Synthesis and crystal structures of two isomeric nitro-α-resorcylic acids

Nitration of α-resorcylic acid (3,5-dihydroxybenzoic acid) to 3,5-dihydroxy-2-nitrobenzoic acid (1) and 3,5-dihydroxy-4-nitrobenzoic acid (2), their separation, and crystallization is described. Crystal structures of 1, 1 · 3H₂O, and 2 were determined by X-ray single-crystal diffraction analysis and compared with DFT-calculated structures. An intramolecular hydrogen bond between the nitro group and o-hydroxyl group(s) occurs in all investigated structures. Intermolecular hydrogen bonds and π···π stacking interactions determine the molecular packing. Additionally, N···O interactions have a crucial role in arrangement of the molecules in the crystal structures of 1 and 2. DFT calculations in the gas phase and in the solution reveal that minor product 2 is a more stable regioisomer than 1, the major product in the nitration reaction.