2-Bromomaleic acid

The title compound, C4H3BrO4, was obtained from a solution of meso-2,3-dibromosuccinic acid and vanadium(IV) oxide. The crystals are isostructural with chloromaleic acid and the molecule has two geometrically different carboxyl groups, one of which has delocalized C—O bonds and is essentially coplanar with the olefinic bond plane [give dihedral angle 15.08 (16)°], whereas the other has a localized C=O bond and forms a dihedral angle of 99.6 (3)° with the C=C bond plane. Two symmetry-independent O—H⋯O hydrogen bonds link the molecules into layers parallel to the bc plane.

The title compound, C 4 H 3 BrO 4 , was obtained from a solution of meso-2,3-dibromosuccinic acid and vanadium(IV) oxide. The crystals are isostructural with chloromaleic acid and the molecule has two geometrically different carboxyl groups, one of which has delocalized C-O bonds and is essentially coplanar with the olefinic bond plane [give dihedral angle 15.08 (16) ], whereas the other has a localized C O bond and forms a dihedral angle of 99.6 (3) with the C C bond plane. Two symmetry-independent O-HÁ Á ÁO hydrogen bonds link the molecules into layers parallel to the bc plane.

Comment
During the ongoing investigation of the bromo-substituted dicarboxylic acids with four carbon atoms, our group tried to prepare and characterize pure acids as well as their metal salts. One of these syntheses involved the reaction of vanadium(IV) oxide with meso-dibromosuccinic acid. It had been shown earlier (Fischer, 2006), that hydrogen bromide can easily be eliminated from racemic 2,3-dibromosuccinic acid, yielding 2-bromofumaric acid. Elimination of hydrogen bromide from meso-2,3-dibromosuccinic acid does not occur as easily. However in the presence of strong bases and at elevated temperature, elimination is observed; it yields 2-bromomaleic acid, whose structure is described here.
There are two symmetry independent O-H..O bonds (Table 1), one of which involves delocalized carboxyl group and is responsible for formation of dimeric centrosymmetric motives traditional to carboxylic acid crystal structures. Another H-bond involves non-symmetric carboxylic group and further links dimeric aggregates into layers parallel to the bc-plane (Fig. 2).
Experimental 89 mg of VO 2 (AlfaAesar, 99%), was added to a solution of 270 mg of meso-dibromosuccinic acid (Sigma Aldrich, 98%) in 4.2 ml of demineralized water. Upon heating to 90°C, vanadium oxide got dissolved, yielding a dark-blue solution, which was put aside for evaporation. Within a week, colourless crystals of the title compound were obtained.

Refinement
H atoms could be located in the Fourier map, however, their isotropic refinement did not yield satisfactory X-H distances.
Therefore, H atoms were placed at calculated positions with d(C-H)=0.93 Å, d(O-H)=0.82 Å and included in the subsequent refinement in riding motion approximation with U iso =1.2U eq of the carrier atom (1.5 U eq for hydroxyl H atoms).   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 Rfactors(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.