Benzene-1,3,5-triyl triacetate

The asymmetric unit of the title compound, C12H12O6, contains two essentially identical molecules related by a pseudo-inversion centre. The three acetoxy groups in each molecule are essentially planar and are tilted, in a regular propeller-style arrangement, with their normals oriented between 56.72 (12) and 76.35 (9)° from the normal to the mean plane of the central C6 ring; in each molecule the three carbonyl O atoms are on the same side of the C6 ring, with the Cring—O—C—Me bonds in a trans conformation. The principal intermolecular contacts appear to be C—H⋯π-ring interactions; each C6 ring has such a contact to both faces of the ring; in addition, each molecule has two intermolecular C—H⋯O contacts with H⋯O distances less than 2.55 Å.

The asymmetric unit of the title compound, C 12 H 12 O 6 , contains two essentially identical molecules related by a pseudo-inversion centre. The three acetoxy groups in each molecule are essentially planar and are tilted, in a regular propeller-style arrangement, with their normals oriented between 56.72 (12) and 76.35 (9) from the normal to the mean plane of the central C 6 ring; in each molecule the three carbonyl O atoms are on the same side of the C 6 ring, with the C ring -O-C-Me bonds in a trans conformation. The principal intermolecular contacts appear to be C-HÁ Á Á-ring interactions; each C 6 ring has such a contact to both faces of the ring; in addition, each molecule has two intermolecular C-HÁ Á ÁO contacts with HÁ Á ÁO distances less than 2.55 Å .
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HB5176).

Comment
Structural factors which enhance the solubility of organic compounds in liquid carbon dioxide are difficult to identify, but a knowledge of these is important in view of the possibility of using liquid carbon dioxide as an environmentally acceptable, cheap, safe and readily available alternative to replace organic-based solvents in the development of so-called "green chemistry". Previous studies  have shown that certain types of acyl group promote the solubilities of per-acylated D-glucopyranose derivatives in liquid carbon dioxide; in particular trimethylacetyl groups promoted solubility, their effect being comparable to acetyl groups and superior to dimethylacetyl groups. In searching for an explanation for solubility differences in this series based on differing intermolecular forces in the solid state, we conducted crystal structure studies on the compounds (Haines & Hughes, 2007), but the results indicated no substantial difference in such intermolecular forces.
Measurement of solubilities in liquid carbon dioxide of the series of 1,3,5-triacetoxybenzene (1) and substituted derivatives, viz 1,3,5-tris-(dimethylacetoxy)benzene (2) and 1,3,5-tris-(trimethylacetoxy)benzene (3), chosen in an attempt to separate the effects on solubility of the number and structure of peripheral substituents in compounds of similar overall molecular dimensions to the carbohydrate derivatives, showed no major differences (Haines, et al., 2009, unpublished results) and prompted an investigation of their crystal structures in order to compare intermolecular interactions in these compounds.
The stucture of the first compound of the series, (1) is shown in Figure 1; other compounds of the series are described in the accompanying paper (Haines and Hughes, 2009). Dimensions are available in the archived CIFs.
Compound (1) was prepared by the acylation of 1,3,5-trihydroxybenzene with acetic anhydride and formed crystals having two essentially identical molecules with very similar orientations in the cell, related by a pseudo inversion centre.
The three acetoxy groups in each molecule are essentially planar and are tilted, in a regular propeller-style arrangement, with their normals at 56.72 (12), 76.35 (9) and 60.73 (12)° from the normal to the mean-plane of the central C 6 ring in one molecule and 58.44 (11), 75.11 (13) and 63.76 (11) ° in the second molecule. In each molecule the three carbonyl O-atoms are on the same side of the C 6 ring, with the C ring -O-C-Me bonds in a trans conformation. The principal intermolecular contacts appear to be C-H···π-ring interactions: each C 6 ring has such a contact to both faces of the ring -the ring C(1-6) has H(12C a ) and H(94 b ) on opposite sides of the ring at 3.25 and 2.63 Å from the ring mean-plane, and the ring of C(91-96) is bounded by H(4 c ) and H(91B d ) at 2.65 and 3.14 Å from the ring mean-plane (the superscripts a-d indicate symmetry operations). Also, each molecule makes two intermolecular C-H···O contacts with H···O distances in the range 2.31-2.54 Å.

Experimental
The title compound was prepared by the conventional acylation of the parent 1,3,5-trihydroxybenzene and has been described previously (Hegetschweiler et al., 1990); the physical data (m.p., 1 H and 13 C NMR spectra) of our product agreed with those reported.