Influences of ClH and BrH on the pyrolyses of neopentane and ethane at small extents of reaction

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Abstract
A symbolic mechanism “μH, YH” has been proposed to account for the homogeneous chain pyrolysis of an organic compound μH in the presence of a hydrogenated additive YH at 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 oftwo 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 ClH and BrH accelerate the pyrolysis of neopentane (into i‐C4H8 + CH4). The experimental results are interpreted by reaction schemes which appear as examples of the mechanism “μH, YH” in the first limiting case (A). The proposed schemes enable one to understand why the accelerating influence of ClH 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‐C5H11 · → i‐C4H8 + CH3 · is discussed.In the case of ethane pyrolysis, BrH inhibits the formation of the majorproducts (C2H4 + 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 “μH, YH” in the second limiting case (B). On the contrary, ClH has no noticeable influence on the reaction kinetics. This result inessentially due to the fact that the bond dissociation energy of ClH(⋍103 kcal/mol) is higher than that of C2H5—H (⋍98 kcal/mol), whereas that of Br—H (⋍88 kcal/mol) is lower.

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