20.21 : 自由基取代：卤代烷与三丁基氢化锡的氢解

Radical substitution reactions are useful for the halogenation of alkanes and alkenes.

Similarly, radical substitutions can remove halogen from alkyl halides using tributyltin hydride, known as the hydrogenolysis of alkyl halides.

Notably, tin hydride has a weak Sn–H bond and can react with alkyl halides to replace the halogen with hydrogen, forming alkane and tin halide.

The reaction is energetically favored as the new bonds formed are stronger than the bonds broken.

The mechanism of hydrogenolysis follows a chain reaction.

Initially, tin hydride produces a tributyltin radical in the presence of light, which then abstracts halogen from an organic halide producing a radical intermediate.

Next, the radical intermediate abstracts hydrogen from tin hydride to finally form an alkane and a tributyltin radical, which propagates the reaction. 

The reactivity of alkyl halides towards hydrogenolysis decreases from iodide to fluoride, where the alkyl fluorides are unreactive.

In particular, the reduction of alkyl chlorides requires a higher concentration of tributyltin radicals, which is achieved by adding an initiator such as AIBN to the reaction.