A computational study of the thermolysis of β-hydroxy ketones in gas phase and in m-xylene solution

Theoretical calculations at the M05-2X/6-31+G(d) level of theory have been carried out in order to explore the nature of the mechanism of the thermal decomposition reactions of the beta-hydroxy ketones, 4-hydroxy-2-butanone, 4-hydroxy-2-pentanone, and 4-hydroxy-2-methyl-2-pentanone in gas phase and in m-xylene solution. The mechanism proposed is a one-step process proceeding through a six-membered cyclic transition state. A reasonable agreement between experimental and calculated activation parameters and rate constants has been obtained, the tertiary?:?secondary?:?primary alcohol rate constant ratio being calculated, at T?=?503.15?K, as 5.9:4.7:1.0 in m-xylene solution and 44.1:5.0:1.0 in the gas phase, compared with the experimental values, 3.7:1.3:1.0 and 13.5:3.2:1.0, respectively. The progress of the thermal decomposition reactions of beta-hydroxy ketones has been followed by means of the Wiberg bond indices. The lengthening of the O1C2 bond with the initial migration of the H6 atom from O5 to O1 can be seen as the driving force for the studied reactions. Calculated synchronicity values indicate that the mechanisms correspond to concerted and highly synchronous processes. The transition states are advanced, nearer to the products than to the reactants. Copyright (c) 2012 John Wiley & Sons, Ltd.