2,3-Dibromo-3-phenylpropanoic acid: a monoclinic polymorph

Bromination of trans-cinnamic acid resulted in the formation of 2,3-dibromo-3-phenylpropanoic acid, C9H8Br2O2. Crystallization from ethanol–water (1:1) gave crystals of different shapes. One is in the form of rods, that crystallized as the orthorhombic polymorph (Pnma), and whose structure has been described [Thong et al. (2008). Acta Cryst. E64, o1946]. The other are thin plate-like crystals which are the monoclinic polymorph (P21/n). The structure of this monoclinic polymorph is similar to that of the orthorhombic polymorph; here the aliphatic C atoms are disordered over three sets of sites (occupancy ratio 0.5:0.25:0.25). In the crystal, molecules are linked by pairs of O—H...O hydrogen bonds, forming inversion dimers with an R22(8) ring motif. The dimers are linked by weak C—H...Br hydrogen bonds, forming chains propagating along the a-axis direction