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 halide ion, resulting in the formation of a cyclic halonium ion intermediate.

Next, the halide ion attacks either carbon of the halonium intermediate from the backside of the ring, causing the ring to open and form the trans-dihaloalkene.

Further addition of another equivalent of the halogen follows a similar mechanism to yield a tetrahaloalkane.

For example, the addition of one mole of bromine to 2-butyne in the presence of acetic acid and lithium bromide selectively forms E-2,3-dibromo-2-butene. Addition of a second mole of bromine yields 2,2,3,3-tetrabromobutane.

Lastly, alkynes are less reactive to electrophilic additions than alkenes. This is because the π electrons are held more tightly in C≡C bonds than in C=C bonds.

Additionally, the halonium ion formed from alkynes is highly strained and more unstable than the corresponding alkene intermediate.