Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

Hydrogenolysis follows a radical chain mechanism initiated by light. Tributyltin hydride undergoes homolytic cleavage to form a tributyltin radical, which abstracts halogen from an alkyl halide, creating an alkyl radical intermediate. The intermediate then abstracts hydrogen from tributyltin hydride, forming an alkane and regenerating the tributyltin radical that propagates the chain reaction.

Tributyltin hydride is effective because it has a weak Sn–H bond that readily undergoes homolytic cleavage. The reaction is energetically favorable since the new bonds formed—the C–H bond in the alkane and the Sn–halide bond—are stronger than the bonds broken. This thermodynamic advantage drives the overall substitution reaction.

Reactivity decreases from iodide to fluoride, with alkyl fluorides being unreactive due to strong C–F bonds. Alkyl bromides and iodides require only daylight for initiation since C–Br and C–I bonds are weaker. Alkyl chlorides require higher concentrations of tributyltin radicals, achieved by adding an initiator like AIBN to the reaction mixture.

AIBN is a thermal initiator that undergoes homolysis above 60 °C to generate nitrile-stabilized radicals. These radicals abstract hydrogen from tributyltin hydride, forming tributyltin radicals that initiate the hydrogenolysis chain reaction. AIBN is preferred over peroxides because peroxide radicals are too reactive and cause unwanted side reactions with organic halides.

Peroxides are unsuitable because the radicals they generate are highly reactive and can abstract hydrogen directly from organic halides, leading to unwanted side reactions. This competing pathway reduces the efficiency of the desired hydrogenolysis reaction and produces undesired products instead of the target alkane.

Both reactions follow radical chain mechanisms, but they serve opposite purposes. Radical substitution halogenation adds halogens to alkanes and alkenes, while hydrogenolysis removes halogens from alkyl halides using tributyltin hydride. Both involve similar propagation steps with radical intermediates abstracting atoms from reagents to form products.

Tributyltin halide is the byproduct formed when tributyltin hydride reacts with alkyl halides during hydrogenolysis. The tin hydride transfers its hydrogen to the alkyl radical intermediate while the tributyltin group bonds with the halogen that was abstracted from the original alkyl halide.