Electrophilic Addition to Alkynes: Halogenation

Electrophilic addition reactions involve the conversion of multiple bonds, such as carbon-carbon double and triple bonds, into other functional groups.

In these reactions, the high electron density around the π bond allows them to function as nucleophiles and attack electrophilic centers. The net result is the addition of a simple molecule across a π bond.

If the simple molecule is a halogen, such as bromine or chlorine, the reaction is called a halogenation reaction. For every mole of the added halogen, one π bond is broken, and two new σ bonds are formed.

Recall that the halogenation of alkenes is a stereospecific reaction that proceeds via an anti addition forming vicinal dihalides.

Halogenation of alkynes follows a similar pattern. However, since alkynes have two π bonds, halogens can add twice across the multiple bonds.

The addition of one equivalent of the halogen forms the trans-dihalide as the major product; another equivalent gives the tetrahaloalkane.

Analogous to alkenes, one of the π bonds in alkynes acts as a nucleophile and attacks the electrophilic center on the polarized halogen molecule.

As this happens, the halogen atom with the partial negative charge leaves as a