Investigations on the thermal decomposition of olefins

Abstract

It has been shown that at 530 °C there is practically no decomposition of olefins not possessing b C—C bond β to the double bond. The rate constants for the decomposition of various olefins at 530 °C over the range of p 0 from 25 to 300 mm have been measured. The relative rates of decomposition and polymerization of ethylene at 530, 600 and 630 °C have been studied. On the basis of chromatographic analysis of the reaction products both in the presence and in the absence of nitric oxide it is concluded that the ethane formed comes mainly from the secondary decomposition of polymers. At 530 °C the inhibitory activity of nitric oxide is exerted only on some olefins and even then it is rather small. All olefins are, however, subject to a remarkable stimulatory activity of nitric oxide. Over the range of pressures investigated this accelerating effect on the decomposition depends linearly on the olefin and nitric oxide pressures r 0 = k 0 [RK] + k 2 [RH] [NO]. The values of the constants for different olefins have been determined. The order of magnitude of the constant k 2 was found to be the same for all olefins investigated except ethylene and propylene, the values lying between 1.0 x 10 -4 and 3.0 x 10 -4 min -1 (mm NO) -1 . Propylene, which possesses an unusually small number (3) of weak C—H bonds and only primary ones, gave (by an indirect method) the lower value (2 ± 1) x 10 -6 min -1 (mm NO) -1 . Ethylene, the only olefin without weak C—H bonds, gave (by an indirect method), a value of k 2 of about (4 ± 1) x 10 -6 min -1 (mm NO) -1 . The reaction of hydrogen abstraction from the olefin molecule by nitric oxide is assumed responsible for the stimulatory phenomenon. The low value of k 2 for ethylene compared with the values for other olefins and for paraffins can be explained as a result of the strengthening influence of the double bond on the C—H bond with the doubly bound carbon atom. The different roles of the inhibitory and accelerative functions of nitric oxide in the decomposition of paraffins and olefins are considered to be mainly responsible for the changes in the shape of the ∆p –time curves as P NO is varied. On this basis the phenomenon of the apparently transistory activity of nitric oxide as an inhibitor in some cases can be explained.