Hydrolysis of 1,8- and 2,3-naphthalic anhydrides and the mechanism of cyclization of 1,8-naphthalic acid in aqueous solutions

Abstract

Naphthalene-1,8-dicarboxylic acid, 1,8-Acid, cyclizes spontaneously in acidic aqueous solution to naphthalene-1,8-dicarboxylic anhydride, 1,8-An, and here we present an ab initio study of the reaction pathway. The effect of pH on the hydrolysis of 1,8-An was analysed and compared with the hydrolysis of naphthalene-2, 3-dicarboxylic anhydride, 2,3-An, to naphthalene-2,3-dicarboxylic acid, 2,3-Acid. The values of the pK_a's of 1,8-Acid and 2,3-Acid were ca. 3.5 and 3.0, for monoanion formation, pK_a1, and 5.5 and 5.0 for dianion formation, pK_a2, respectively. Fluorimetric titration demonstrated that the diprotonated 2,3-Acid, AH₂, was further protonated to yield AH₃⁺. The pH–rate constant profile for 2,3-An hydrolysis showed a water reaction between pH's 1.0 and 6.0 and a base catalysed hydrolysis above pH 7.0. Under no condition was 2,3-An formed from 2,3-Acid. The pH dependent decomposition kinetics of 1,8-An is complex and, below pH 6.0, the pH–rate constant profile was fitted by assuming that both AH₂ and AH₃⁺ are in equilibrium with 1,8-An. The values of the equilibrium constants for 1,8-An formation from AH₂ and AH₃⁺ were ca. 4 and 100 in dilute and concentrated acid, respectively. Ab initio calculations for a possible reaction pathway connecting the undissociated 1,8-Acid to 1,8-An show a transition state where an intramolecular proton transfer is concerted with oxygen alignment towards the carbonyl centre. The planar intermediate is then dehydrated yielding a complex between water and 1,8-An.