Amine oxidation. Part II. The oxidation of some trialkylamines with alkaline potassium hexacyanoferrate(III)

Abstract

N-Methyl- and NN-dimethyl-tertiary amines are easily oxidised at room temperature by aqueous alkaline potassium hexacyanoferrate(III). For substrate in excess, the reaction has a stoicheiometry of two moles of hexacyanoferrate(III) to one of amine and the major product is the demethylated secondary amine. Which alkyl group is lost is controlled predominantly by the acidity of the α-protons. In excess of alkaline hexacyanoferrate(III) dialkylformamides are also produced. Kinetic studies indicate that the rate law is –d[Fe(CN)₆ ^3–]/dt=k₂′[amine][Fe(CN)₆ ^3–]. The rate constants for the amines are correlated using a Taft free-energy relationship and give ρ*=–5·45. A mechanism accounting for the results in which an irreversible electron abstraction from the nitrogen lone pair is the rate-determining step is suggested.