Homogeneous, unimolecular gas-phase pyrolysis kinetics of 4- and 2-hydroxyacetophenone

- Pyrolysis of 4- and 2-hydroxyacetophenones are homogeneous, unimolecular, and first-order.
- Main products formation from pyrolysis of hydroxyacetophenones are phenol and ketene.
- A semi-polar concerted four-membered cyclic transition state type of mechanism for the formation of phenol and ketene.
- The molecular elimination of a very difficult leaving group as the acetyl group is reported.

The pyrolyses kinetics of 4- and 2-hydroxyacetophenones in the gas phase were determined in a static system, where the reaction vessel was deactivated with allyl bromide, and in the presence of the free radical inhibitor propene when necessary. The working temperature range was 464-485 °C, and the pressure range was 44-133 Torr. The reactions were found to be homogeneous and unimolecular and to obey a first-order rate law. The products for 4-hydroxyacetophenone are phenol and ketene, while for 2-hydroxyacetophenone are phenol and ketene, with smaller amounts of phenyl acetate and benzofuran. The Arrhenius expression of the pyrolyses was found for 4-hydroxyacetophenone: log k1 (s−1) = (13.83 ± 0.08) − (258.1 ± 1.1) kJ mol−1(2.303RT)−1(r = 0.9999); for 2-hydroxyacetophenone: log k1 (s−1) = (13.81 ± 0.43) − (251.9 ± 6.2) kJ mol−1(2.303RT)−1(r = 0.9991). Deacetylation may be considered as the primary reasonable mechanism, and the process appears to proceed through a semi-polar concerted four-membered cyclic transition state.

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