Kinetics and mechanism of the gas phase reactions of thermally and chemically activated alkyloxiranes

The gas phase thermal decompositions of 2, 3-epoxy-1.1, 1-trifluoropropane (A) (677-741 K. 6-130 torr), 2,3-direthyl-2,3-epoxyhutane (B) (642-733 K. 6-25 torr) and 2,3-dimethyl-2.3-epoxypentane (C) (638-727 K, 4-14 torr) have been investigated in aged reaction vessels Arrhenius parameters were determined for the homogeneous, unimolecular formation of the major products. Pyrolysis of A, in the presence of 8% nitric oxide to reduce the amount of fragmentation, gave 1.1, 1-trifluoroproparone and 3. 3. 3-trifluoropropanal as major products. Some pressure dependence was observed in the measured rate constants which could be explained in terms of chemical activation effects. When pyrolysed B gave propene, propanone, 3,3-dimethylbutan-2-oneand 2,3-dimethylbut-l-en-3-ol as major products with rate constants for their formation independent of pressure.The pyrolysis of C gave the following major products ; propene,propanone, but-l-ene, hut-2-ene, butanone, 2,2-dimethylpentan-3-one, 3,3-dimethylpentan-2-one. 2,3-dimethylpent-l-en-3-ol, 3-ethyl-2--methylbut-3-en-2-ol and 2,3-dirrethylpent-3-en-2-ol. The rate constants for the formation of these products were independent of pressure. The mechanism for formation of the major products is discussed with a biradical mechanism proposed for formation of isomeric ketones (and/ or aldehydes) and a concerted mechanism for formation of allylic alcohols. Some degree of ionic character in the transition states for these reactions is supported. Vinyl ethers are also thought to be a major primary product formed via a biradical mechanism, but at the temperature of the study these ethers rapidly decomposed to an alkene and a carbonyl compound. This was confirmed in the current work by a separate investigation of the thermal decomposition of 2-(1-methylethoxy)propene (554-610 K, 12-70 torr) to form propene and propanone. The reaction of B with methylene in the presence of oxygen wasinvestigated with ketene as the methylene precursor. Four major r products were found ; C, propanone. 2, 3-dimethyl-2, 3-epoxybutanalrand 2,3-dimethylbut-2-ene. When diazomethane was the methylene precursor, only C and 2,3-dimethyl-2,3-epoxybutanal resulted from photolytic reactions. 2, 3-dimethylbut-2-ene was formed by a dark reaction. The mechanism of the formation of each of the products is discussed. No products resulting from the chemically activated decomposition of C were observed and RRKM calculations showed that this was the expected behaviour.