The Rate-Constant of the Reaction of Hydroxyl Radicals With Methanol, Ethanol and (D₃)Methanol

Abstract

The rate coefficients for hydrogen abstraction by hydroxyl radicals from methanol and ethanol have been determined in the temperature ranges 260-803 K and 255-459 K respectively. Flash photolysis combined with resonance absorption detection of OH was used to obtain results which may be described by the Arrhenius expressions:  

Methanol     k(T) = (8.0±1.9)×10^-12 exp[-(664±88)K/T]cm³ s^-1         (1)  

Ethanol      k(T) = (1.25±0.24)×10^-11 exp[-(360±52)K/T]cm³ s^-1 (2)  

The results obtained for methanol are in excellent agreement with results obtained by other workers, but the Arrhenius parameters for ethanol are markedly different to those obtained in the only other study of the temperature dependence of this reaction.

The rate constant for reaction with (D₃)methanolhas been determined at 293 K: (D₃)

Methanol       k(293) = 5.0±0.2×10^-13 cm³ s^-1 (3) 

The presence of an isotope effect confirms that the predominant process is abstraction of hydrogen from the carbon rather than the oxygen.

The results obtained for the reaction with ethanol were analysed by using a 21 reaction scheme to determine the effect of [OH]_O and secondary reactions on k(T). The simulations indicate that secondary reactions involving OH are relatively unimportant in determining the bimolecular rate coefficients found in this study.