Outer‐sphere reduction of hexacyanoferrate(III) by enolizable and nonenolizable aldehydes in alkaline medium

Outer‐sphere reduction of hexacyanoferrate(III) by some enolizable/nonenolizable aldehydes (viz., aliphatic, heterocyclic, and aromatic aldehydes) in alkaline medium has been studied spectrophotometrically at λ max = 420 nm. The reactions are first order each in [aldehyde] and [Fe(CN) 6 3− ]. The rate increases with an increase in [OH − ] in the oxidation of aliphatic and heterocyclic aldehydes, whereas it is independent of [OH − ] in the reaction with aromatic aldehydes. The intervention of free radicals in the reaction mixture was carried out using both acrylonitrile and acrylamide scavenger in two different experiments. The kinetic results indicate that the oxidation of benzaldehyde in aqueous medium proceeds at a slower rate than the aliphatic aldehydes (other than formaldehyde) and furfural. The values of third‐order rate constant ( k 3 ) at 308 K in the oxidations of some aliphatic aldehydes and furfural follow the order (CH 3 ) 2 CH > CH 3 CH 2  > CH 3  > C 4 H 3 O > H. The rate constants correlate with Taft's σ* value, the reaction constant being negative (–9.8). The pseudo–first‐order rate constants in the oxidations of benzaldehyde and substituted benzaldehydes follow the order NO 2 > H > Cl > OCH 3 . The Hammett plot is also linear with a ρ value (0.6488) for meta‐ and para‐substituted benzaldehydes. The kinetic isotope effect for benzaldehyde ( k H / k D = 1.93 at 303 K) was obtained. The rate‐determining step is the outer‐sphere formation of Fe(CN) 6 4− and free radicals, which is followed by the rapid oxidation of free radicals by Fe(CN) 6 3− to give products. The kinetic data and hence thermodynamic parameters have been used to distinguish enolizable and nonenolizable aldehydes. An attempt has also been made to correlate kinetic data with hydration equilibrium constants of some aliphatic aldehydes.