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Substituent effects in the photohydration of styrenes and phenylacetylenes. An attempt to establish a σ h ν scale for excited‐state reactions
Author(s) -
Mcewen John,
Yates Keith
Publication year - 1991
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610040402
Subject(s) - chemistry , singlet state , quenching (fluorescence) , substituent , excited state , reaction rate constant , photochemistry , medicinal chemistry , computational chemistry , fluorescence , atomic physics , kinetics , physics , quantum mechanics
The acid‐catalysed rate constants ( k H + ) for the photohydration of 18 meta ‐ and para ‐substituted styrenes and phenylacetylenes were determined in H 2 SO 4 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 σ scales, and were used to try to establish a σ h ν 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 σ hv scale gave reasonable correlations ( r = 0.957–0.966). The problems associated with establishing linear free energy relationships for photoreactions are discussed.