from abstract ideas...fundamental knowledge at Esso Research
Electronic excitation of weak organic acids by ultra-violet light gives them chemical properties that are unpre dictable by ordinary ground-state rules, according to the findings ofDrs. William Bartok and P. J . Lucchesi, two Esso Research scientists. T h e enhanced rate of protolytic dis sociation of weak organic acids excited by absorption of U V light in aqueous solutions has been established. Values of the dissociation constants of a-naphthol or /3-naphthol, for example, are greater for the first excited singlet than for the ground state by a factor of 106-107^ To learn more about the chemistry of excited states, the effects of U V light on the dissociation of phenols and their parasubstituents (para-cresol
and para-chlorophenol) and the corre sponding anions have been measured in neutral water and in 0.1 M aqueous sodium hydroxide solution, as pic tured above. Values of the dissoci ation constants for the excited acids were calculated from the spectral data:
pKa Excited Ground Singlet State State 5.7 Phenol 10.0 p-Cresol —0.6 10.3 9.4 p-Chlorophenol 3.1
Apart from the large increases in acid ity with excitation, no correlation was found for the dissociation constants of the excited phenols. T h e inductive effects of the para-substituents are
small compared with the influence of electronic excitation. Thus, the large fluctuations of the excited state values are unexpected. These fluctuations in dicate that chemical properties of ex cited states cannot be predicted on the basis of substituent effects as measured in ground-state chemistry. These studies add three new organic molecules to the number whose acidity in the excited state are known. This knowledge opens the way to reactions that are presently unknown and pos sibly to a whole new field of organic chemistry. ...adapted from the scientists' notes at
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