Molecular interaction between naphthalene and 2,4-dinitrophenol in solid state

Article abstract of DOI:10.1080/15421400701693484

Kinetics of the solid-state reaction between naphthalene and 2,4-dinitrophenol have been studied at different temperatures (30-50C), and it was found that naphthalene diffuses toward 2,4-dinitrophenol. When naphthalene enters into the crystal lattice of 2,4-dinitrophenol, a 1:1 addition compound is formed with an orthorhombic crystal lattice, and the unit cell volume is much higher than the sum of the unit cell volumes of the components. As a result of this, cracks are formed and the reaction goes to completion. Computer calculations have shown that charge-transfer interaction occurs between the two components during the reaction.

Full text of DOI:10.1080/15421400701693484

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Molecular Interaction between
Naphthalene and 2,4-
Dinitrophenol in Solid State
N. B. Singh a , A. Srivastava a , N. P. Singh b &
Ashutosh Gupta b
a Department of Chemistry , Deen Dayal Upadhya
Gorakhpur University , Gorakhpur, India
b Department of Chemistry , Udai Pratap Post
Graduate Autonomous College , Varanasi, India
Published online: 22 Sep 2010.
To cite this article: N. B. Singh , A. Srivastava , N. P. Singh & Ashutosh Gupta
(2007) Molecular Interaction between Naphthalene and 2,4-Dinitrophenol
in Solid State, Molecular Crystals and Liquid Crystals, 474:1, 43-54, DOI:
10.1080/15421400701693484
To link to this article: http://dx.doi.org/10.1080/15421400701693484
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Mol. Cryst. Liq. Cryst., Vol. 474, pp. 43–54, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701693484
Molecular Interaction between Naphthalene
and 2,4-Dinitrophenol in Solid State
N. B. Singh
A. Srivastava
Department of Chemistry, Deen Dayal Upadhya Gorakhpur University,
Gorakhpur, India
N. P. Singh
Ashutosh Gupta
Department of Chemistry, Udai Pratap Post Graduate Autonomous
College, Varanasi, India
Kinetics of the solid-state reaction between naphthalene and 2,4-dinitrophenol
have been studied at different temperatures (30–50ꢀC), and it was found that
naphthalene diffuses toward 2,4-dinitrophenol. When naphthalene enters into
the crystal lattice of 2,4-dinitrophenol, a 1:1 addition compound is formed with
an orthorhombic crystal lattice, and the unit cell volume is much higher than
the sum of the unit cell volumes of the components. As a result of this, cracks
are formed and the reaction goes to completion. Computer calculations have shown
that charge-transfer interaction occurs between the two components during the
reaction.
Keywords: 2,4-Dinitrophenol; interaction; kinetics; naphthalene; solid state reactions
INTRODUCTION
Organic solid-state reactions are important both from a synthetic and
mechanistic point of view. During recent years, a large number of
such reactions have been studied [1]. Reaction products and the mech-
anism of reactions depend very much on the nature of interactions.
Naphthalene is a planar and symmetrical molecule with zero dipole
moment, whereas 2,4-dinitrophenol is unsymmetrical and dipolar.
During a solid-state reaction, a very weak interaction would be
Address correspondence to N. B. Singh, Department of Chemistry, Deen Dayal Upadhya
Gorakhpur University, Gorakhpur 273 009, India. E-mail: dr_n_b_singh@rediffmail.com
43

