A polymorph of 2,4-dinitrophenylhydrazine

The crystal structure of a previously unreported polymorph (form II) of 2,4-dinitrophenylhydrazine (DNPH), C6H6N4O4, was determined at 90 K. The first polymorph (form I) is described in the monoclinic space group P21/c [Okabe et al. (1993 ▶). Acta Cryst. C49, 1678–1680; Wardell et al. (2006 ▶). Acta Cryst. C62, o318–320], whereas form II is in the monoclinic space group Cc. The molecular structures in forms I and II are closely similar, with the nitro groups at the 2- and 4-positions being almost coplanar with the benzene ring [dihedral angles of 3.54 (1) and 3.38 (1)°, respectively in II]. However, their packing arrangements are completely different. Form I exhibits a herringbone packing motif, whereas form II displays a coplanar chain structure. Each chain in form II is connected to adjacent chains by the intermolecular interaction between hydrazine NH2 and 2-nitro groups, forming a sheet normal to (101). The sheet is stabilized by N—H⋯π interactions.

The crystal structure of a previously unreported polymorph (form II) of 2,4-dinitrophenylhydrazine (DNPH), C 6 H 6 N 4 O 4 , was determined at 90 K. The first polymorph (form I) is described in the monoclinic space group P2 1 /c [Okabe et al. (1993). Acta Cryst. C49, 1678-1680; Wardell et al. (2006). Acta Cryst. C62, o318-320], whereas form II is in the monoclinic space group Cc. The molecular structures in forms I and II are closely similar, with the nitro groups at the 2-and 4-positions being almost coplanar with the benzene ring [dihedral angles of 3.54 (1) and 3.38 (1) , respectively in II]. However, their packing arrangements are completely different. Form I exhibits a herringbone packing motif, whereas form II displays a coplanar chain structure. Each chain in form II is connected to adjacent chains by the intermolecular interaction between hydrazine NH 2 and 2-nitro groups, forming a sheet normal to (101). The sheet is stabilized by N-HÁ Á Á interactions.

A polymorph of 2,4-dinitrophenylhydrazine Kiichi Amimoto and Hiromitsu Nishiguchi Comment
The nature and reactivity of carbonyl group is one of the most important topics in organic chemistry. 2,4-Dinitrophenylhydrazine (DNPH) is often used as qualitative test for carbonyl groups in the field of chemical education (Brady & Elsmie 1926;Williamson et al., 2006). DNPH also produces the 2,4-dinitrophenylhydrazone derivatives, which offer a variety of functional organic dye crystals. The crystal structure (I) of DNPH at room temperature and 120 K were reported (Okabe et al. 1993;Wardell et al., 2006). In the course of our studies on the development of teaching materials for organic chemistry and novel crystalline materials of organic dyes, we have found the new polymorph (II) of DNPH.
The molecular structure in II is almost the same to that in I. The molecular structure in II adapts the planar conformation: the dihedral angles of nitro groups at the 2-and 4-positions to the benzene ring are 3.54 (1)° and 3.38 (1)°, respectively.
In both I and II, there is an intermolecular interaction between hydrazine NH 2 and 4-nitro group, forming a chain structure. The distinguished difference between I and II originates their molecular arrangements in the chain structure. In

Experimental
Crystals of title polymorph II were obtained by slow evaporation with commercially available DNPH using 1,4-dioxane as solvent.

Refinement
All hydrogen atoms were found in a difference Fourier map and refined isotropically.

Special details
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.