The Mercury (3P1) Photo-Sensitized Reaction of Ethane at High Temperatures

Abstract

The mercury photo-sensitized decomposition of ethane has been carried out with particular emphasis on the high temperature reaction. Evidence is given that under these experimental conditions the ethyl radical becomes unstable around 400°C. An activation energy of 39.5 kcal is assigned to the reaction C2H5=C2H4+Hon the basis of the proposed mechanism.

A search for a gas-phase free-radical inversion displacement reaction at a saturated carbon atom

“…Previous studies in this laboratory (3) established that the initial stages of the decomposition from 995 to 1103 K are well accounted for by reactions [4], [5], and [6], with reaction [4] exhibiting some unimolecular pressure dependence, and no evidence for the occurrence of reaction [2]. The present experiments have extended these measurements to 899 and 802 K at pressures of about 1000 Torr where k, should be close to its high-pressure limit.…”

“…2 at a methane concentration of 8 x M, much too low for the inversion reaction. Because of uncertainties in the efficiencies of reactions [5], [6], and [7], it is impossible to calculate a meaningful upper limit for reaction [2] from this system, but there is no positive evidence for its occurrence.…”

“…These data are in essential agreement with our earlier studies, with quantum yields rising with increasing pressure and temperature. If reactions [7], [5], and [6] were quantitative, the quantum yield should have been unity, independent of temperature and pressure. The observed values, and their variation, probably reflect as before an incomplete and temperature-dependent quenching of Hg('P,) in reaction [7], with perhaps some loss of CH,, and thence CzH6, by the incomplete suppression of the combination reaction,…”

“…In pure azomethane this is followed by the reaction and reaction [6]. Subsequent reactions of CH,NNCH, and other radicals with azomethane lead to a short-chain reaction with a chain length of 2 or 3, yielding N2, CH,,, C2H6, and some higher products (7 If to this mechanism we now add the inversion reaction, [2], followed by reaction [5] the net result would be, as in the methane pyrolysis, a chain reaction yielding H2 and C2H6.…”

“…the mercury-photosensitized decomposition of ethane at high temperature (6) in which a chain decomposition is propagated by the decomposition of the ethyl radical). An activation energy of about 6 kcal/mol can be estimated from Fig.…”