Hydrohalogenation and Hydration of Alkynes

Hydrogen chloride adds to aryl alkynes in acetic acid to give mixtures of a-chlorostyrenes and the corresponding vinyl acetate. A vinyl cation, which is stabilized by the aryl substituent, is believed to be an intermediate. The ion pair formed by 

Alkene/alkyne pair Bromination, Chlorination, Acid-catalyzed 

Alkyl-substituted acetylenes can react with HCI by either the Ad S or the Ad 2 mechanism. The Ad S mechanism leads to anti addition. The preference for one or the other mechanism depends on the reactant structure and the reaction conditions. Added halide ion promotes the Ad S mechanism and increases the overall rate of reaction. Reaction of 4-octyne with TFA in CH2CI2 containing O.l-l.OMBr leads mainly to Z-4-bromo-4-octene by an anti addition. The presence of Br greatly accelerates the reaction as compared to reaction with TFA alone, indicating the involvement of the Br in the rate-determining protonation step.  

1-Octyne and 2-octyne also give more than 95% anti addition under these conditions. The reactions are formulated as concerted Ad S processes, involving bromide attack on an alkene-acid complex. 

Under these conditions, 1-arylalkynes react by a mixture of Ad 2 and Ad S mechanisms with the proportion of Ad 3 reaction increasing with increasing bromide concentration and decreasing effective acidity. The Ad S reaction of 1-phenylpropyne gives the anti-Markovnikov product. This is believed to be due to a steric effect of the phenyl group. 

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