Gas-phase thermal decomposition of dimethyl mercury. Part 2.—The homogeneous reaction

Abstract

The homogeneous gas-phase pyrolysis of dimethyl mercury has been studied in a wide variety of conditions. The products and kinetic characteristics of the reaction are shown to be more complex than has been supposed. An abridged short-chain radical mechanism which accounts for the main features of the reaction is suggested. Evidence is presented which indicates that the initial unimolecular breakdown of dimethyl mercury is not in its first-order region in the usual range of pressures and temperatures employed and the pressure dependence of the overall first-order rate constant is shown to be in qualitative accord with the predictions of the Kassel-Slater theories. A mechanistic explanation of the so-called type I and type I1 modes of mercury dialkyl decomposition is offered and the experimental conditions for transition of behaviour for dimethyl mercury are stated. The results indicate that the breakdown of the radical *HgCH3 proceeds without measureable activation energy.The experimental conditions in which the pyrolysis of DMM exhibits the characteristics of a homogeneous gas-phase reaction were described in part 1.1 For a detailed study of this reaction a Pyrex vessel (S/Y = 1 cm-1) washed with nitric acid was used. Following exposure to the atmosphere at least six pyrolyses were conducted before measurements were renewed.l The apparatus and experimental procedures employed were as described earlier.1 RESULTS PYROLYSIS OF DMM ALONEA list of the products of reaction and an indication of their abundance at the extremes of the temperature range employed was given in table 1 of part 1.1 Fig. 1 illustrates a gas chromatogram obtained with the product of the homogeneous decomposition at 440°C of 20 mrn Hg of DMM carried to 5 % reaction. The sample 103

Determination of the bond dissociation energy, D(CH3Hg—CH3), by the toluene carrier method

“…The available high pressure Arrhenius parameters for the pyrolysis of dimethylmercury in the presence of suitable inhibitors are in reasonable agreement (1)(2)(3)(4). The results of Russell and Bernstein (I), k = 5.0 x 10" exp (-57 900/RT) s-', and of Kallend and Purnell (2), k = 5.0 x 101j exp (-57 600/RT) sC1 are in excellent agreement.…”

Mechanisms of Photoreactions in Solution. XXI.1 Quenching of Excited Singlet States of Benzophenone

“…It has been shown that benzophenone acts as a sensitizer for the cis-trans photoisomerization of the stilbenes.12,18 Benzophenone is among the group of compounds that are classified as "high-energy" sensitizers. 14 With such a sensitizer the mechanism of isomerization can be simplified to the following scheme in which T represents the triplet group that decays to groundstate molecules.18…”

Kinetic Order Determination in the Thermal Decomposition of Dimethylmercury

“…Since the submission of this study for publication, a comprehensive study of the decomposition in the vicinity of 400°has been published by Kallend and Purnell. 14 Their proposed mechanism gives the rate equation = 2fci[DMM] + B[DMM]'A (13) where fci is the rate constant for the elementary reaction (CH,),Hg -CH3 + CH3Hg and B is a combination of several rate constants in their postulated mechanism. Their data are found to fit this expression rather well.…”