Electrophilic Addition to Alkynes and Cumulenes

The electrophilic additions discussed above also occur with alk)Ties and cumulenes, but there can be significant differences in reactivity between alkenes and these other compounds. The reaction of bromine and 1-phenylpropyne (56) in acetic acid led to the formation of four primary products, 57, 58, 59, and 60 (equation 9.56).  

In the absence of Br and at low concentrations of Br2, the kinetic expression is first order in alkyne and first order in bromine. A p value of —5.17 for bromine addition to substituted derivatives of 56 is consistent with the intermediacy of a vinyl cation-bromide ion pair that can combine to form both cis and trans dibromide adducts or can react with solvent to form cis- and traMS l-acetow-2-bromo-l-phenylpropene, as shown in equations 9.58 through 9.60. -  

Schmid, G. H. in Patai, S., Ed. The Chemistry of the Carbon-Carbon Triple Bond Wiley-Interscience Chichester, England, 1978 pp. 275-341. 

Pincock, J. A. Yates, K. /. Am. Chem. Soc. 1968,90,5643. In addition to the products shown, 1,1-dibromoethyl phenyl ketone was formed as a result of subsequent reaction of products 59 and 60. 

For a discussion of vinyl cations and electrophilic additions to alkynes, see Stang, P. J. Rappoport, Z. Hanack, M. Subramanian, L. R. Vinyl Cations Academic Press New York, 1979. 

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