DISORDER IN 13-CIS-RETINOIC ACID

13-cis-Retinoic acid [(2Z,4E,6E, 8E)-3,7-dimethyl-9-(2,6,6-trimethylcy clohex- 1-enyl)-2,4,6,8-nonatetraenoic acid, C20H28O2] was found to ad opt the 6-s-trans conformation, with the ring double bond and all the C atoms of the polyene chain coplanar. The plane of the double-bond sy stem is coincident with a crystallographic mirror plane. The crystal d isplays crystallographic symmetry arising from disorder, indeed the cy clohexene ring, in a half-chair conformation, is present in two equall y occupied conformations related to one another by the mirror plane. T he carboxylic acid group is also disordered, lying on opposite sides o f the plane of the double-bond system. In the crystal, the molecules f orm dimers connected by hydrogen bonds involving the carboxylic acid g roups, and the dimers pack in parallel layers.