The formation and destruction of pentenes during the combustion of pentane

Abstract

Studies have been made of the quantitative rôle of pent-1-ene and pent-2-ene in the combustion of pentane at temperatures below 400°C. The present results show that, under these conditions, pentenes are initial and direct products of pentane combustion and pent-2-ene is the principal conjugate alkene formed [1- 14 C]Pent-1-ene and [2- 14 C]pent-2-ene have been synthesized and the combustion of pentane has also been investigated in the presence of small concentrations of these specifically labelled com­pounds; control experiments have shown that the pentenes, in the amounts added, do not interfere appreciably with the kinetics of pentane combustion. Measurements of the variation with time of the specific activities and concentrations of the pentenes enable the separate rates of formation and destruction of the conjugate alkenes to be determined. Hence it is possible to calculate the total quantities of these compounds formed at different stages of reaction and to show to what extent these are greater than the net amounts revealed by conventional analytical measurements; the differences are found to be most marked at small conversions. The reactivity ratios of pentane and the pentenes have also been determined. The rates of destruction of both pentenes are much greater than that of pentane; pent-2-ene is removed from the system roughly twice as fast as pent-1-ene. An important contrast between the behaviour of pentane and butane is that, between 300 and 400°C, but-1-ene is a much more abundant product than but-2-ene. However, with both alkanes, the relative amounts of the alk-1-enes formed become greater as the temperature is increased; indeed, the rate of formation of pent-1-ene considerably exceeds that of pent-2-ene at the instant of the passage of a strong cool flame. Comparison of the total amounts of conjugate alkenes formed from the two alkanes at 315°C shows that only ca . 30% of the pentane consumed is converted to pentenes, whereas nearly 75% of the butane which has reacted is converted to butenes. Thus there is clearly a sharp decrease in the quantitative importance of conjugate alkenes as the carbon chain length is increased from C 4 to C 5 . This suggests that the predominant reaction of pentyl radicals, under the conditions used, is to add on oxygen to form pentylperoxy radicals.