Enol–keto tautomerism of 9-anthrol and hydrolysis of its methyl ether

Abstract

The equilibrium constant for keto–phenol tautomerisation of anthrone to 9-anthrol (K_E = [phenol]/[ketone]) has been determined as pK_E (−log K_E) = 2.10 from ratios of rate constants for ketonisation of anthrol and phenolisation of anthrone in aqueous acetic acid buffers at 25 °C. Combining this value with pK_a = 10.0 for the ionization of anthrone, measured spectrophotometrically in piperazine and borate buffers, gives pK_a = 7.9 for the phenolic hydroxy group of anthrol. Measurements of rate constants for tautomerisation showed acid catalysis by H₃O⁺ in aqueous HCl but by the base component only in buffer solutions of weaker acids. The H₃O⁺-catalysed reaction is subject to a solvent isotope effect k_H3O/k_D3O = 4.8, consistent with protonation of 9-anthrol at the 10-carbon atom of the anthracene ring in the rate-determining step. Comparison with hydrolysis of the methyl ether of anthrol showed that ketonisation is faster by a factor of 3000. This large rate difference is consistent with NMR measurements which show that deuterium isotope exchange at the 10-position of the anthryl methyl ether occurs in competition with hydrolysis. This accounts for a 60–70 fold of the rate difference. The residue is attributed to (a) a normal difference of 16-fold in protonation rates of phenols and the corresponding methyl ethers and (b) a minor contribution from steric hindrance to resonance stabilisation of the anthracen-9-onium ion intermediate in the hydrolysis reaction from interaction of the conjugating methoxy group with the 1,8-hydrogen atoms of the adjacent phenyl rings.