Spectroscopic and theoretical investigations of electrophilic bromination reactions of alkynes : The first evidence for complexes as reaction intermediates.

The first direct detection of a 1:1 Br2–alkyne complex as an intermediate during the addition of Br2 to alkynes is reported. The measured negative apparent activation energies for bromination of arylalkylacetylenes are only compatible with the formation of a reactive complex that precedes the rate-determining step. Quantum-chemical calculations suggest that the Br2 in the 1:1 complex functions as a donor for a second Br2 molecule as depicted here, giving a trimolecular complex structurally very similar to a stable Br2–Br2–alkene adduct reported some time ago