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PHYSICA Registered in U. S . Patent Office 0 Copjright, 1975, by the American Chemical Society
VOLUME 79, NUMBER 1
JANUARY 2, 1975
Rate Constants for the Reaction of 0- Radicals with Organic Substrates in Solutionf P. Neta and Robert H. Schuler" Radiation Research Laboratories, Center for Special Studies and Department of Chemistry, Meiion institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 152 13 (Received June 2 1, 1974) Publication costs assisfed by Carnegie-Meiion University and the U.S. Atomic Energy Commissbn
In strong base OH radical is rapidly converted into 0- so that reactions of the latter radical can be conveniently studied by radiolyzing solutions a t high pH. The absolute rate constants for reaction of 0- with unsaturated compounds have been measured in 1 M NaOH by following the increase in absorption of the allylic radicals resulting from H atom abstraction. The radical produced from 3-hexenedioic acid has a very strong absorption a t a relatively long wavelength (X(max) 266 nm, ~ ( 2 6 6 16,350 ) M - l cm-l) so that this compound was chosen as an appropriate reference for competitive experiments. Detailed examination of the competition against ethanol indicates that relative rate constants for the reaction of 0- can be determined accurately (&lo%) and placed on an absolute scale by reference to the measurements on 3-hexenedioic acid. The rate constant measured for reaction of 0- with ethanol has been determined to be 1.2 X lo9 M - l sec-l and this reaction can, in turn, be used as a secondary reference in cases where the radical produced from a second solute absorbs significantly. Measurement of the rate constants for H abstraction from a number of saturated alcohols and monocarboxylic acids by 0- gives values -50% of those for abstraction by OH. More highly charged anions exhibit markedly lower rate constants. Partial rate constants for various H atom configurations are suggested. Rate constants for reaction with most unsaturated compounds having allylic hydrogens are higher than expected from the partial rates for the saturated system showing that a main path of reaction involves abstraction of these hydrogen atoms. The data indicate that the rate for abstraction from allylic positions is increased by a factor of 2-3 over the rates in similar saturated systems. Addition of 0- to double bonds and aromatic systems has again been shown to be relatively slow (
