The pyrolysis of methyl iodide

Abstract

There is an extensive literature on the pyrolysis and photolysis of organic iodides. The main reason for the popularity of these substances as subjects for kinetic investigations is the relative weakness of the carbon-iodine bonds, the comparatively ready-splitting of these bonds providing a possible initiating process for the decomposition.

The object of the present work was to establish the kinetics of the pyrolysis of methyl iodide in the presence of hydrogen iodide, with a view to determining the carbon-iodine bond dissociation energy. Ogg¹ carried out a kinetic investigation into the pyrolysis of methyl, ethyl, and n-propyl iodides in the presence of hydrogen iodide. However, apart from the fact that the bond dissociation energies derived from Ogg’s analysis of his data are about 12 k.cals per mole lower than values from other sources, work done in St. Andrews, on the pyrolysis of trifluoromethyl iodide² and benzyl iodide⁵, suggested that a mechanism different from that postulated by Ogg was operative.

In the light of these considerations, it seemed that a reinvestigation of the thermal reaction between methyl iodide and hydrogen iodide was necessary. Before describing this present work, a summary is given of published work on the decompositions of iodides, and on carbon-iodine bond dissociation energies.