Reactions of free radicals with sulphur dioxide. Part I - methyl radicals

The reaction of sulphur dioxide with methyl radicals, generated from the photolysis of azomethane at λ >3400 Å, has been studied in the temperature range 25-164°C. At low temperatures, the addition reaction to form methyl-sulphonyl radicals is fast but as the temperature increases methylsulphonyl radicals become increasingly unstable and appear not to be formed at 164°C. Arrhenius parameters (based upon a value of 10^13.34 mole^-1 cm³ sec^-1 for the rate of combination of methyl radicals) have been measured or deduced using a material balance method for the following reactions:                                                                                                      log A (mole^-1 cm³ sec^-1) E  (kcal mole^-1)
CH₃ + CH₃N₂CH₃ → CH₄ + CH₂N₂CH₃              11.55                   8.7
            CH₃ + SO₂ → CH₃SO₂                           10.8                     1.5
               CH₃SO₂ → CH₃ + SO₂                        13.0 (sec^-1)        22.4

A value of 0-04 has been obtained for the disproportionation : combination ratio of methyl and methylsulphonyl radicals. The addition reaction of methyl radicals with sulphur dioxide has been compared with their reaction with oxygen, carbon monoxide and ethylene. Although the reaction with oxygen requires zero activation energy the three-body limitation for the oxygen reaction leads to the conclusion that in the gas phase at pressures of ∼50 mm Hg the overall rates of addition of methyl radicals to sulphur dioxide and oxygen are similar.