A kinetic study of the reactions of 2-propyl radicals in the liquid phase in the presence and absence of oxygen

Article abstract of DOI:10.1002/(SICI)1097-4601(1996)28:3<201::AID-KIN6>3.0.CO;2-Z

2-Propyl radicals have been generated from the photolysis of solutions of 2,2-azopropane and 2,4-dimethyl-3-pentanone in decane in a glass and a metal cell. The time course of their reactions in the presence and absence of oxygen has been monitored between 323 and 373 K. The primary process involves the formation of solvent-caged radical pairs, two 2-propyl radicals and a 2-propyl and a 2-methylpropanoyl radical from the azo and ketone precursors, respectively. Subsequently these radicals are partitioned between cage escape and dimerization and disproportionation within the cage. In oxygenated solution the free 2-propyl radicals are effectively trapped as 2-propylperoxyl radicals. However, oxygen does not react with the solvent-caged radicals. This leads to a major difference in the hydrocarbon products from the two precursors. 2,2′-Azopropane gives propane, propene, and 2,3-dimethylbutane from the start of the reaction whereas the ketone only gives propene. Following the depletion of oxygen, or in the absence of oxygen, both precursors behave analogously and give all three hydrocarbons. The 2-propylperoxyl radicals undergo self-reaction and hydrogen abstraction from the solvent to give 2-propanol, propanone, and 2-propyl hydroperoxide and, under conditions of low oxygen concentration, by reaction with 2-propyl radicals they give 2,2′-dipropylperoxide. Although the two cells lead to different overall rates of reaction, the relative rates and product distributions are unaffected by the cell design. A unified mechanism is described and the known and best estimates of rate constants for the individual steps are used to simulate the time dependence of the product yields from the photolysis of both precursors.