2,5-Dimethyl-1,3-dinitrobenzene

The title compound, C8H8N2O4, was prepared via the nitration of p-xylene. The molecules are stacked along the c axis in an antiparallel manner. The two nitro groups are rotated relative to the benzene ring with dihedral angles of 44.50 (7) and 31.67 (8)°. The tilt of the nitro groups allows the formation of C—H⋯O interactions between the ring C—H and nitro groups of adjacent molecules creating puckered sheets perpendicular to the c axis. The H atoms of the methyl group in the 5-position are disordered (60° rotation) with an occupancy of 0.616 (19) for the major component. The crystal was found to be a non-merohedral twin with a twin law [−1 −0.002 0.005, 0.00031 −1 0.002, 0.118 −0.007 1] corresponding to a rotation of 180° about the reciprocal axis (001) and refined to give a minor component fraction of 0.320 (2).

The title compound, C 8 H 8 N 2 O 4 , was prepared via the nitration of p-xylene. The molecules are stacked along the c axis in an antiparallel manner. The two nitro groups are rotated relative to the benzene ring with dihedral angles of 44.50 (7) and 31.67 (8) . The tilt of the nitro groups allows the formation of C-HÁ Á ÁO interactions between the ring C-H and nitro groups of adjacent molecules creating puckered sheets perpendicular to the c axis. The H atoms of the methyl group in the 5-position are disordered (60 rotation) with an occupancy of 0.616 (19) for the major component. The crystal was found to be a non-merohedral twin with a twin law [À1 À0.002 0.005, 0.00031 À1 0.002, 0.118 À0.007 1] corresponding to a rotation of 180 about the reciprocal axis (001) and refined to give a minor component fraction of 0.320 (2).

Experimental
Data collection: APEX2 and GIS (Bruker, 2009); cell refinement: SAINT (Bruker, 2009)  Nitro-derivatives of para-xylene have been prepared as synthetic intermediates and as energetic materials. There are three possible isomers for the dinitro derivative of p-xylene, with studies showing that the major product is the 2,3-dinitro isomer, with exact amounts dependent on reaction conditions (Kobe & Hudson, 1950, Johnson & Northcott, 1967. The title compound was prepared as a solid derivative of para-xylene for a qualitative organic analysis laboratory course. The intended product was the mono-nitro derivative, but it appears the major product was the dinitro product. Large (~1 cm) needle crystals were obtained by vapor diffusion of n-pentane into a diethyl ether solution of the compound. The lower solubility of the 1,3-dinitro product relative to the other isomers (Kobe & Hudson, 1950) likely favored formation of crystals of the single isomer.
The molecules are packed along the c axis ( Fig. 2) with the rings nearly parallel to each other with an interplane angle of 0.63 (2) ° and interplane spacings (centroid to plane) of 3.648 Å and 3.659 Å. The positioning of the nitro groups enables the formation of non-conventional hydrogen bonds (Desiraju, 2005) between the aromatic C-H and nitro group oxygen atoms of adjacent molecules as illustrated in Figures 3 and 4 (for measurements see Table 1). This type of C-H···O interaction is often found in the structures of simple nitroarenes (Gagnon et al., 2007). These interactions combine to create a network of puckered sheets perpendicular the c axis.

Experimental
Concentrated nitric acid (4 ml) and concentrated sulfuric acid (4 ml) were placed in a round-bottom flask equipped with a Claisen adapter, thermometer and condenser. Approximately 4.5 ml of p-xylene was slowly added to the nitric/sulfuric acid mixture, ensuring that the internal temperature did not exceed 323-328 K. After the addition was complete, the mixture was heated for an additional 15 min at 323-328 K. The reaction mixture was cooled to room temperature, poured into 40 ml of cold water and cooled to produce the crystals of the crude nitration product.
The product was recrystallized by vapor diffusion of n-pentane into a diethyl ether solution. Large translucent needles formed after two weeks. The melting point of this crystal was determined to be 398.4 (1) K by DSC, in agreement with literature values (Johnson & Northcott, 1967, Liu et al., 2005a. unique overlapping reflections, or 37 percent overlapping reflections. The structure was solved using the non-overlapping reflections from both domains (HKLF 4). The structure was refined using corrected reflections from only the major component including overlaps (HKLF 5). Refinement produced a minor twin component fraction of 0.320 (2).
All hydrogen atoms were located in the difference map and refined with the atom positions constrained to appropriate positions with C-H distances of 0.95 Å (aromatic carbon atoms) or 0.98 Å (methyl groups). The methyl group in the 5-position was modeled as an idealized disordered methyl group with hydrogen atoms in two positions rotated 60° from each other. The occupancy for the major methyl group orientation was 0.62 (2). A riding model was used for all H atoms with U iso (H) = 1.2 times U iso (aromatic) or 1.5 times U iso (methyl carbon atoms). Fig. 1. The molecular structure of the title compound drawn with 50% probability displacement ellipsoids for non-H atoms and showing the atom labeling scheme.   Table 1.