Kinetics and mechanism of electrophilic bromination of acetylenes

The rates of addition of molecular bromine in acetic acid to a number of acetylenes have been found to follow the general equation[Formula: see text]In the absence of bromide ion and at low bromine concentrations (< 2 × 10−4 M), only the k2 process is observable. These k2 values for a series of ring-substituted phenylacetylenes are correlated well with σ+ values and give a ρ value of −5.17 which is interpreted in terms of a transition state leading to a vinyl cation intermediate. As expected from this intermediate, both cis and trans dibromide products are formed and the bromoacetates are only of the Markownikoff type (1-acetoxy-1-phenylethylene derivatives). An ion pair scheme has been presented to account for the variation in product composition with substrate structure.In contrast to these results for phenylacetylenes, a cyclic bromonium ion intermediate is postulated for alkyl acetylenes on the basis of only trans dibromide formation for 3-hexyne and 1-hexyne.The kBr− values have also been obtained for the ring-substituted phenylacetylenes as well as 3-hexyne. For all the substrates studied, this kBr− process represents a bromide ion catalyzed attack of molecular bromine in an AdE3 mechanism. Thus only trans dibromide products are formed from this rate process. The non-linear σ+ plot for these values has been interpreted in terms of change in transition state structure with substituent.