Kinetics of the formation of hemicyanine dyes by the condensation of Fischer's base aldehyde with anilines

The kinetics of the condensation of Fischer's base aldehyde with anilines, yielding hemicyanine dyes, was investigated spectrophotometrically. The study was carried out in aqueous solutions with 30%, 45%, or 60% acetic acid. It was found that the condensation is reversible and first order with respect to each reactant. The apparent second-order rate constant of the process decreased slightly for most of the used anilines, when the acetic acid content of the solvent was increased from 30% to 45% and 60%. The electron-withdrawing substituents on the aromatic ring of anilines were found to increase the observed reaction rate, whereas the electron-donor substituents were found to decrease it. However, when the “true” second-order reaction constants are considered, the reverse is valid. The “true” second-order reaction constants were calculated taking into account that part of the total aniline is protonated (and thus, not available for the condensation reaction). The Hammett plots obtained with these “true” second-order rate constants showed satisfactory correlation coefficients and gave a reaction constant of ρ = −(1.77 ± 0.10) in 30% AcOH, at 30 °C. The obtained kinetic data, and the reaction constants obtained from the Hammett plots, indicate that the rate-determining step of the studied process is the nucleophilic attack of the free amine on the carbonyl group of Fischer's base aldehyde. Activation parameters of the studied condensation reaction were also determined.