Hydrogen-bonded complexes of 4-aminoantipyrine with 2-chloro-3,5-dinitrobenzoic acid and monochloroacetic acid

1:1 Hydrogen-bonded complexes of 4-aminoantipyrine (4-amino-2,3-dimethyl-1-phenyl-3-pyrazoline-5-one; 4AAP) with 2-chloro-3,5-dinitrobenzoic acid (CNBA) and monochloroacetic acid (MCAA), were prepared in the solid state. Fourier transform infrared spectra confirmed the formation of proton transfer complexes between 4AAP and the two acids under investigation. Analysis of the spectra in the v(NH2) and v(C=O) regions strongly suggest that the interaction occurs at the -NH2 site for the 4AAP-MCAA system while it occurs at the -C=O site for the 4AAP-CNBA system. The equilibrium constants, K-eq, Of proton transfer between 4AAP and the two acids were determined in methylene chloride. 1,2-dichloroethane and acetonitrile by using UV spectroscopy. The results are in good agreement with the FT-IR results. It was found that K-eq in acetonitrile is affected by short range solvation. The thermodynamic parameters for the proton transfer complex formation were estimated in 1,2-dichloroethane. The proton transfer process is entropy and enthalpy controlled. The thermodynamic parameters are consistent with the FT-IR and UV results.