Abstract
It have been described a general mechanism for the attack step of the reactions between nitrogen nucleophiles and carbonyl compounds to form a neutral tetrahedral addition intermediate. This general mechanism is based on the relative importance of the "concerted" hydronium-ion catalyzed attack of the nitrogen base on the carbonyl compound with respect to the "step wise" mechanism that involves the previous formation of a zwitterionic tetrahedral intermediate (T±) and the posterior proton transfer to it from the hydronium-ion. The relative importance of this pathway gives origin to three kinds of profile rates vs pH for these reactions. On the other hand, the importance of the "stepwise" mechanism was attributed to a dependence on the stability of the T± intermediate that is related to the pKa of the nitrogen base and the Kad (the equilibrium constant for the formation of the neutral tetrahedral addition intermediate (T0)). The comparison of the reactions of p-chlorobenzaldehyde and norcamphor with different nitrogen nucleophiles led to the observation that the general mechanism depends on each type of carbonyl compound. This is due to the great influence of the steric and electronic effects of the compounds structure, and the fact that the pKa value of the nitrogen base, which is more related to the stability of the T± than the T0 intermediate, is more important as a factor of prediction of the profiles than the value of Kad, this value being more related to the stability of the T0 intermediate.
Highlights
Sayer et al.[1] have described a general mechanism for the reaction between nitrogen nucleophiles and carbonyl compounds in aqueous solution, based on Scheme 1. This generalization is based on the relative importance of the hydronium-ion catalyzed concerted attack of the nitrogen nucleophile on the carbonyl compound with respect to a mechanism that involves the previous formation of a zwitterionic tetrahedral intermediate (T±) and the posterior proton transfer to it from the hydronium-ion
The importance of stepwise pathway II depends on the stability of the intermediate T± which is related to the pKa of the nitrogen nucleophile and to the equilibrium constant value Kad for the formation of the neutral addition tetrahedral intermediate T0 from the reagents
The equilibrium constants - Kad - for the addition intermediate formation from several amines with norcamphor and hydroxylamine with ciclopentanone in aqueous solution and ionic strength 0.5 M (KCl) were determined spectrophotometrically[6], from the initial decrease in absorbance of the carbonyl group at 285 nm extrapolated to zero time, on mixing a solution of ketone with several concentrations of amines, at pH 8.5 (0.01 M borate buffer)
Summary
Sayer et al.[1] have described a general mechanism for the reaction between nitrogen nucleophiles and carbonyl compounds in aqueous solution, based on Scheme 1. This generalization is based on the relative importance of the hydronium-ion catalyzed concerted attack of the nitrogen nucleophile on the carbonyl compound (pathway I - “concerted mechanism”) with respect to a mechanism that involves the previous formation of a zwitterionic tetrahedral intermediate (T±) and the posterior proton transfer to it from the hydronium-ion (pathway II - “step wise mechanism”).
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