Redetermination of 3-methylbenzoic acid

The asymmetric unit of the title compound, C8H8O2, contains two crystallographically independent molecules, which form dimers linked by O⋯H—O hydrogen bonds. The benzene rings in the dimers are inclined at a dihedral angle of 7.30 (8)° and both methyl groups display rotational disorder. This redetermination results in a crystal structure with significantly higher precision than the original determination [Ellas & García-Blanco (1963 ▶). Acta Cryst. 16, 434], in which the authors reported only the unit-cell parameters and space group, without any detailed information on the atomic arrangement. In the crystal, dimers are connected by weak C—H⋯O interactions, forming R 2 2(10) and R 4 4(18) rings along [110] and an infinite zigzag chain of dimers along the [001] direction also occurs.

RMF is grateful to the Spanish Research Council (CSIC) for the use of a free-of-charge licence to the Cambridge Structural Database. RMF also thanks the Universidad del Valle, Colombia, and the Instituto de Química de Sã o Carlos, USP, Brazil for partial financial support.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SJ5096).

organic compounds
Redetermination of 3-methylbenzoic acid R. Moreno-Fuquen, R. De Almeida Santos and A. R. Kennedy

Comment
The title compound (I) was investigated in a continuation of our studies on the formation of molecular complexes from N-oxide derivatives with different hydrogen bond donors (Moreno-Fuquen et al., 1997, 2009, 2011. The structure of a similar molecule was taken to compare with the title molecule (Barcon et al., 1997). A perspective view of the dimeric hydrogen-bonding in (I), showing the atomic numbering scheme, is given in Figure 1. Both molecules of m-toluic acid are held together by intermolecular O···H-O hydrogen bonds of moderate character (Desiraju & Steiner, 1999). Indeed, carbonylic O4 and O2 atoms are linked to O1 and O3 atoms with O···O distances of 2.6149 (16) and 2.6205 (16)Å respectively. The two aromatic rings in (I) form a dihedral angle of 7.30 (8)°. Both methyl groups display rotational disorder. The rotamer ratio found in this modeling was 50:50. In (I), Fig. 1, bond lengths and bond angles are in normal ranges (Allen et al., 1987). The title crystal structure also exhibits other weak C-H···O interactions (see Table 1, Nardelli, 1995). Indeed, in a first substructure, atom C7 acts as hydrogen bond donor to O1 i in the molecule at (-x + 1,-y + 1,-z) and the C16 atom acts as hydrogen bond donor to O1 ii in the molecule at (-x + 2,-y + 2,-z). The propagation of these interactions generates rings with graph-set notation R 2 2 (10) and R 4 4 (18) (Etter, 1990), running along the [110] direction (see Fig. 2). In a second substructure, atom C14 acts as a hydrogen bond donor to O2 iii in the molecule at (x,-y + 1/2 + 1,+z -1/2). The propagation of this interaction forms an infinite zigzag chain of dimers along the [001] direction (see Fig. 3). All of these interactions in the [110] and [001] directions define the bulk structure of the crystal.
Experimental 3-Methylbenzoic acid (0.545 g, 4 mmol) (Aldrich) was disssolved in ethanol (200 ml). The solution was left to evaporate slowly at room temperature. After three days, colourless crystals of a good quality suitable for X-ray analysis were obtained.