44
N. B. Singh et al.
expected between the two components. Weak interactions control the
reaction kinetics in the solid state. For this reason, kinetics of the
solid-state reaction between naphthalene and 2,4-dinitrophenol has
been investigated, and the mechanism is discussed in this article.
EXPERIMENTAL
2,4-Dinitrophenol was purified by repeated crystallization from hot
water. The purified sample melts at 114ꢀC. Naphthalene was purified
by distillation under reduced pressure. The melting point of the pure
sample of naphthalene was 80ꢀC.
Preparation of Reaction Product
From Solution
Solutions of 2,4-dinitrophenol and naphthalene in benzene were
mixed in a 1:1 molar ratio. A light-brown-coloured reaction product
precipitated, which was dried and stored in a desiccator. The reaction
product melts at 94ꢀC.
By Solid-State Reaction
Solid powder of 2,4-dinitrophenol and naphthalene in equimolar
amounts were ground together in an agate mortar with pestle. The
entire mass was then kept in an incubator at 50ꢀC for a week with
occasional grinding. The reaction product melts at 94ꢀC.
Kinetic Studies of Reaction between Naphthalene
and 2,4-Dinitrophenol in Solid State
Capillary Technique
The solid-state reaction between naphthalene and 2,4-dinitrophenol
was studied by the capillary technique [2]. A glass capillary sealed at
one end was half-filled with 2,4-dinitrophenol (particle size < 350
mesh). For uniform packing of 2,4-dinitrophenol, each capillary was
tapped for 5 min. The surface was made smooth by a thin glass
rod. The remaining half of the capillary was filled with naphthalene
(particle size < 350 mesh). At the junction of the two reactants, the
reaction started with a color change (light brown). The kinetics was
followed by measuring the thickness of the product layer at different
intervals of time with the help of a traveling microscope. The experi-
ment was performed at 30, 35, 40, and 50ꢀC.

Naphthalene and 2,4-Dinitrophenol
45
Attempts were made to study the kinetics by creating air gaps of dif-
ferent lengths between the components at 60ꢀC. The reaction started
at the surface of 2,4-dinitrophenol, but only a thin layer of the product
was formed, the thickness of which did not show measurable change,
and hence kinetic studies could not be made.
Gravimetric Technique
The gravimetric study was performed in a way described earlier [3].
A known amount of 2,4-dinitrophenol was taken in a separate glass
tube fitted with a B19 female joint and was connected with another
tube fitted with B19 male joint containing naphthalene. The assembly
was kept in an incubator maintained at constant temperature. The
vapors of naphthalene started reacting with solid 2,4-dinitrophenol,
forming light-brown-colored product. The kinetics was studied by mea-
suring the change in weight of the tube containing 2,4-dinitrophenol
at different intervals of time. The experiments were carried out at
different temperatures.
Microscopic and Microphotographic Studies
A small amount of 2,4-dinitrophenol was placed on a glass slide at
116ꢀC and allowed to melt. The cover slip was glided over the melt
and allowed to undercool. Nucleation started from one side, and care
was taken to have unidirectional freezing. The microphotograph was
taken with the help of camera attached to the microscope at a suitable
magnification. In a small glass beaker, solid naphthalene was heated
at 60ꢀC so that the beaker was full of vapor. The slide of 2,4-dinitro-
phenol was kept on the top of the beaker. 2,4-Dinitrophenol reacted
with the vapors of naphthalene, and the change in microstructure
was photographed as a function of time.
Study of Phase Diagram
Phase-diagram studies were made by the thaw melt method [4].
Powder X-ray Diffraction Studies
Powder X-ray diffraction (XRD) pattern of the reaction product obtained
from the solid-state reaction was recorded at Regional Sophisticated
Instrumentation Center (RSIC), Nagpur, using CuKa radiation.
UV-Visible Spectra
UV-visible spectra of the components and the mixture in 1:1 molar
ratio were recorded in methanol.

46
N. B. Singh et al.
Electronic Structure Calculations
Gaussian 03 [5] program suite was used for the electronic structure
calculations. Geometries of the complex under investigation were fully
optimized at the RHF level of theory using a 6-31G (d,p) basis set. The
interaction energy (DE) was computed using the supermolecule approach,
DE ¼ EAB ꢁ EA ꢁ EB;
where E is the energy of a molecule in a particular geometry.
The electrostatic potential maps were generated for an isosurface of
electron density for individual molecules as well as for the complex
using Gauss View [6]. The blue color denotes regions of strong positive
potential, and the red color denotes the regions of strong negative
potential. The other colors, namely green and yellow, represent inter-
mediate values of electrostatic potential. Because there are a large
number of geometries under consideration for different molecules
and complexes, the scales are not shown in each case.
RESULTS AND DISCUSSION
The reaction product obtained from solution and solid-state reactions
have the same melting point (94ꢀC), and the elemental analysis shows
that naphthalene reacts with 2,4-dinitrophenol in a 1:1 molar ratio.
The phase diagram studies (Fig. 1) show the formation of a 1:1
FIGURE 1 Solid–liquid equilibrium data of 2,4-dinitrophenol–naphthalene
system.

Naphthalene and 2,4-Dinitrophenol
47
addition compound. Because the maxima is flat, it appears that the
addition compound is stable in the solid state and dissociated in the
solution or molten state.
When solid powders of the two reactants (naphthalene and
2,4-dinitrophenol) were kept in contact in a glass capillary, a light-
brown-colored reaction product was formed at the surface of
2,4-dinitrophenol, the thickness of which increased with time. It was
inferred that naphthalene molecules diffused toward 2,4-dinitrophenol.
The kinetics was studied by measuring the thickness of the product
layer (n) as a function of time. The kinetic data obeyed Eq. (1):
n ¼ k1tn
ð1Þ
where n is the thickness of the product layer at any time t, k1 is the
apparent rate constant, and n is another constant. When log n is
plotted against log t, straight lines are obtained (Fig. 2), indicating
the validity of Eq. (1). From the intercepts and the slope of the
lines, the values of k1 and n were calculated and are given in Table 1.
Equation (1) is empirical in nature, and the value of n does not have
much theoretical background.
When log k1 was plotted against 1=T, a straight line was obtained
showing the validity of the Arrhenius equation. The energy of acti-
vation was found to be 26.4 kJ=mol. The diffusion of naphthalene
molecules toward 2,4-dinitrophenol may take place via vapor phase,
grain boundary, defects, or bulk and surface migration. Because the
energy of activation is very low, the contributions of grain boundary,
defects, and bulk diffusion might be very small. The process of dif-
fusion might take place via vapor phase and surface migration. The
heat of sublimation of naphthalene is 63 kJ=mol, which is much higher
than the energy of activation. Therefore, the diffusion is not a vapor-
phase-dominated process. Further, when the two reactants were sepa-
rated by air gaps of different length, the reaction started at the surface
of 2,4-dinitrophenol, but the increase in the thickness of the product
layer was very small. This clearly shows that the diffusion is not
taking place exclusively via the vapor phase. Because the energy of
activation for the solid-state reaction is much less than the heat of sub-
limation of naphthalene, it appears that diffusion occurs through some
easier method. Because the gliding of organic molecules [7] over the
surface of glass is not uncommon, it may also glide over the surface
of 2,4-dinitrophenol. Thus diffusion may also occur via surface
migration. In the capillary method, kinetics is studied only by seeing
the color change at the surface. Therefore, the inference in favor of
surface migration is more probable. Further, when k1 was plotted
against r (r ¼ radius of particles), a straight line is obtained indicating

48
N. B. Singh et al.
FIGURE 2 Verification of Eq. (1).
that as the particle size increased, the rate of surface migrations is
increased. The values are given in Table 2. This supports the idea that
naphthalene might be diffusing toward 2,4-dinitrophenol via surface
migration. The surface migration is slow because the 2,4-dinitrophenol
molecule is neither planar nor symmetrical. It has a very high value
of dipole moment (lcis ¼ 3.09 D, ltrans ¼ 5.51 D) [8], and as a result
it hinders the surface migration of naphthalene molecules over the
surface of 2,4-dinitrophenol.

