have the initiation step:

The dot in the top right hand corner represents a free radical. In the next stage a free radical removes a hydrogen atom from the alkane to form a hydrogen halide molecule. This produces an alkyl free radical, that is an alkane molecule missing a hydrogen atom and consequently having an outer, unpaired electron. This new free radical will react with another halogen molecule to form the halogenoalkane and another halogen free radical. The new halogen free radical can start off this sequence again, and these two stages can repeat many times over. Such a process, where an end product can start off the reaction again is called a chain reaction. These two steps are called propagation steps:

In the final stages two free radicals may join together and this will stop this part of the chain. These are called termination steps. However, new free radicals are being made all the time by the uv light. There are always three possible termination steps. One of which leads to an unusual product. Presence of this product (in this case dodecane, C₁₂H₂₆) in the reaction mixture is evidence of this particular mechanism.

The free radicals are very reactive and fairly indiscriminate. Any of the hydrogen atoms on the hexane can be substituted for bromine in this way. Also, multiple substitution is common - that is, more than one of the hydrogen atoms can be replaced.