Addition of HCl to alkynes twice to give geminal dichlorides
Description: Addition of two equivalents of hydrogen chloride (HCl) to alkynes results in geminal dichlorides (1,1-dichlorides)
Notes: The reaction proceeds with Markovnikoff selectivity. “Geminal” means that the two chlorides are attached to the same carbon.
Examples:
Notes: Note how in examples 1-3 the chlorides add to the more substituted carbon (Markovnikoff selectivity). In example 4 both carbons are equally substituted, but the reaction still produces one product since the alkyne is symmetrical. In example 5 two different products are obtained.
Mechanism: Attack of the alkyne π bond on HCl leads to the vinyl carbocation at the more substituted position (Markovnikoff selectivity) (Step 1, arrows A and B) followed by attack of the chloride on the carbocation (Step 2, arrows C) to give the vinyl chloride. When a second equivalent of HCl is present, the process then repeats itself. Protonation of the alkene (Step 3, arrows D and E) gives a carbocation, which is then attacked by chloride (Step 4, arrow F).
Is this anti addition?
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
That makes sense. Thanks for the detailed reply, James!
Dan
Is this the same reaction as adding 2 HBr and under the same arrow no peroxides is written? The same geminal product is shown in my class notes but no explanation of why no peroxides is under the reagent 2 HBr.
Hi Samantha –
Yes, no peroxides are used in this reaction. It’s just like the “no peroxides” version of the addition of HBr to alkenes, except here there are 2 pi bonds, so the addition happens twice.
In the “peroxide” version with alkenes, the Br and H would switch places since the addition is “anti-Markovnikov”
Hope this helps
James