On the other hand, anti -addition of hydrogen to the triple bond of alkynes occurs when they are treated with lithium or sodium metal in ammonia or ethylamine at low temperatures (Scheme 8). In this process, sodium in liquid ammonia produces solvated electrons that can reduce the alkyne to trans -alkene. The anti- stereochemistry results from the greater stability of the vinyl radical in the trans -configuration, where the alkyl groups are farther apart.

Scheme 8

4.9.2 Addition of Halogens

Bromine and chlorine can add to alkynes just as they add to alkenes. If 1 mole of halogen adds to 1 mole of an alkyne, the product is a dihaloalkene. The stereochemistry of addition may be either syn or anti , and the products are often mixtures of cis and trans isomers. If 2 moles of halogen add to 1 mole of an alkyne, a tetrahalide results (Scheme 9).

Scheme 9

4.9.3 Addition of Hydrogen Halide

When a hydrogen halide adds to a terminal alkyne, the product has the orientation predicted by Markovnikov's rule. When two moles of a hydrogen halide add to an alkyne, the second mole also adds with the same orientation as the first. The reaction of an internal alkyne, such as 2-pentyne, gives a mixture of products (Scheme 10).

Scheme 10

The mechanism is similar to the mechanism of hydrogen halide addition to alkenes. The vinyl cation, formed in the first step, is more stable with the positive charge on the more highly substituted carbon atom. Attack by halide ion gives Markovnikov product (Scheme 11).

Scheme 11