Methyl radical reactions with ethanol and deuterated ethanols

Abstract

The rates of hydrogen abstraction by methyl radicals from the three different sites in ethanol have been measured by studying three isotopically different ethanols : CH₃CH₂OH, CH₃ CD₂ OH and CH₃CH₂ OD. Photolysis of acetone and acetone-d₆ over the temperature range 130–250° has been used as the source of methyl radicals and velocity constants have been obtained for five elementary reactions.

Abstraction takes place from all three sites. The methylene group is the most reactive site in the molecule, and the methyl group the least. At 150°C, 75 % of the hydrogen comes from CH₂, 20 % from OH and about 5 % from CH₃. Arrhenius parameters for methyl radical attack based on Shepp's rate-constant for methyl radical combination are as follows : [graphic omitted] Isotope effects have been determined for the methylene and hydroxyl groups; at 150°, (k_CH2/k_CD2) is 6.3 and (k_OH/k_OD) is 3.2. The activation energy difference E_CD2–E_CH2 is somewhat greater than would be expected from the zero-point-energy difference, and tunnelling may contribute to this.

A hydrogen exchange reaction between acetone and the hydroxyl group of ethanol has been found to occur. Its importance is to limit the validity of abstraction studies to isotopically compatible systems. Such pairs are studied here.