2-sec-Butyl-1-(2-hydroxyethyl)-1H-benzimidazole-5-carboxylic acid

In the title compound, C14H18N2O3, the carboxylic group is tilted by 12.00 (4)° with respect to the mean plane throught the benzimidazole ring system. The alcohol and carboxyl hydroxy groups are involved in intermolecular O—H⋯O and O—H⋯N hydrogen bonds, forming a two-dimensional network extending parallel the ab plane. The network is further stabilized by weak C—H⋯O interactions. The sec-butyl group is disordered over two sets of sites with refined occupancies of 0.484 (4) and 0.516 (4).


Related literature
The title molecule ( Fig. 1) is benzimidazole carboxylic acid derivative and is similar to other benzimidazole ethyl ester (Arumugam et al., 2011;Hamzah et al., 2012) derivatives. The benzimidazole ring system is essentially planar, with atom C2 deviating 0.040 (1) Å from its mean plane. The dihedral angle it forms with the carboxylic group is 12.00 (4)°. The bond lengths (Allen et al., 1987) and angles are in normal ranges. The atoms of the sec-butyl group (C12/C13/C14) are disordered over two sets of sites, with refined occupancies of 0.484 (4) and 0.516 (4).

Experimental
To a solution of 2-sec-butyl-1-(2-hydroxy-ethyl)-1H-benzimidazole-5-carboxylic acid ethyl ester (136 mg, 0.47 mmol) in THF (2 ml) was added NaOH (4 N, 0.5 ml). The mixture was refluxed at 66 °C untill all reactants fully converted to the desired acid. The progress of the reaction was monitored by TLC (EtOAc/hexane 4:1 v/v). Upon completion, THF was removed under pressure and the mixture acidified using 2 M HCl to raise pH to 6-7. Removal of excess water in vacuo gave a white precipitate which was later dissolved in butanol to separate the carboxylic acid and the salt. The filtrate was again evaporated under reduced pressure to obtain the crude product. The title compound was recrystallized using MeOH to afford colourless single crystals.

Refinement
X-ray data were collected at 100 K (Cosier & Glazer, 1986). Hydroxyl H-atom [O2-H2 = 0.92 (2) Å and O3-H3 = 0.94 (4) Å] were located in a difference Fourier map and refined freely. The remaining H atoms attached to C atoms were fixed geometrically and refined as riding model with C-H= 0.95-1.00 Å and with U iso (H)=1.2 or 1.5U eq (C). A rotating group model was applied to the methyl groups. The C12, C13 and C14 atoms of the sec-butyl group are disordered over two sites and refined with site occupancies of 0.484 (4) and 0.516 (4). The disordered atoms were refined with the C-C distances restrained to be 1.54 (1) Å.

Figure 1
The molecular structure of the title compound, with displacement ellipsods drawn at the 50% probability level. Both disorder component are shown. Atoms labelled with suffix X denote the major component of disorder.

Figure 2
The molecular packing of the title compound viewed down the c axis. The minor disorder components have been omited for clarity.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open=flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (