SUBSTITUENT EFFECT ON THE STABILITY OF THE HYDROXYL RADICAL ADDUCT OF ALPHA-PHENYL N-TERT-BUTYL NITRONE (PBN)

Alpha-Hydroxy-(3- and 4-substituted)-benzyl tert-butyl aminoxyl radicals, the hydroxyl radical spin adducts of 3-and 4-substituted alpha-phenyl N-tert-butyl nitrones (R-PBN's), decay rapidly (in seconds) by first order kinetics in acetonitrile (ACN). The Hammett equation is obeyed with rho = -1.36 and r = 0.90. Polar solvents produce an increase in rate of decay: water (fast) > ACN (fast) > benzene (slow). Thus substantial charge separation in the transition state is indicated. The mechanism proposed is a unimolecular cleavage reaction producing the substituted benzaldehyde and tert-butyl nitroso anion radical: [GRAPHICS] Support for this mechanism is provided by the detection of tert-butyl hydroaminoxyl when the decay is studied in water.