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The reaction of singlet oxygen with olefins Ashford, Robert Douglas

Abstract

The reaction of singlet oxygen with a number of substituted mono-and di-olefins was studied here in the gas phase. The reaction took place in a conventional discharge flow system and was followed by a mass spectrometer which measured the decay of the olefin concentration along the tube. .The singlet oxygen was monitored photometrically using a calibrated photomultiplier tube. By determination of the rate constant at a series of temperatures up to 200°C, it was possible to obtain a value for the activation energy and entropy change for each compound, and so gain an idea of the different chemical influences that govern the rate of reaction. It was found that increased methyl substitution caused a lowering of the activation energy, and that the mono-methyl and unsubstituted derivatives of cyclohexene appeared to have an anomalously high activation energy in comparison with the cyclopentene or butene-2 analogues. These influences were interpreted in the light of the 'ene' mechanism for the mono-olefins. In the case of the dienes, the evidence points to a normal Diels-Alder type addition reaction. A detailed description of the transition states for these two reactions is given. The entropy change for the reaction was found to remain approximately constant with the different olefins and was correlated with the calculated values for the different intermediates and transition states that have been proposed for these reactions. It was pointed out that the disparity with the data obtained in solution is such that a simple transfer of these conclusions to other systems must be treated with a considerable amount of caution.

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