No abstract

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.

No abstract

Calculations were carried out on several data sets to study the mechanism of hydrogen abstraction from phenols by peroxy radicals: Rate constants, k values, were collected for the reactions of cumyl‐, 1‐phenylethyl‐ and tert‐butyl‐peroxy radicals with ortho‐para‐substituted phenol inhibitors. The rate constants were recalculated for the same temperature. Solvent effects were neglected because the solvents used were similar in nature. The phenol ortho substituents were characterized by an indicator variable ItBu accounting for the presence or absence of di‐tert‐butyl groups. The phenol para substituents were characterized by Charton's σI, σR, and σ italicR+ substituent constants. The dependence of log k values on Itbu, σI, σR, σ italicR+ was investigated using stepwise linear regression analysis. The combined data set of 32 reactions gives: and The results suggest that hydrogen abstraction from phenols by peroxy radicals proceeds by an electrophilic mechanism, and that neither the peroxy‐radical nor the ortho‐di‐tert‐butyl groups have considerable effect on the rate of reaction (1).

No abstract