This paper is dedicated to Professor Ross Stewart on the occasion of his 65th birthday ROBIN A. Cox, EWART GRANT, TODD WHITAKER, and THOMAS T. TIDWELL. Can. J. Chem. 68, 1876 (1990). The excess acidity method has been used to analyse the hydration kinetics of the phenylacetylenes Y-C6H4-C-C-Z in aqueous sulfuric acid mixtures; Z = CF3 (I), H (2), COC6H4-X (3), and COzH (4). All substrates gave acetophenone-type products consistent with the normal hydration mechanism involving rate-determining vinyl cation formation. Standard-state log &intercepts, and m*m* slopes, were both used in linear free energy relationship plots against the substituent u + values. Solvent isotope effects and activation parameters were obtained in some cases. The deactivating Z substituents in 1 , 3, and 4 all cause reaction to be some 100 times slower than that of the parent phenylacetylene 2. Compounds 2,3, and 4 all have p+ values of about -3.8, but 1 is more substituent sensitive, with a p+ of -5.3. A u+ value of 0.38 is calculated for the CF3C-C substituent. The p+ values were found to be acidity independent for 1 and 2, and probably for 3, but not for 4. Proton transfer at the transition state was found to be most advanced for the fastest reaction, that of 2, contrary to intuition.Key words: alkyne hydration, excess acidity, phenylacetylenes, vinyl cations, deactivated carbocations. ce n'est toutefois pas le cas avec le compose 4. Contrairement ace que l'on pouvait croire d'une faqon intuitive, on a trouvC que le transfert de proton dans l'ttat de transition est le plus avanck avec la reaction la plus rapide.

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