HX Addition to Alkynes
Addition of Halogens to Alkynes
In contrast to alkenes, alkynes can add two halogen molecules as they have two π bonds. If only one molar equivalent of the halogen is applied, only one halogen molecule is added to each alkyne molecule, because the dihalogen alkene is less reactive in the addition reaction than the alkyne. Only the tetrahalogen alkane is formed if two or more molar equivalents of the halogen are applied.
Why do the dihalogen alkenes show less reactivity in addition reactions than alkynes?
Alkenes are more reactive in electrophilic halogenations than alkynes. However, alkynes can be selectively dihalogenated, because the product of alkyne dihalogenation is not a simple alkene but a dihalogen alkene. The π electron cloud of a dihalogen alkene is more electron-deficient and therefore less nucleophilic because of the -I effect of the halogen substituents. As a result, a dihalogen alkene is less reactive in the addition reaction than an alkyne.
The following table illustrates these facts by using the electrostatic potentials of the molecular surfaces of 2-butyne, 2,3-dibromo-2-butene, and 2-butene. The triple bond of 2-butyne is considerably richer in electrons than the double bond of 2,3-dibromo-2-butene. Therefore, the triple bond of 2-butyne more easily attacks the electrophilic bromine. In comparison, the π bond of the non-brominated 2-butene shows a considerably higher electron density than the π bond of 2,3-dibromo-2-butene.