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The Electrophilic Addition to Alkynes Revisited
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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.
Keywords (Audience):
Second-Year UndergraduateKeywords (Domain):
Organic ChemistryKeywords (Subject):
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This article has been cited by 1 ACS Journal articles (1 most recent appear below).
Further Comments upon the Electrophilic Addition to Alkynes: A Response to Criticism from Professor Thomas T.Tidwell
Hilton M. WeissJournal of Chemical Education1996 73 (11), 1082Further Comments upon the Electrophilic Addition to Alkynes: A Response to Criticism from Professor Thomas T.Tidwell
Hilton M. WeissJournal of Chemical Education1996 73 (11), 1082This paper responds to the preceeding article by Professor T. Tidwell in which he provides arguments for vinyl cations being an intermediate in most electrophilic additions to alkynes. This paper underlines the different experimental results obtained from ...
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- Received: August 03, 2009
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