Crystal structure of 2-benzoyl-3,4,5,6-tetrachlorobenzoic acid.

Abstract

intermediates in the synthesis of a variety of phthaleins and fluorans. Besides their importance in synthetic chemistry, this class of compounds also furnishes interesting research topics in structural chemistry. Lalancette et al.1 studied the X-ray structure of the simplest acid, 2-benzoylbenzoic acid (BBA). They demonstrated that BBA crystallizes in dimorphic (anhydrous and hydrated) forms, where BBA molecules take the open conformation as shown in Fig. 1(a), and form centrosymmetric dimers through intermolecular hydrogen bonds between carboxylic acid groups. This type of intermolecular hydrogen bonding is normally seen in carboxylic acids. Recently we have analyzed the X-ray structure of o-(p-N,Ndimethylaminobenzoyl)benzoic acid (DMABBA) and pointed out that intermolecular hydrogen bonds are formed between the carboxyl OH and the ketone oxygen of adjacent DMABBA molecules to form catemeric chains.2 In solution, 2benzoylbenzoic acids also exist as the lactone form (Fig. 1(b)) as well as the open form. Bhatt et al. investigated the effect of the benzene ring substitution on the tautomeric equilibrium between the open and the lactone form.3 Interestingly, he suggested that 2-benzoyl-3,4,5,6-tetrachlorobenzoic acid (TCBBA) takes the lactone form in the crystalline state. In this work, we have undertaken the X-ray analysis of TCBBA in order to clarify the crystal and molecular characteristics of 2-benzoylbenzoic acids having a typical lactone form. The title compound was prepared as follows. Anhydrous aluminum(III) chloride (2.320 g:8.1 mmol) was added portionwise to tetrachlorophthalic anhydride (2.160 g:16.2 mmol) in benzene (20 ml). The mixture was refluxed for 20 h. The reaction was stopped by adding 1 M HCl (10 ml) and the raw product was extracted with ethyl acetate. After evaporation of the extract in vacuo, the residue was purified by column chromatography (hexane/ethyl acetate = 1/3). Colorless crystals suitable for X-ray diffraction analysis were obtained by crystallization from aqueous acetone solution at room temperature (m.p. 196.0 C). All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located by a difference Fourier synthesis and a geometrical calculation, with the parameters of H atom bonded to O3 also being refined. The crystal and experimental data are listed in Table 1. The final fractional atomic coordinates and equivalent isotropic thermal parameters for non-hydrogen atoms are given in Table 2. Selected bond 615 ANALYTICAL SCIENCES MAY 2002, VOL. 18 2002 © The Japan Society for Analytical Chemistry