First direct observation of tautomerism of monohydroxynaphthazarins by IR-spectroscopy

Some substituted monohydroxylated naphthazarins (5,8-dihydroxy-1,4-naphthoquinones) were synthesized and studied by IR-spectroscopy in aprotic organic solvents at ambient temperature. Two narrow stretching mode bands in the high frequency range 3540–3410 cm−1 due to a β-hydroxy group were observed; it was established that this effect was caused by tautomerism. This allowed the creation of a convenient and accurate method for the measurement of a small amount of tautomer engaged in rapid exchange with the principal tautomer. Solvent and substituent effects were estimated.