Electrophilic Additions to Alkynes:
A current claim (Weiss, H. J. Chem. Ed. 1993, 70, 873 - 874) which vinyl cations are not the predominant intermediates in the electrophilic addition to alkynes in disputed on the following grounds:
(1) These are a linear free energy correlation among the rates of acid-catalyzed hydration of alkenes and alkynes, and because carbocations are accepted like intermediates in the former reaction, they are implicated in the latter also;
(2) Rearrangements are known to be energetically much less favorable in vinyl cations as compared to alkyl cations, and thus the lesser observed tendency for rearrangement in the former case does not argue for the nonexistence of vinyl cation intermediates;
(3) There is proof that alkenes and alkynes react along with hbr and hcl in a number of cases with anti addition and a kinetic term in [hx]2, but this is not an argument for a variation in behavior among the two, or for a pi-complex mechanism;
(4) Thermochemical calculations depict that vinyl cations are not prohibitively destabilized as compared to analogous alkyl cations;
(5) The study of an hcl or acetylene pi-complex in the gas phase is not an argument that this presents a rate-limiting transition state in solution.