Rate determining step oxime formation

It has been shown that the rate of formation of oximes is at a maximum at a pH that depends on the substrate but is usually 4, and that the rate decreases as the pH is either raised or lowered from this point. We have previously seen (p. 425) that bell-shaped curves like this are often caused by changes in the rate-determining step. In this case, at low pH values step 2 is rapid (because it is acid catalyzed), and step 1... 

The rate-determining step of the oxime formation from carbonyl compounds (equation 187) changes with increasing pH from formation of the addition intermediate to its dehydration to the products. 

Formation of oxime (41) from 4-dimethylaminobenzaldehyde (X = NMc2) and 4-trimethylammoniobenzaldehyde (X = NMesP) has been interpreted in terms of a standard two-step mechanism initial carbinolamine formation, followed by dehydration. pH-rate profiles have been constructed for both compounds the amine shows evidence of a change in rate-determining step at low pH (and of the intervention of the reaction of the protonated amine), while the ammonium salt shows a change from acid-catalysed to uncatalysed carbinolamine formation. Equilibrium constants for oxime formation are also reported and discussed. 

Other studies involving nickel(IV) complexes have been published. Schlemper and Murman prefer a nickel(II)-diradical description of a formal nickel(IV)-oxime complex. Panda and co-workers have examined substitution properties of the inert [Ni(dmg)3] ion and the kinetics of reduction by [FeCCN) ] . The product is [Ni(dmgH)2] and the mechanism involves rate-determining outer-sphere one-electron transfer from [Fe(CN)6] to [Ni(dmg)3] and its protonated forms with formation of a nickel(III) intermediate which subsequently is reduced in a fast step. 

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