Halogens addition to alkynes

Predicting the outcome of electrophilic additions to alkynes from an extension of alkene reactivity usually works well, and can be applied to halogenations and hydrations. Hydration of an alkyne has a subtle twist, however the product is a ketone This can still be rationalized quite readily, though. 

The present situation in regard to addition of halogen azide to alkynes is that the synthetic aspects are well established but that mechanistic evidence is sparse and equivocal. Clearly, some rate studies of diese reactions would be of value in elucidating the mechanisms actually involved. 

A. General Reaction Equation for Addition to Alkynes Alkynes add hydrogen, hydrogen halides, and halogens (chlorine... 

Addition of halogens to unsaturated carbon-carbon bonds is a classical example of electrophilic addition. Addition to alkynes generally leads to 1,2-dihaloalkenes. Various mechanisms of addition have been discussed in the earlier review by Schmid. No major changes in the mechanistic picture have been published since. The actual addition might be preceded by complex formation. Ault applied the matrix isolation technique to study complex formation between GIF and acetylene, propyne and 2-butyne and also between CI2 and acetylene and propyne. The data seem to support a T-shaped structure. 

Halides (CI2 or 612) add to alkynes in an analogous fashion as for aUcenes. When one mole of halogen is added, a dihaloalkene is produced, and a mixture of syn and anti addition is observed. 

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