Estimation of Phenols by the 4-Aminoantipyrine Method: Identification of the Colored Reaction Products by Proton Magnetic Resonance Spectroscopy

The products from the oxidation of several phenols in the presence of 4-aminoantipyrine have been isolated and their p.m.r. spectra examined. The results confirm the proposed formation of p-quinoneimide adducts and the elimination of the para-group in the case of reactive p-substituted phenols. Data from ortho substituted phenols reveal the formation of syn and anti geometrical isomers of the quinoneimide. In contrast, when the starting phenol has a meta-substituent, only a single isomer, the less hindered anti form, is obtained. An unusually large anisotropic effect of the antipyryl group, causing a deshielding of the closest proton on the quinoneimide ring, has been found. In the reaction between p-benzoquinone and 4-aminoantipyrine, the color obtained is due to the formation of the p-quinoneimide, and not to a charge-transfer complex.