Photochemical Processes of Chlorotoluene in Methanol: Methylanisole Formation Yields and Photosubstitution Reaction Studied by Laser Flash Photolysis

Article abstract of DOI:10.1021/j100325a012

The photosubstitution reaction of chlorotoluene in methanol was studied.The formation yield of methylanisole was measured in the presence of biacetyl, and it was found that two processes simultaneously take place in the photosubstitution of p-, m-, and o-chlorotoluene.One is the triplet route that was quenched by the addition of biacetyl, and the other is a short-lived excited state.The formation yield for each process was determined to be on the order of 10^-2-10^-3, while the dominant process in a photolysis was found to be the photodecomposition, whose quantum yield ranged from 0.1 to 0.5.The smallest value of the photodecompositio quantum yield was for p-chlorotoluene, which has a higher molecular symmetry than the meta or ortho isomer.The transient absorption spectra and decay profiles of the triplet chlorotoluene in methanol were measured with photostationary and flow conditions.The formation of the photoproduct dimethylbiphenyl was confirmed in the spectrum.The effect of biacetyl addition was also examined, and the results indicated the significance of the triplet route for the photosubstitution reaction.The quenching rate of the triplet p-chlorotoluene by biacetyl was determined to be 8.3*10⁸ M^-1 s^-1.The sensitized triplet-state formation of naphthalene by the triplet chlorotoluenes was studied in cyclohexane solution.The T-T absorption coefficients of chlorotoluenes were determined, and the triplet yields were also determined to be 0.5, 0.4 and 0.2 for p-, m-, an o-chlorotoluene, respectively.From these values, the formation rate of methylanisole in the triplet state was estimated to be on the order of 10⁴ s^-1, which is 2 orders of magnitude smaller than the decay rate constant in methanol (1*10⁶ s^-1).