20.10 : Radical Reactivity: Concentration Effects

In a radical reaction, the concentration of starting materials governs the selectivity of a radical. For example, the reaction between an alkyl halide and an alkene, in the presence of tin hydride and AIBN, begins with the generation of a tin radical. The generated radical then abstracts halogen from the alkyl halide, producing an alkyl radical. This alkyl radical can either react with tin hydride, yielding an alkane, or add to an alkene, generating a nitrile-stabilized radical, eventually forming the addition product. The formation of the alkane and the addition product has an equal possibility because the rate constant for the reaction between the alkyl radical and tin hydride is almost the same as for the reaction between an alkyl radical and the alkene. But, the reaction towards the addition product can be driven by increasing the alkene concentration at least ten times higher than that of the tin hydride. This is because the higher alkene concentration will increase the reaction rate between the alkyl radical and the alkene by ten times, thereby favoring the formation of the addition product over the alkane.