Tautomerism of anthraquinones: V. 1,5-Dihydroxy-9,10-anthraquinone and its substituted derivatives

1,5-Dihydroxyanthraquinone and its substituted derivatives are capable of existence in the states structurally described as 9,10-, 1,10-, and 1,5-quinoid tautomerism, and as rotational isomerism involving a cleavage of intramolecular hydrogen bond. 1,5-Quinoid tautomers are characteristic only of substituted derivatives, and also appear in some metal complexes. The considerable color changes on introducing into the 1,5-dihydroxy-anthraquinone methyl, methoxy, and sulfo groups are caused by the shift in tautomeric and conformer equilibria.