Alkyne Reactions

For each of the reactions on the left, predict the major organic product. Pay particular attention to the regiochemistry of the reaction and the possibility of carbocation rearrangement.
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The reaction of an alkyne with two moles of HCl results in the formation of a 1,1-dichloro group. The reaction involves the intermediate generation of a vinyl halide, which adds the second mole to give the final product. The regiochemistry of the reaction is Markovnikov (the chlorines are bonded to the alkyne carbon which would form the most stable carbocation).

The reaction of an alkyne with BH₃ in THF proceeds through the formation of an organo-borane intermediate with the boron bonded adjacent to the alkyne carbon which would form the most stable carbocation (anti-Markovnikov addition). Work-up with alkaline peroxide results in the formation of an anti-Markovnikov enol> which rapidly reverts to the corresponding carbonyl compound.

Alkynes can be partially reduced to trans-alkenes using a "dissolving metal reduction", in which the alkene is formed by a radical mechanism in the presence of Li or Na metal, dissolving in liquid ammonia. Please remember that this differs from the base sodium amide, which is formed from sodium metal previously dissolved in liquid ammonia.

The addition of halogen to alkynes is a stepwise process involving a "halonium" ion intermediate. The formation of this intermediate is initiated through attack of halogen on the alkyne p-system, to form the cyclic halonium ion (i.e., bromonium or chloronium ion) and expel the halogen anion (i.e., bromide or chloride). This intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system; that is, the halide anion expelled in the previous step. Attack by halide generates a vinyl halide, which is an alkene and can undergo a second addition of halogen. The final product of the reaction is therefore a 1,1,2,2-tetrahalide.

The reaction of an alkyne with BH₃ in THF proceeds through the formation of an organo-borane intermediate with the boron bonded adjacent to the alkyne carbon which would form the most stable carbocation (anti-Markovnikov addition). Work-up with alkaline peroxide results in the formation of an anti-Markovnikov enol which rapidly reverts to the corresponding carbonyl compound.