Naphthalene and 2,4-Dinitrophenol
49
TABLE 1 Kinetic Parameter of Eq. (1)
Temperature (ꢀC)
k1 (cm=h)
n
30
35
40
50
0.012 ꢂ 0.001
0.014 ꢂ 0.001
0.017 ꢂ 0.002
0.022 ꢂ 0.002
0.33 ꢂ 0.02
0.32 ꢂ 0.01
0.29 ꢂ 0.01
0.28 ꢂ 0.01
When the kinetics of the reaction between naphthalene (vapors)
and 2,4-dinitrophenol (solid) were studied gravimetrically, the kinetic
data obeyed Eq. (2):
DW ¼ k2t;
ð2Þ
where DW is the increase in weight at any time t and k2 is rate con-
stant. The reactions at 60 and 70ꢀC go to completion after 936 and
816 h respectively, and show that the reaction between naphthalene
and 2,4-dinitrophenol occurs in a 1:1 molar ratio. When DW was
plotted against t, straight lines are obtained, showing the validity of
Eq. (2). The energy of activation for the reaction determined by this
technique was found to be 35 kJ=mol. This value is nearly one and half
times higher than the value obtained by the capillary technique.
Because the reaction goes to completion, it appears that the reaction
product is porous or cracks and voids are formed in the product
layer as a result of volume changes.
The XRD lines for the 1:1 addition compound formed between naph-
thalene and 2,4-dinitrophenol were indexed for an orthorhombic
system. The lattice parameters and unit cell volumes are given in
Table 3. When naphthalene molecules enter into the crystal lattice
of 2,4-dinitrophenol, the following possibilities may occur:
(i) When there is no interaction between the components, the unit
cell volume of the addition compound will be sum of the unit cell
volumes of the components.
TABLE 2 Effect of Particle Size on the Rate Constant
Particle-size (mesh)
r (cm)
k1 (cm=h)
140–200
200–240
240–300
300–350
0.0424
0.0374
0.0346
0.0300
0.39
0.34
0.32
0.29

50
N. B. Singh et al.
TABLE 3 Crystal Data
Compound
Crystal data
˚
Reference
˚
Naphthalene
Monoclinic, a ¼ 8.010 A, b ¼ 5.884 A,
9
9
3
˚
˚
c ¼ 8.536 A z ¼ 2, V ¼ 402.31 A
˚
˚
2,4-Dinitrophenol
Orthorhombic, a ¼ 6.122 A, b ¼ 23.290 A,
3
˚ ˚
c ¼ 5.189 A z ¼ 4, V ¼ 739.85 A
˚
˚
Naphthalene–
2,4-dinitrophenol
Orthorhombic, a ¼ 23.042 A, b ¼ 11.968 A,
3
˚ ˚
c ¼ 6.762 A z ¼ 6, V ¼ 1864.73 A
(ii) When there is some sort of interaction between the reacting
components, the unit cell volume of the addition compound may
be higher or lower than the sum of the unit cell volumes of the
components.
One can write it as
Udiff ¼ UN:DNP ꢁ ðUN þ UDNPÞ;
ð3Þ
where Udiff, UN:DNP, UN, and UDNP are the difference of the unit cell
volumes, unit cell volumes of naphthalene–2,4-dinitrophenol addition
compound, naphthalene, and 2,4-dinitrophenol respectively. On
putting the numerical values in Eq. (3), one gets the value
3
3
3
3
ꢀ
ꢀ
ꢀ
ꢀ
Udiff ¼ 1864:73 A ꢁ ð402:31 A þ 739:85 A Þ ¼ 722:57 A
There is an increase in the unit cell volume as a result of interaction
between the two components. As a result of this, there is a possibility
of formation of cracks and voids during the course of the reaction.
The reaction product is a fine crystalline material having different
morphology than those of the reactants. It appears that in the crystal
structure of 2,4-dinitrophenol, there are appropriate cavities where
the naphthalene molecule can enter easily, resulting in the separation
of reaction product. For completion of the reaction, naphthalene mole-
cules must enter into the crystal lattice of 2,4-dinitrophenol, which is
possible only when the diffusion occurs through defects, and as such it
will involve higher activation energy.
Further, the microstructure of 2,4-dinitrophenol is shown in Fig. 3a.
When this was exposed to vapors of naphthalene at 60ꢀC for 10 min,
the microstructure changed (Fig. 3b). It appears that naphthalene
vapors react rapidly on the surface of 2,4-dinitrophenol, forming a very
thin layer of reaction product, which cracks. When the slide with
2,4-dinitrophenol is exposed to the vapors of naphthalene for 1 h, the

