Vinylic carbocation, electronic from alkynes

If we consider the protonation of 2-butyne by HBr, then we obtain an unstable vinyl cation, 11.15. This is linear because the carbon atom is sp-hybridized, with an empty p-orbital. Thus, it can be attacked by bromide ion from either side, resulting in the formation of both E- and Z-alkene products (Figure 11.32). The instability of the vinyl cation intermediate means that alkynes react more slowly with electrophiles than do alkenes. The addition of a second mole of HBr to the alkene generally gives the geminal dibromide the intermediate carbocation is stabilized by interaction with the lone pair of electrons from bromine (Figure 11.33). 

A reasonable mechanism for the bromination reaction is presented below. One of the alkyne groups reacts with bromine, resulting in the formation of a new C-Br bond, and a vinyl carbocation (see below for details on this). The n electrons from the other alkyne attack this carbocation, resulting in the formation of a new C-C bond, and a new vinyl carbocation. Nucleophilic attack of the bromide gives the product. 

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