Addition of HCl, HBr, or HI (once)

Addition of HCl, HBr, or HI (once) Definition:

 

The addition of hydrogen chloride, hydrogen bromide, or hydrogen iodide to alkynes follows Markovnikov rule which means that the electrophile adds to the terminal (less substituted) carbon atom.

 

 

Addition of HCl, HBr, or HI Explained:

 

When it comes to the addition of hydrogen halide, a similar mechanism of addition to alkenes can be proposed for the addition of HX to a triple bond: (1) protonation to form a carbocation followed by (2) nucleophilic attack:

 

 

Alkynes react similarly to alkenes. Alkynes are somewhat less reactive than alkenes toward many electrophiles. A major reason for this difference in reactivity is the substantially higher energy of the vinylic carbocation (vinyl = a carbon atom bearing a double bond) intermediate that is formed by an electrophilic attack on an alkyne and from which we can successfully explain the observed regioselectivity. The differences in the rate of addition between alkenes and alkynes depend upon the specific electrophile and the reaction condition.