Methyl radical reactions with isopropanol and methanol, their ethers and their deuterated derivatives

Abstract

The reactions of methyl radicals with three isotopically different isopropanols three isotopically different methanols and the undeuterated ethers (diisopropyl and dimethyl ethers) have been investigated. The photolysis of acetone or acetone-d₆ has been used to generate methyl radicals or trideuteromethyl radicals. An isotopic exchange reaction between acetone and alcohols makes it necessary to work with isotopically compatible pairs. Values for Arrhenius parameters, based on Shepp's rate-constant for the combination of methyl radicals, are as follows : [graphic omitted]

Primary isotope effects in the alkyl groups of these alcohols are larger than expected solely on the basis of zero-point-energy difference : at 150°, (k_CH/k_CD) is ca. 8 for methanol and ca. 6.5 for isopropanol. These values, which resemble those for ethanol and for isobutane, taken together with the corresponding activation-energy differences, suggest that tunnelling may again be important.

A secondary reaction, disproportionation between the radicals · C(CH₃)₂OH and CH₃, is important in attack on isopropanol. It gives rise to an apparent enhancement of reactivity of the hydroxyl groups, to a spurious secondary isotope effect and, unless allowance is made, to a low value for the A-factor. The corresponding reaction does not appear to be significant in methanol or ethanol systems. In dimethyl ether, the reactivity of each methyl group is the same as in methanol but in diisopropyl ether there is steric hindrance and the alpha hydrogen atoms are less accessible than in the alcohol.