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  Home > JCE Print > Journal of Chemical Education > Issues > 1996  > November  >
Information • Textbooks • Media • Resources
The Electrophilic Addition to Alkynes Revisited
Thomas T. Tidwell
Department of Chemistry, University of Toronto, Toronto, ON M5S 1A1, Canada
Cover
November 1996
Vol. 73 No. 11
p. 1081
Abstract
A recent claim (Weiss, H. J. Chem. Ed. 1993, 70, 873 - 874) that vinyl cations are not the predominant intermediates in the electrophilic addition to alkynes in disputed on the following grounds: (1) these is a linear free energy correlation between the rates of acid-catalyzed hydration of alkenes and alkynes, and since carbocations are accepted as intermediates in the former reaction, they are implicated in the latter as well; (2) rearrangements are known to be energetically less favorable in vinyl cations compared to alkyl cations, and so the lesser observed tendency for rearrangement in the former case does not argue for the absence of vinyl cation intermediates; (3) there is evidence that alkenes and alkynes react with HBr and HCl in some cases with anti addition and a kinetic term in [HX]2, but this is not an argument for a difference in behavior between the two, or for a pi-complex mechanism; (4) thermochemical calculations show that vinyl cations are not prohbitively destabilized compared to analogous alkyl cations; (5) the observation of an HCl/acetylene pi-complex in the gas phase is not an argument that this represents a rate-limiting transition state in solution.
More Information
*  Citation
 Tidwell, Thomas T. J. Chem. Educ. 1996 73 1081.
*  Keywords
 Organic Chemistry
*  History
 Created:
Last Updated:		 August 5, 1999
February 21, 2006
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