The thermal decomposition of azomethane. III. Kinetics of the noninhibited reaction

Abstract

The thermal decomposition of azomethane (A) has been studied in a static system at temperatures between 250" and 320°C and at pressures between 5 and 402 torr, with particular attention to identification of products. Major products, in decreasing order of importance, were nitrogen, methane, ethane, methylethyldiimide, dimethylhydrazone, propane, tetramethylhydrazine, ethylene, methylpropyldiimide, and methylethylhydrazone. Carbon balance at the lowest pressure and highest temperature was 92%, but decreased with increasing pressure and decreasing temperature owing to the formation of a polymer. A fairly simple mechanism accounts reasonably well for a short chain in the decomposition, propagated by the radical CH3N2CH2 (B), and for the five most abundant products, except ethane. It turns out that there is a second source of ethane, arising by C2Hs + A + CzHs + B; this explains an anomalously high apparent activation energy for the reaction CH3 + A + CH4 + B. Ethyl radicals are also shown to be responsible for the formation of propane, ethylene, methylethylhydrazone, and methylpropyldiimide. The radical B decomposes to CHI + CH2 + Nz, and the methylene radical (probably both singlet and triplet) is shown to yield CZHS at low pressure and high temperature, and mostly polymer at high pressure and low temperature.

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

“…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,…”

“…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.…”

Ideal Gas Thermodynamic Properties of CH3, CD3, CD4, C2D2, C2D4, C2D6, C2H6, CH3N2CH3, and CD3N2CD3

“…The decomposition was originally thought to be a true unimolecular process and the reaction pioneered experimental verification of developing theories r61. Although it is now recognized that the decomposition is a complex chain reaction [7], there is still considerable interest in the molecule as a clean supplier of methyl radicals in both photochemical [8] and thermal [9] systems. In the course of a shock tube investigation of the decomposition of azomethanel deuterated hydrocarbon mixtures by two of us (R&S), it was discovered that there were no tables of thermodynamic properties for azomethane or' azomethane.,.d 6 -in thelitemture.-As-ffi"dinarily prepared and-un~.der normal conditions, azomethane exists only in the trans form [10], and this paper deals exclusively with that configuration .. figuration.…”

Radical equilibrium studies; the thermodynamic parameters of n‐propyl

“…I n a comprehensive study of the pyrolysis of pure azomcthane, Paquin and Forst [31] showed that ethyl radicals were formed in a complex series of reactions starting with CH3N2CH2. The presence of ethyl radicals in our reaction system could have serious implications for the interpretation of our results in that the reactions (9) (10) 2 C2H5 * n-C,Hio…”