Tributyltin hydride ( Bu3SnH ) is used synthetically to reduce alkyl halides, replacing...

Tributyltin hydride ( Bu₃SnH ) is used synthetically to reduce alkyl halides, replacing a halogen atom with hydrogen. Free radical initiators promote this reaction, and free-radical inhibitors are known to slow it or stop it. Your job is to develop a mechanism, using the following reaction as the example.

The following bond-dissociation enthalpies may be helpful:

Br- Br 192 kJ/mol

H- Br 368 kJ/mol

Bu₃Sn-H 310 kJ/mol

Bu₃Sn-Br 552 kJ/mol

(a) Propose initiation and propagation steps to account for this reaction.

(b) Calculate values of for your proposed steps to show that they are energetically feasible. (Hint: A trace of Br₂ and light suggests it’s there only as an initiator, to create Br. radicals. Then decide which atom can be abstracted most favorably from the starting materials by the Br. radical. That should complete the initiation. Now decide what energetically favored propagation steps will accomplish the reaction.)