X2 Addition

Much like alkenes, alkynes are susceptible to electrophilic addition. As we saw with the addition of H-X to alkynes, the reaction proceeds through a carbocation intermediate, and the principles of Markovnikov's rule often apply, particularly with terminal alkynes. The addition of halogens to alkynes follows a similar mechanistic pathway, although it can lead to multiple addition products due to the presence of two pi bonds.

The overall transformation involves the conversion of a carbon-carbon triple bond (C≡C) to a carbon-carbon double bond substituted with two halogens (X-C=C-X), and potentially, with excess halogen, further addition to a tetrahalide (X₂C-CX₂).

Reagent : Typically, the halogen (e.g., Br₂, Cl₂, I₂) is used as the electrophile. These reactions are generally carried out in an inert solvent, such as methylene chloride (CH₂Cl₂) or carbon tetrachloride (CCl₄), to avoid side reactions.
In the presence of 1 equivalent of the halogen, a 1,2-dihaloalkene X-C=C-X is formed, but in the presence of an excess, the C=C reacts to form a 1,1,2,2-tetrahalide X₂C-CX₂.
Regioselectivity : not relevant since all the new bonds are the same, C-X.

A few mechanism have been considered and come into play depending upon the structure of the alkyne.

Aryl Alkynes

Aryl alkynes, where the triple bond is attached to an aromatic ring, often proceed through a vinyl cation intermediate. The adjacent aryl group stabilizes this vinyl carbocation through resonance, making its formation more favorable. Because the bromide (or other halide) anion can attack the vinyl cation from either side, a mixture of syn and anti addition products is typically observed. This loss of stereospecificity supports the formation of a free carbocation.

The bromide anion can attack either side of the vinyl cation resulting in both syn and anti-products.

Alkyl Alkynes

The reactivity of alkyl alkynes, where the triple bond is attached to alkyl groups, depends on whether they are internal or terminal.

Internal Alkynes: Internal alkynes tend to undergo anti addition, which is consistent with a termolecular mechanism or a bridged halonium ion intermediate. In the termolecular process, two Br₂ molecules react simultaneously with the alkyne. One Br₂ molecule acts as the electrophile, while the other assists in the departure of the bromide ion. This concerted mechanism leads to anti addition. The bridged halonium ion is similar to what is seen in alkene addition and also leads to anti addition.
Terminal Alkynes: Terminal alkynes, on the other hand, more likely proceed through a vinyl cation intermediate. While the initial addition may show some preference for anti addition, the second addition in the presence of excess halogen often leads to a mixture of stereoisomers, suggesting that a carbocation intermediate is involved at some point. The terminal alkyne is more acidic than an internal alkyne and is therefore more likely to react through a carbocation intermediate.

Termolecular Process

Bridged Halonium ion