Ethyl 3-carboxy-5-nitrobenzoate

In the title compound, C10H9NO6, the carboxy, ethoxycarbonyl and nitro groups form dihedral angles of 3.8 (1), 4.5 (1) and 164.8 (1)°, respectively, with the mean plane of the benzene ring. In the crystal structure, molecules lying about inversion centers are linked through O—H⋯O hydrogen bonds. C—H⋯O interactions are also present.

In the title compound, C 10 H 9 NO 6 , the carboxy, ethoxycarbonyl and nitro groups form dihedral angles of 3.8 (1), 4.5 (1) and 164.8 (1) , respectively, with the mean plane of the benzene ring. In the crystal structure, molecules lying about inversion centers are linked through O-HÁ Á ÁO hydrogen bonds. C-HÁ Á ÁO interactions are also present.

Comment
The title compound is an important intermediate for the preparation of iodinated X-ray contrast media, such as iotalamic acid, ioxitalamic acid, and ioxilan, which are used clinically all over the world (Morin et al., 1987;Singh et al., 1980;Stacul, 2001). We report here the crystal structure of the title compound.
The crystal data show that the bond lengths and angles in the title compound ( Fig. 1) are within expected ranges and agree well with the corresponding molecular dimensions reported for a similar compound (Zou et al., 2009). The carboxylic acid group (O5/C10/O6) attached at C3 and the ester group (O1/C7/O2) attached at C1 are nearly coplanar with the benzene ring (C1-C6) (dihedral angles of 3.8 (1) and 4.5 (1)°, respectively), while the nitro group (O3/N1/O4) attached at C5 forms a dihedral angle of 164.8 (1) ° with the benzene ring. In the cyrstal structure, the molecules lying about inversion centers are linked through O-H···O hydrogen bonds (Table 1).
Experimental 5-Nitroisophthalic acid (2.1 g, 0.01 mol) was dissolved in 1.5 M ethanolic hydrochloric acid solution (7.5 ml) at 323 K. The mixture was stirred at 323 K for 6 hr. Then sodium chloride (1.8 g, 0.03 mol) in water (20 ml) was added. An oily liquid separated and crystallized on cooling. The precipitate was suction filtered and washed with water until neutral. The solid was suspended in sodium bicarbonate (1.0 g, 0.01 mol) in water (10 ml) and the undissolved diester was filtered off. The filtrate was acidified with 1 M hydrochloric acid to a pH of 4. The precipitate was filtered and washed with cold water. The crude product was purified by recrystallization from ethanol (yield: 41%). Single crystals were grown by slow evaporation of a ethanol/water(v/v 1:1) solution: colourless prismatic crystals were formed after several days.

Refinement
All the H atoms could have been discerned in the difference electron density maps. With the exception of the hydrogen belonging to the hydroxyl group of the carboxyl the H atoms were situated into the idealized positions and refined in riding motion approximation. The hydroxyl hydrogen was refined freely. The used constraints: C aryl -H = 0.95 Å, C methyl -H = 0.98 Å and C methylene -H = 0.99 Å, U iso (H) = 1.2U eq (C).
supplementary materials sup-2 Figures   Fig. 1. A view of the title compound with the atomic numbering scheme. Displacement ellipsoids were drawn at the 30% probability level.