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.
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).