The reaction betw een propylene and oxygen has been exam ined betw een th e tem peratures of 340 and 400° C in a static system by detailed analysis supplem ented by pressure-tim e data. The reaction is of th e degenerately branched-chain type, an d acetaldehyde is m ainly responsible for branching. The aldehyde is probably produced by th e decom position of a peroxide w hich is th e initially form ed m olecular product. Inhibition of certain of th e elem entary processes w hich occurs a t high

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... oncentrations of hydrocarbon is ascribed to th e form ation of an unreactive allyl radical. R eaction processes which account for th e m ain features of th e reaction and the m ore im p o rta n t interm ediates and products are discussed. I n t r o d u c t i o n Numerous recent studies of gas-phase autoxidation of organic compounds ha\ fully substantiated the view th a t a t moderate tem peratures the reaction proceed via the interm ediate formation of peroxidic substances. Hinshelwood (1948) an others (Cullis & Hinshelwood 1947; Mulcahy 1949 a; Cullis & Mulcahy 1949) hav studied the effect of structure on the overall rate of oxidation of saturated hydro carbons and also the effect of substituents (Cullis, Hinshelwood & Mulcahy 1949 Cullis & Willsher 1951). The oxidation of the olefinic hydrocarbon has receivet very little attention, which is surprising since studies of these substances in solutioi have proved particularly rewarding. The ethylene-oxygen reaction has been examined in detail (for example, b;< Lenher 1931 and H arding & Norrish 1952), and shows behaviour not characteristii of unsaturated hydrocarbons, since full olefinic properties are not developed unti propylene and the higher olefins are reached. Thus, im portant changes in reactivity arise when there is a CH2 group adjacent to the double bond. This work is part 0 a systematic study being made in these laboratories of both the low temperatur< gaseous combustion and the solution catalyzed oxidation of olefins. The first paper of this series deals specifically with the oxidation of propylene The results are critically compared w ith those of the oxidation of the higher olefim butene-2 {cis and trans forms) and of hexene-1 in the second and th ird papers L ater publications will deal with the oxidation of other related unsaturated sub stances, and to compounds in which the possibility of formation of resonanct stabilized allyl-type radicals formed by hydrogen abstraction does not exist. E x p e r i m e n t a l Materials Cylinder propylene (I.C.I. Billingham) wras condensed in a trap cooled in liquic nitrogen while being subjected to continuous pumping to remove impurities volatile under these conditions. The tem perature of the trap was raised to -78° C