Solvent effects on the absorption spectrum of the product and the equilibrium constant for the proton transfer reaction from 1-phenacylquinolinium bromide to amines

The equilibrium constants, K₂, have been determined for the proton-transfer reactions of 1-phenacylquinolinium ion, PHQ⁺, with several amines {triethylamine (TEA), N,N,N′,N′- tetramethylethylenediamine (ED), N,N,N′,N′-tetramethylpropanediamine (PD), N,N,N′,N′-tetramethylbutanediamine (BD), and 1,8-bis(dimethylamino-naphthalene (DMAN)} in acetonitrile (AN), AN-tetrahydrofuran (THF) and AN-ethanol (EtOH) mixtures. The reaction was followed spectrophotometrically using a stopped-flow technique. The K ₂ value decreased for DMAN and increased for TEA with increasing vol-% of THF in AN-THF mixtures. The changes in the K₂ value for ED, PD and BD changed in the order: ED, PD and BD from a pattern similar to TEA to a pattern similar to DMAN. The change in the K₂ value for DMAN with increasing vol-% of THF in AN-THF mixtures was explained by the effect of polarity on the stability of P^-Q⁺ (the deprotonated product of PHQ⁺). The effect of THF on the K₂ value is consistent with that of the peak wavelength of the absorption spectrum of P ^-Q⁺. The change in the K₂ value for TEA, ED, PD and BD depended on the structures of the protonated bases, one of the products for this reaction. The effect of EtOH on the K₂ value for DMAN was examined in ternary EtOH-THF-AN mixtures that contain different amounts of EtOH and whose relative permittivities were adjusted to that of EtOH. The K ₂ value increased with increasing vol-% of EtOH because of the stabilization of P^-Q⁺ upon the formation of the hydrogen-bonded complex with EtOH. The absorption spectrum of P ^-Q⁺ demonstrated a blue shift as the vol-% of EtOH increased.