Geminal vinyl dibromide

Another common method for the conversion of an aldehyde into an alkyne is the transformation of the aldehyde via a Corey-Fuchs reaction into a geminal vinyl dibromide followed by reaction with nbutyllithium and aqueous work-up.22... 

Pd - and PT-catalyzed carbonylation of geminal vinylic dibromides gives diynes or dicarboxylic acids using benzene or t-amyl alcohol as a solvent, respectively (eq. (10)) [135]. 

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). 

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