Aromatic substitution. Part XXVI. The alkaline ferricyanide oxidation of pyridinium salts. Effects of substituents and mechanism

Abstract

The mechanism of alkaline ferricyanide oxidation of pyridinium salts has been studied as has the quantitative effect of a 3-substituent upon orientation and reactivity of the nucleus. A3-methyl group activates slightly (_H^MeK= 1·1₃) and directs mainly to C(2); a 3-cyano-group activates appreciably (_H^CNK= 20·2) and also directs mainly to C(2); a 3-methoxycarbonyl group deactivates (_H^CO₂MeK= 0·04₇) and directs exclusively to C(6). The C–H bond cleavage step is not rate-determining in the oxidation of 3-picoline methiodide. Rate-determining addition of OH^– cannot explain the substituent effects. Base-catalysed H–D exchange has been studied; nuclear proton abstraction cannot account for the observed orientation. It is suggested that the rate-determining step is the formation of a complex which reacts with more ferricyanide, oxidation taking place within a second complex. A 3-methyl group facilitates the approach of the polarisable Fe(CN)₆^3–. 3-Substituents exert both electronic and steric effects.