Amine oxidation. Part III. The oxidation of some aralkyl tertiary amines with alkaline potassium hexacyanoferrate(III)

Aralkyl tertiary amines are oxidised slowly at room temperature by aqueous alkaline potassium hexacyanoferrate-(III). Kinetic studies using a series of para-substituted NN-dimethylbenzylamines show that the rate law for these oxidations is –d[Fe(CN)63–]/dt=k2′[Amine][Fe(CN)63–]. The second-order rate constants are correlated by use of the Hammett, Bronsted, and Taft linear free-energy relationships and give ρ=–0·989, α= 0·990, and ρ*=–4·24, respectively.The quantitative estimation of the products is complicated by the Cannizzaro disproportionation of the benzaldehydes. However, the course of the oxidation is controlled by the acidity of the α-hydrogens; thus, whether an alkyl or an aralkyl group is lost is rationalised in terms of the effect of the substituent on the relative acidities of the N-methyl and N-benzyl hydrogens, respectively.The results show that the mechanism of the oxidation of these amines by potassium hexacyanoferrate(III) is identical to that suggested previously for trialkylamines.