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 or stop it.
Your job is to develop a mechanism, using the following reaction as the example.
<IMAGE> + BU3SnH trace Br2 hv —> <IMAGE> + Bu3SnBr
The following bond-dissociation enthalpies may be helpful:
<IMAGE> 413 kJ/mol
<IMAGE> 309 kJ/mol
Br—Br 190 kJ/mol
H—Br 366 kJ/mol
Bu3Sn—H 310 kJ/mol
Bu3Sn—Br 552 kJ/mol
b. Calculate values of
ΔH 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. Finally, decide what energetically favored propagation steps will accomplish the reaction.)