SUBSTITUENT EFFECTS IN THE PHOTOHYDRATION OF STYRENES AND PHENYLACETYLENES - AN ATTEMPT TO ESTABLISH A SIGMA-HV SCALE FOR EXCITED-STATE REACTIONS

The acid-catalysed rate constants (k(H) + ) for the photohydration of 18 meta- and para-substituted styrenes and phenylacetylenes were determined in H2SO4 solutions by means of fluorescence quenching measurements and the standard Stern-Volmer approach. Singlet lifetimes of the substrates were determined in the reference solution (water at pH 7) by single photon counting. The log k(H) + values obtained did not correlate well with any previously reported sigma scales, and were used to try to establish a sigma-hv scale (for H-, p-F, m-F, p-Me, m-Me, p-Et, m-Et, p-OMe and m-OMe substituents) for possible use in other photochemical reactions. Correlation of all available literature data versus the new sigma-hv scale gave reasonable correlations (r = 0.957-0.966). The problems associated with establishing linear free energy relationships for photoreactions are discussed.