Titrations in non-aqueous media. Part XIV. Redoximetric titrations of aldehydes with manganese(III) acetate in glacial acetic acid

Abstract

Twenty-five aromatic and nine aliphatic aldehydes were titrated with manganese(III) acetate in glacial acetic acid. The end-points were determined potentiometrically using a redox platinum and silver-silver chloride electrode assembly. The titrations were carried out both directly and indirectly at 60 °C. The times required for the indirect titrations were much shorter than those necessary for the direct titration of these compounds. A hydroquinone solution was used as the back-titrant in the indirect titrations. Of the 34 aldehydes titrated in this work only eight underwent a two-electron redox reaction with manganese(III) acetate; these included the hydroxybenzaldehydes and some of their methyl, hydroxy and bromo derivatives. The nitro derivatives of the hydroxybenzaldehydes did not undergo the redox reaction. For a redox reaction between an aldehyde and manganese(III) acetate to occur two protons must be available in the reaction medium and the aromatic ring must have a certain degree of electron density. All the hydroxybenzaldehydes, including the nitrohydroxybenzaldehydes, were also titrated with tetrabutylammonium hydroxide in order to determine their acidities. It was found that the acidities of the nitro derivatives of the hydroxybenzaldehydes were very different from those of the other hydroxybenzaldehydes (at least 3.7 pK units less). In the nitrohydroxybenzaldehydes, the presence of the nitro group causes the carbonyl carbon atom of the aldehyde group to attract the electron pair of the C–H bond very strongly and therefore the manganese(III) acetate cannot accept the two electrons from the aldehyde group. The titratable hydroxybenzaldehydes and their derivatives gave the corresponding acetyl ester derivatives of benzoic acid, whereas 2-hydroxybenzaldehyde yielded acetylsalicyclic acid (aspirin). These acids were formed in the reaction medium as their manganese(II) salts.