Thermodynamic dissection of solvene effects in the schiff reaction

Rate constants for the ractions of p-substituted benzaldehydes with aniline and some 2-alkyl- and 2,6-dialkyl-anilines to give imines have been measured in acetonitrile and N,N-dimethylformamide at 25 and 75°C. The contributions of solvation energies to the free energy of activation have been measured at 25°C in cyclohexane,carbon tetrachloride,methanol,N,N-dimethylformamide,acetonitrile,ethylene glycol,dimethyl sulphoxide and N-methylformamide for the reaction of benzaldehyde and aniline.The observed reactivity depends on both polar and steric effects of the alkyl groups.Thermodynamic dissection of solvent effects points out that acidic or basic properties of solvents are important features in determining the rate of the process.However, the changeover of the rate limiting step from the formation of the intermediate alpha-amino alcohol(with electron-donating groups)to its dehydration(with electron withdrawing groups)seems scarcely affected by solvent variation. H-NMR measurements in dimethyl sulphoxide allowed the detection of the intermediate alpha-amino alcohol.