Tributylin 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. You job is to develop a mechanism, using the following reaction as an example.

The following bond-dissociation enthalpies may be helpful:

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

(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 Brradicals. Then decide which atom can be extracted most favorably from the starting materials by the Brradical. That should complete the initiation. Finally, decide what energetically favored propagation steps will accomplish the reaction).

An atom or group of atoms containing odd or unpaired electron is known as the free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

In a free-radical chain reaction, free radicals are generally created in the initiation steps. A free radical and a reactant is combined to yield a product and another free radical in the propagation steps. Lastly, the number of free radicals generally decrease in the termination steps.

It may be defined as the amount of enthalpy required to break a bond homolytically in such a way that each bonded atom retains one of the bond’s two electrons.

Mathematically, ΔH⁰ = Σ(BDE of bonds broken) -Σ(BDE of bonds formed)

(a) The mechanism consists of three parts which are initiation step, propagation step I and propagation step II.

Initiation step

Propagation step I

Propagation step II

(b)

breaking C-Br bond: +285kJ/mol

formation Sn-Br bond: -552kJ/mol

total: -267 kJ/mol

breaking Sn-H bond: +310kJ/mol

formation C-H bond: -397kJ/mol

total: - 87kJ/mol

Overall enthalpy can be calculated by adding the enthalpies calculated for the two steps.

Overall ΔH = -267kJ/mol + (-87 kJ/mol)

Overall ΔH = -354kJ/mol

Overall enthalpy is found to be as -354kJ/mol.