3-[5-(4-Chlorophenyl)-1-(4-methoxyphenyl)-1H-pyrazol-3-yl]propionic acid and the corresponding methyl ester.

The synthesis of 3-[5-(4-chlorophenyl)-1-(4-methoxyphenyl)-1H-pyrazol-3-yl]propionic acid, C(19)H(17)ClN(2)O(3), (I), and its corresponding methyl ester, methyl 3-[5-(4-chlorophenyl)-1-(4-methoxyphenyl)-1H-pyrazol-3-yl]propionate, C(20)H(19)ClN(2)O(3), (II), is regiospecific. However, correct identification of the regioisomer formed by spectroscopic techniques is not trivial and single-crystal X-ray analysis provided the only means of unambiguous structure determination. Compound (I) crystallizes with Z' = 2. The propionic acid groups of the two crystallographically unique molecules form a hydrogen-bonded dimer, as is typical of carboxylic acid groups in the solid state. Conformational differences between the methoxybenzene and pyrazole rings give rise to two unique molecules. The structure of (II) features just one molecule in the asymmetric unit and the crystal packing makes greater use than (I) of weak C-H...A interactions, despite the lack of any functional groups for classical hydrogen bonding.