2336 Nithya et al.
Asian J. Chem.
hexagonal crystals. From the benzoin, about 1 g was taken
and treated with conc. nitric acid by heating in a water bath
inside a fume cupboard for about 3 h until it is free from the
smell of nitrates. It is then cooled and the obtained benzil is
recrystallized using hot ethanol. Pure crystals of benzil 1a
separate out (Scheme-I). The yield is about 70-80 %. The
melting point of the benzil was found to be 80.2 °C. The other
polymorph 1b of the crystal is obtained by recrystallization
using THF. The melting point of the benzil was found to be
113.8 °C. The molecular formula and structure of benzil was
determined by IR, NMR and mass spectral analysis. The thermal
behaviour of crystal was analyzed using thermogravimetric
analysis. The structure of the crystal was confirmed by single
crystal XRD analysis for compound 1a and 1b.
(a)
Crystal structure determination: Single crystal X-ray
diffraction data were collected at room temperature (25(2) °C),
using monochromatized MoKα radiation (0.71073 Å), on a
Bruker axs kappa apex2 CCD diffractometer for forms 1a and
1b. The structures were solved using program SIR92 [15] and
refined using the full-matrix LS procedure with SHELXL-97
[16].
TG-DTA studies were carried out with a NETZSCH STA
409 C/CD instrument. Samples weighing 3-5 mg were heated
in opened platinum pans at a rate of 10 K/min under nitrogen
gas flow of 40 mL/min.
(b)
Fig. 1. Ortep diagram of polymorph 1a and polymorph 1b
IR spectra were recorded on a Perkin Elmer Spectrum
RX I spectrometer with KBr discs in the 4000-450 cm-1 region.
For diffuse reflectance analysis, samples weighing approxi-
mately 2 mg were mixed with 200 mg KBr by means of an agate
mortar and pestle and placed in sample cups for fast sampling.
FT-RAMAN spectrometer is a Bruker 110/S spectrometer
with a multi RAM, stand alone model. The spectral range is
4000-50 cm-1. The laser source is Nd:YAG 1064 nm. The spectro-
meter has a large sample compartment to accommodate different
sample formats, from powder to liquid in vials.
The polymorph 1a crystallizes in monoclinic c2/c space
group and the other polymorph 1b was found to crystallize in
the monoclinic space group P21/c. Thus, the molecules exhibit
conformational polymorphism as the conformations of the
molecule are described by the different sequence of torsion
angles listed in Table-1. The title compound contains two ring
systems, viz. chloro phenyl moiety. The torsional angle for
the C=O groups were found to be C(1)-C(6)-C(7)-O(1)
144.84(15), C(5)-C(6)-C(7)-O(1) -35.29(19) and C(1)-C(6)-
C(7)-O(1) 167.7(2), C(5)-C(6)-C(7)-O(1) -12.1(3) for 1a and
1b respectively. The torsional angles between Cl(1)-C(1)-C(2)-
C(3) -177.81(12) and Cl(1)-C(1)-C(2)-C(3) -178.27(17) for
1a and 1b shows a wide difference denoting the presence of
polymorphism. ORTEP diagram of the complex is presented
in Fig. 1 and crystal packing diagram is illustrated in Fig. 2.
Polymorphic structures stabilized by strong C=O inter-
actions and close packing were considered. The compound
2,2’-dichloro benzil has distance angle scatter plots of C=O
bonds extracted from the Cambridge Structural Database
indicate that the polymorphs with a large number of symmetry
independent molecules (high Z’) generally have better
interactions when compared with the polymorphs with lower
RESULTS AND DISCUSSION
Synthesis and characterization: The polymorph 1a and
1b were obtained by recrystallization using ethanol and THF
respectively. The melting point of the benzil 1a and 1b were
found to be 80.2 and 113.8 °C respectively. Both polymorphs are
insoluble in water, but soluble in ethanol, chloroform and
acetone. Crystals of both compounds were found to be yellow
cubes. Compound 1a crystallizes in monoclinic c2/c space
group and 1b in monoclinic P21/c space group.
Crystal structures: The structures of polymorph 1a and
1b were determined and the crystallographic data viz., selected
bond distances, bond, torsion angles and refinement are pre-
sented in Table-1.
Cl
Cl
O
O
C
H
Conc. HNO3
KCN
C
C
O
2
CHO
C
OH
Cl
2,2'-Dichloro benzil
Cl
2-Chloro benzaldehyde
Cl
2,2'-Dichloro benzoin
Scheme-I: Preparation of 2,2’-dichloro benzil
Nithya
