20.10 : 自由基反应性：浓度效应

The concentration of starting materials significantly affects the selectivity of radical reactions.

Consider the reaction between an alkyl halide and an alkene. In the presence of tributyltin hydride and AIBN, a radical initiator, the alkyl group is added to the alkene.

Typically, the reaction begins with the generation of a tin radical, which subsequently abstracts halogen from the alkyl halide, producing an alkyl radical.

The alkyl radical now has a choice. It can either abstract hydrogen from the tin hydride producing an alkane, or add to the alkene forming a nitrile-stabilized radical, which further reacts to form the addition product.

This is because the rate constants for the two reactions are about the same.

To drive the reaction towards the addition product, the alkene concentration should always be at least 10 times higher than that of the tin hydride.

This would increase the rate at which alkyl radical adds to the alkene.

In conclusion, the concentration of the starting materials governs the selectivity of radical reactions by favoring one reaction over the other.