Stereochemistry of additions to a triple bond

Summary

• 1. Steric orientation in the bromination of monosubstituted acetylenes depends on the bromination conditions and the structure of the acetylenic compound. In acetic acid, ionoid reaction predominates and leads to trans-dibromo olefins, though small amounts of cis-isomers are formed at the same time. In the dark in presence of large amounts of lithium bromide trans-addition of bromide can be carried out selectively. In photochemical bromination in nonpolar solvents considerable amounts of cis-dibromo olefins are formed.

• 2. cis-Dibromides arise not only as a result of the isomerization of trans-isomers under the conditions of the reaction, but also directly by cis addition of bromine atoms. In the series of monosubstituted acetylenes the tendency for cis addition increases with increase in the size of the substitutent.

• 3. When irradiated or allowed to stand for a long time in the dark, monosubstituted trans-dibromoethylenes are partially converted into their cis-isomers.