Addition of HCl to alkynes twice to give geminal dichlorides

Re: addition of HCl to alkynes.

This is taught different ways at different schools.

Some profs teach this reaction as going through a vinyl carbocation, in which case addition can be either syn or anti.

Others teach a more complex, “termolecular” (tri-molecular) transition state which does not involve a pure vinyl carbocation. Indeed they cite evidence that under certain conditions addition is largely syn.

When preparing this I went off March’s Advanced Organic Chem 5th edition which cites vinyl carbocations as intermediates in work of Stang and others from the 1980s. Later work seems to be supporting the termolecular mechanism.

This is a long and complicated way of saying that it’s not 100% clear from the literature what the mechanism is – science is still developing – and it is taught in different ways.

I realize this doesn’t really answer your question, but I hope it explains why the diagram isn’t very clear.

Thanks for leaving the comment – James

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