SolutionsHalogenated derivatives

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Exercise 1

Write the initiation and propagation steps of the reduction of bromocyclohexane with Bu₃SnH and AIBN. Why is a small amount of AIBN enough?

Initiation: on heating, AIBN decomposes into N₂ and two 2-cyanoprop-2-yl radicals, which abstract H from Bu₃SnH and form Bu₃Sn•. Propagation: (1) Bu₃Sn• + C₆H₁₁Br → Bu₃SnBr + C₆H₁₁•; (2) C₆H₁₁• + Bu₃SnH → C₆H₁₂ + Bu₃Sn•. Each initiating radical starts a chain that converts many molecules before two radicals meet and terminate it.

Exercise 2

Which compound is obtained by treating 2-bromoacetophenone (PhCOCH₂Br) with DMSO and a mild base? Why can neopentyl bromide not be oxidized in this way?

Phenylglyoxal, PhCOCHO (Kornblum oxidation). The first step is an S_N2 reaction of the DMSO oxygen, very fast at an α-halocarbonyl. Neopentyl bromide is primary but very hindered and hardly undergoes S_N2 reactions, so the alkoxysulfonium salt is never formed.