Anodic oxidation of amines. Part III. Cyclic voltammetry and controlled potential electrolysis of 4-dimethylaminoantipyrine (4-dimethylamino-2,3-dimethyl-1-phenyl-Δ3-pyrazolin-5-one) in acetonitrile

Abstract

The anodic oxidation of 4-dimethylamino-, 4-methylamino-, 4-amino-, and unsubstituted antipyrines has been investigated in acetonitrile at a glassy-carbon electrode. The first step in the anodic oxidation of 4-dimethyl-aminoantipyrine is a quasi-reversible electron transfer from the lone-pair electrons on the 4-dimethylamino-nitrogen atom to form the cation radical. The blue-violet cation radical decays in a second-order manner with a rate constant of 12·0 ± 0·4 l mol^–1 s^–1 at 25 °C as determined by reversal coulometry. The second-order rate constant determined by spectrophotometry increases with increase in concentration of 4-dimethylaminoantipyrine. A well resolved e.s.r. spectrum of the cation radical was obtained and interpreted on the basis of HMO calculation. A mechanism for the first wave of the anodic oxidation of 4-dimethylaminoantipyrine is suggested.