A symbolic mechanism "pH, YH" has been proposed to account for the homogeneous chain pyrolysis of an organic compound pH in the presence of a hydrogenated additive YII a t small extents of reaction. An analysis of this mechanism leads to two limiting cases: the thermal decomposition of neopentane corresponds to the first one (A), that of ethane to the second one (B). Previous experimental work has shown that this mechanism seems to account for a number of experimental observations, especially the inhibition of alkane pyrolyses by alkenes.Experimental investigations were extended by examining the influences of two hydrogen halides (ClH and BrH) upon the pyrolyses of neopentane (at 480°C) and ethane (around 540°C). The experiments have been performed in a conventional static Pyrex apparatus and reaction products have been analyzed by gas-liquid chromatography.The study shows that C1H and BrH accelerate the pyrolysis of neopentane (into i-CIH, + CHI). The experimental results are interpreted by reaction schemes which appear as examples of the mechanism "pH, YH" in the first limiting case (A). The proposed schemes enable one to understand why the accelerating influence of CIH is lower or higher than that of BrH, depending on the concentration of the additive. An evaluation of the rate constant of the elementary steps neo-C5HI1 * + i-C4Hs + CH, . is discussed.In the case of ethane pyrolysis, BrH inhibits the formation of the major products (C2Hl + H2) and, even more, that of n-butane traces. The experimental results are interpreted by a reaction scheme which appears as an example of the mechanism "pH, YH" in the second limiting case (B). On the contrary, CIH has 110 noticeable influence on the reaction kinetics. This result in essentially due to the fact that the bond dissociation energy of C1-H (=lo3 kcal/niol) is higher than that of GH5-H (~9 8 kcal/mol), whereas that of Br--K (h.88 kcal/niol) is lower.
The pyrolysis of neopentane, at small extents of reaction, was studied by gas chromatography, in Pyrex reaction vessels between 450' and 530°C and in the initial pressure range 25-200 mm Hg. At initial time, this thermal decomposition can be essentially represented by a homogeneous long-chain radical mechanism. The rate constant of the unimolecular initiation process is approximately given by the expressionThe initial rate constant of the global reaction (order s/z) is nearly equal to This reaction is strongly inhibited by propene or isobutene and self-inhibited by the isobutene formed; an interpretation of all these inhibition phenomena of the neopentane pyrolysis is proposed. Our observations and conclusions, which have been summarized in communications during 1968 and 1969, are compared to those of other authors, particularly to the recent ones of Purnell and colleagues [13] and of Taylor and colleagues [14], [15].
Res~rlts obtained in an analytical and kinetic study of propane or isopetltane pyrolysis, either pure or in presence of trace amounts of oxygen, are reported. I t is shown particularly, that oxygen accelerates or inhibits the reaction, depending upon wall conditions; a n interpretation of thts dual effect is proposed. The fact that the pyrolysis of these hydrocarbons can be inhibited almost completely under given conditions proves that any molecular 111echanism of decornposition is negligible as compared to the chain reaction.
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