Radiolyse des hydrocarbures. 21^e communication. Mécanismes radicalaires et non‐radicalaires dans les n‐alcanes en phase liquide

Abstract

( 24 VII 1973) 21e communication1) Summary. Contributions of radical and non-radical processes have been determined in the formation of radiolysis products of n-heptane, n-octane, n-nonane and n-decanc in a large range of temperature. Calculations are based on the combination and the dismutation of radicals, both reactions having nearly the same importance. Hydrogen abstraction reactions become important above -25". Intermediate molecular weight products and dimers are formed by statistical combination of the various radicals resulting from C-C and C-H scission. A t low temperature, low molccular weight products are formed by both radical and non-radical processes, the second one bcing more important (3/4 for alcanes and 2/3 for olefins). The yield of radicals increases with the chain lcngth of the irradiated u-alkane and amounts to 4.5 for n-heptane and 6.8 for n-dccanc at -25". This increase is due only to radicals from C-H scission, while the yield of radicals from C-C scission remains constant. Scission of CH,-CH, bonds is favored for bonds inside the molecule, but this affect diminishes with chain length and CH,-CH, rupture is equally probable at all positions for 92-alcanes heavier than decane. Methyl C-H scission is 2.7 times less probable than methylene C-H scission. The radiolysis of mixtures of protonatcd and deuterated n-alcanes is shown t o be able t o give information concerning basic processes in radiation chemistry. [56] ont CtC faits sans tenir compte de la dismutation. On considkre aujourd'hui que cette dernikre rCaction est une cause importante de disparition des radicaux alkyles [7-151, surtout aux basses tempkratures, ou l'on peut envisager la combinaison et la dismutation des radicaux comme Ctant les seules rCactions radicalaires. La probabilitC de fragmentation a CtC calculCe, en tenant compte, cette fois, de la dismutation pour la shie des 12-alcanes, de l'heptane au dCcane. L'influence de la tempCrature sur la fragmentation et sur le rendement radiolytique en radicaux a ktk plus particuligrement CtudiCe dans le cas de l'heptane; son bas point de fusion permet la radiolyse en phase liquide dans un large domaine de tempkrature o h l'arrachement d'un atome d'hydrogkne dQ aux radicaux est nkgligeable. Bien que tous les produits de la radiolyse des n-alcanes [21] puissent Ctre formellement obtenus par rCactions entre radicaux, il est nCcessaire de considCrer aussi d'autres mkcanismes, comme le montrent la radiolyse de n-alcanes l)20e Communication, Helv. 56, 531 (1973).