4-Bromo-3,5-dihydroxybenzoic acid monohydrate

The crystal packing of the title compound, C7H5BrO4·H2O, is influenced by O—H⋯O hydrogen bonds.

Compound (I) possesses normal geometric parameters (Allen et al., 1987). The dihedral angle between the mean plane of the C1-C6 benzne ring and the plane of the C7/O1/ O2 grouping is 4.5 (5) .
The crystal packing of (I) is influenced by O-HÁ Á ÁO hydrogen bonds (Table 1). First, classical inversion dimers involving O4-H4Á Á ÁO3 iii and O4 iii -H iii Á Á ÁO3 bonds of Figure 1 A view of the molecular structure of (I), showing 50% probability displacement ellipsoids for the non-hydrogen atoms. The hydrogen bond is indicated by a double-dashed line. adjacent head-to-head carboxylic acid groups are formed (Fig. 2) [symmetry code: (iii) 1 À x, 1 À y, 1 À z]. Atom H4 appeared to be well ordered in a difference map, but the C7-O4 bond [1.288 (5) Å ] is shorter and the C7 O3 bond [1.252 (5) Å ] is longer than expected for a well ordered carboxylic acid group: the mean C-O and C O bond lengths in carboxylic acid groups bound to an aromatic ring are 1.226 Å ( = 0.020 Å ) and 1.305 Å ( = 0.020Å ), respectively (Allen et al., 1987). This perhaps suggests partial disordering of H4 in (I), i.e. it is bound to both O3 and O4, but this was not visible in a difference map.
These dimeric pairs of 4-bromo-3,5-dihydroxybenzoic acid molecules are then linked into an infinite sheet by way of O1-H1Á Á ÁO4 i bonds (Fig. 2). This results in distinctive R 6 6 (32) supramolecular loops (Bernstein et al., 1995). The O2-H2 hydroxyl group forms a hydrogen bond to a water molecule O atom. In turn, the water molecule acts as a donor for two more O-HÁ Á ÁO interactions, to result in a three-dimensional network (Fig. 3).
O-bound H atoms were located in a difference map and their positions were refined with the distance restraint O-H = 0.85 (2) Å . U iso (H) was set equal to 1.2U eq (O). C-bound H atoms were placed in idealized positions, with C-H = 0.95 Å , and refined as riding, with U iso (H) = 1.2U eq (C).

Figure 3
Unit-cell contents for (  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.