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 d(Br,)/dt = [acetylene](k, [Br,] + k3 [Br212 + k,,[Br,] [Br1) In the absence of bromide ion and at low bromine concentrations (< 2 x M), only the k 2 process is observable. These k 2 values for a series of ring-substituted phenylacetylenes are correlated well with o+ values and give a p 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 (I-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 k,,values have also been obtained for the ring-substituted phenylacetylenes as well as 3-hexyne. For all the substrates studied, this k,,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 a+ plot for these values has been interpreted in terms of change in transition state structure with substituent.