PULSE-RADIOLYSIS STUDIES .14. RATE CONSTANTS FOR REACTIONS OF HYDROGEN ATOMS WITH AROMATIC COMPOUNDS IN AQUEOUS SOLUTION

Absolute rate constants have been determined for the addition of hydrogen atoms to aromatic compounds in water by observation of the formation rate curves of the cyclohexadienyl radicals. The values of these rate constants for the compounds studied are: C6H5COOH, 1.00 × 109; C6H5CN, 0.68 × 109; C6H5NO2, 1.04 × 109; C6H5SO3H, 0.82 × 109; C6H5CH2COOH, 1.01 × 109; C6H5COCH3, 1.10 × 109; p-Cl-C6H4COOH, 1.13 × 109; p-NO2-C6H4COOH, 0.98 × 109; p-OH-C6H4COOH, 1.45 × 109 M-1 sec-1 at 25°. The uncertainty in these values is about ±15%. The specific rate constant for the reaction of hydrogen atoms with methanol, determined by competition kinetics with several of these compounds, is 1.6 × 106 M-1 sec-1 at 25°. The isotope effect for the reaction of H and D atoms with two of these compounds was found to be kH/kD = 1.32 ± 0.05 for C6H5COOH and C6H5SO3H.