Constant Selectivity in Proton Exchange Reactions Between Aqueous Sulphuric Acid and Acetylenes

Styrene hydration and stilbene isomerization in strong acid media. An excess acidity analysis

Rate constants obtained for the hydration of some ring-substituted styrenes, YC6H4CR=CH2 (R = H, CH3, CF3) (1-3), and for the isomerization of some cis-stilbenes substituted in both rings, YC6H4CH=CHC6H4Z (4), in aqueous H2SO4, D2SO4, and HClO4 media at 25°C and, in some cases, at other temperatures, have been subjected to an excess acidity analysis. Data obtained by different groups and in different media for the same substrates agree very well. The m‡ values obtained show that all the substrates react via the A-SE2 mechanism, rate-determining proton transfer to carbon; the transition state is late, with the proton transfer being about three-quarters complete. The reactivities of 1-3 in the aqueous standard state are obtained by extrapolation; they are 1:103:10-7, respectively. The stilbenes are relatively unreactive; log k0 values for compounds with substituents in the ring adjacent to the developing positive charge show a good correlation with σ+ with a large negative ρ+ value, but those for compounds with substituents in the other ring correlate with σ with a small negative ρ. Extrapolated log k0 values for 1 and 3 show a good correlation with σ+, but those for 2 correlate with neither σ+ nor σ, because the α-CH3 group twists the molecule and reduces the resonance interaction of ring substituents with the developing positive charge. On the other hand, this effect is not seen in 3, when the strongly deactivating α-CF3 group is present, meaning that the transition state is forced to be planar in this case. The activation parameters, solvent isotope effects, and excess acidity slope parameters m‡m* obtained in the analysis are tabulated and discussed. The parameters obtained for the parent styrenes 1 are very similar to those previously obtained for the phenylacetylenes YC6H4Ctriple bondCH, meaning that reactions with vinyl cation intermediates and reactions with benzyl cation intermediates can be quite similar. Key words: styrenes, stilbenes, hydration, excess acidity, LFER, mechanism.