Ethane rate constant with hydroxyl radical

Since work with the radical clock substrate probes indicated important differences in the hydroxylation mechanisms for M. capsulatus (Bath) and M. trickosporium OB3b, work with (R) and (S)-[1-2H,1-3H]ethane with both enzymes was carried out (93, 94). With M. tri-chosporium OB3b, approximately 65% of the product displays retention of stereochemistry (93). A rebound rate constant of 2 - 6 x 1012 s-1 was calculated, assuming a free energy change of 0.5 kcal mole-1 for rotation about the C-C bond (94). This estimate approaches the value obtained from the radical clock substrate probe analysis (59). 

Hsu, K. J., and W. B. DeMore, Rate Constants and Temperature Dependences for the Reactions of Hydroxyl Radical with Several Halogenated Methanes, Ethanes, and Propanes by Relative Rate Measurements, J. Phys. Chem., 99, 1235-1244 (1995). 

The latter reaction which has the activation energy about 85 kJ/mol seems to be more probable at higher temperatures at lower temperatures hydroxy radicals may appear in other reaction pathways as in the monomolecular decomposition of hydroperoxides, etc. Hydroxyl which are very mobile and very reactive propagate the reaction site only to a short distance (the rate constant of transfer reaction of HO radicals with CH3 group of ethane is equal 108 dm3 mol-1 s approximately at 20 °C). 

J. H. Lee and I. N. Tang, Absolute rate constants for the hydroxyl radical reactions with ethane, furan, and thiophene at room temperature, J. Chem. Phys. 77(9), 4459-4463 (1982). 

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