The hydration of 1-aryl-3,3,3-trifluoropropynes and some other phenylacetylenes in sulfuric acid. An excess acidity analysis
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 (1), H (2), COC6H4-X (3), and CO2H (4). All substrates gave acetophenone-type products consistent with the normal hydration mechanism involving rate-determining vinyl cation formation. Standard-state log k0 intercepts, and m≠m* slopes, were both used in linear free energy relationship plots against the substituent σ+ 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 ρ+ values of about −3.8, but 1 is more substituent sensitive, with a ρ+ of −5.3. A σ+ value of 0.38 is calculated for the CF3C≡C substituent. The ρ+ 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...