Crystal structure of 2,4,6-trimethylbenzoic anhydride

The title compound was isolated as a by-product from a reaction incorporating 2,4,6-trimethylbenzoic acid under basic conditions.


Chemical context
Benzoic anhydrides have traditionally been used in synthetic organic chemistry for the preparation of aromatic esters, amides and carboxylic acids. Aromatic anhydrides have also been shown to be effective acylating agents (Shiina, 2004;Shiina & Nakata, 2007). The title compound has been used to trap deprotonated 3,4-epoxy-2,3,4,5-tetrahydrothiophene 1,1dioxide, forming 3-(2,4,6-trimethylbenzoyloxy)-2,3-dihydrothiophene 1,1-dioxide (Alonso et al., 2004(Alonso et al., , 2005. The synthesis of the compound we report here, 2,4,6-trimethylbenzoic anhydride (common name: mesitoic anhydride), was first published in 1941, where it was formed in the reaction between (2,4,6-trimethylphenyl)sodium and 2,4,6-trimethylbenzoic acid in the presence of pyridine (Fuson et al., 1941). Recently, several new approaches for the syntheses of symmetric acid anhydrides, including the title compound, have been reported (Kazemi et al., 2004;Li et al., 2012;McCallum & Barriault, 2015). The most recent report involves the in situ generation of a Vilsmeier-Haack reagent for the coupling of symmetric carboxylic acids (McCallum & Barriault, 2015). Due to the structural similarities between this reagent and the 1,3-dichloro-1,3-bis(dimethylamino)propenium salt used here, it is possible that the title compound was formed via a similar method in our reaction. The crystal structure that we report is the first example of a benzoic anhydride where the aryl rings are substituted with only alkyl groups, although several other substituted benzoic anhydrides are known.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. It crystallizes in the monoclinic space group C2/c with one half of the molecule uniquely present in the asymmetric unit. The two C-O bond lengths are significantly different, as would be expected for anhydrides, with lengths of 1.1934 (12) (C1-O2) and 1.3958 (11) Å (C1-O1). The C1-C2 distance is normal for an sp 2 -sp 2 bond, with a length of 1.4873 (13) Å . The second half of the molecule, which is generated by rotation about the twofold axis passing through O1 (0, y, 0.25), forms a dihedral angle of 54.97 (3) between the equivalent aromatic rings. If the planes of the two overlapping CO 2 groups are chosen instead, the dihedral angle becomes 59.30 (11) . The C-C bonds in the aromatic ring are not all statistically equivalent. Unsurprisingly, the longest C-C bonds in the ring are adjacent to the electron-withdrawing anhydride group, C2-C3 [1.4032 (13) Å ] and C2-C7 [1.4059 (13) Å ]. The remaining C-C bonds are statistically equivalent, averaging 1.3942 (8) Å . All of the C-CH 3 bond lengths are statistically equivalent with an average length of 1.5102 (8) Å .

Supramolecular features
The packing of the molecules, when viewed in projection down the a axis, forms wavy chains that run parallel to the c-axis direction (Fig. 2). Within the chains, the molecules are oriented in a alternating up and down fashion, shifting by 1 2 along [001] each time, such that they overlap slightly. There are no close stacking interactions between the phenyl rings in various planes. However, if the packing is viewed down the b axis, the C5-C6-C7-C10 fragment of one trimethylphenyl group lies directly above/below the same fragment running in the opposite direction, C10-C7-C6-C5, in the plane above/ below it.
There are short intramolecular contacts between the aromatic H atoms H4 (2.37 Å to H8A and 2.37 to H9C) and H6 (2.39 Å to H10A), and the designated methyl H atoms, which close five-membered rings in the molecule.
In the crystal, molecules are linked by weak intermolecular C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á contacts (Table 1, Fig. 3). It is notable that these contacts involve one H atom from each of the three methyl substituents on the phenyl ring. All of these contacts occur between the chains that run parallel to the c axis and not within the individual chains, thus consolidating the overall structure.

Figure 1
The molecular structure of the title compound. Only half of the molecule is crystallographically unique (labelled atoms). Displacement ellipsoids are drawn at the 50% probability level.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 2. H atoms were included in calcu-lated positions (C-H = 0.95-0.98 Å ) and refined as riding with U iso (H) = 1.2 or 1.5U eq (C).

2,4,6-Trimethylbenzoic anhydride
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.