The azo-hydrazone tautomeric equilibrium of 4-(phenylazo)-1-naphthol is compared in various liquid and supercritical fluid solvents. The less polar azo tautomer is dominant in the dilute gas phase, compressed ethane, and liquid alkanes. In liquid and supercritical CO2, the equilibrium shifts toward the more polar hydrazone, to yield similar amounts of the two tautomers. This shift is attributed to the Lewis acidity and large quadrupole moment of CO2. The dominance of the hydrazone tautomer in fluoroform (> 90%) can be attributed to that solvent’s large dipole moment and ability to act as a strong electron acceptor (hydrogen bond donor). Since acid-base interactions are prevalent at the lowest pressure studied (1000 psia), changes in the equilibrium constant as a function of pressure have been assigned primarily to increases in the nonspecific polar interactions. The large differences in the polarities, acidities, and basicities of these fluids, despite their similar polarizabilities per volume, are of interest for manipulating chemical processes and for practical applications of supercritical fluid science and technology.
Oshea, ke kirmse, km fox, ma johnston, kp