Substituent effects in the photohydration of styrenes and phenylacetylenes. An attempt to establish a σhν scale for excited‐state reactions

The acid‐catalysed rate constants (kH+) 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 values obtained did not correlate well with any previously reported σ scales, and were used to try to establish a σhν 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 σhv scale gave reasonable correlations (r = 0.957–0.966). The problems associated with establishing linear free energy relationships for photoreactions are discussed.