Naphthalene and 2,4-Dinitrophenol
51
FIGURE 3 (a) Microstructure of 2,4-dinitrophenol, ꢃ150. (b) Microstructure
of 2,4-dinitrophenol exposed to the vapors of naphthalene for 10 min at 60ꢀC,
ꢃ150. (c) Microstructure of 2,4-dinitrophenol exposed to the vapors of naph-
thalene for 1 h at 60ꢀC, ꢃ150.

52
N. B. Singh et al.
FIGURE 4 Optimized geometry of the complex formed between naphthalene
and 2,4-dinitrophenol.
microstructure changed completely and the reaction product appeared
in the form of small particles having a rod-type morphology (Fig. 3c).
This indicates the possibility of structural change during the solid-
state reaction.
The optimized geometry of the complex formed between naphtha-
lene and 2,4-dinitrophenol is shown in Fig. 4, which clearly shows that
there is an alternate stacking of the two molecules but that the two
planes containing naphthalene and 2,4-dinitrophenol respectively
are not parallel to each other. There is a possibility of van der Waals
or charge-transfer interaction between the two molecules.
Molecular electrostatic potential surface (MESP) of naphthalene,
2,4-dinitrophenol, and the complex naphthalene–2,4-dinitrophenol
were generated (Figs. 5–7). On comparing the top view of
FIGURE 5 Molecular electrostatic potential surface (MESP) of naphthalene.
(See COLOR PLATE I)

Naphthalene and 2,4-Dinitrophenol
53
FIGURE 6 Molecular electrostatic potential surface (MESP) of 2,4-dinitro-
phenol. (See COLOR PLATE II)
naphthalene–2,4-dinitrophenol complex MESP (in which naphthalene
is seen from above, (Fig. 7b) with naphthalene top view MESP (Fig.
5b), the color change is noticeable. In Fig. 7b, the color is yellow above
the ring, implying lack of electron density, whereas in Fig. 7b, the
color is dark red above the ring, which implies that electron density
is high. It should be noted that when electron density is high then it
is shown by red color, and if there is less electron density, it is shown
by blue color. Such observation implies that electron density is getting
transferred from naphthalene moiety to 2,4-dinitrophenol, showing
the formation of the charge-transfer complex. The same conclusion
can be obtained if Figs. 7f and 6g are compared.
In Fig. 7f, the naphthalene–2,4-dinitrophenol complex is seen such
that the plane of 2,4-dinitrophenol lies above. The color of the MESP is
less blue with a few areas of green color. Such color pattern is not
present in Fig. 6g (isolated 2,4-dinitrophenol). Some areas of green
color in Fig. 7f imply that electron density has increased because of
transfer of electrons from naphthalene to 2,4-dinitrophenol. Thus it
FIGURE 7 Molecular electrostatic potential surface (MESP) of complex
naphthalene–2,4-dinitrophenol. (See COLOR PLATE III)

54
N. B. Singh et al.
can be seen that electron-charge transfer complex has formed between
naphthalene and 2,4-dinitrophenol. The calculated interaction energy
for charge-transfer complex is ꢁ2.80 kcal=mol [RHF=6-31G(d,p) for
optimized geometry]. This shows the possibility of charge-transfer
interaction between the two components in the addition complex.
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[2] Rastogi, R. P., Bassi, P. S., & Chaddha, S. L. (1963). J. Phys. Chem., 67, 2569.
[3] Rastogi, R. P. & Singh, N. B. (1966). J. Phys. Chem., 70, 3315.
[4] Rastogi, R. P. & Verma, K. T. R. (1956). J. Chem. Soc., 2097.
[5] Frisch, M. J. (2003). Gaussian 03, Revision A. 1, Gaussian, Inc., Pittsburgh, PA.
[6] Dennington II, R. (2003). Gauss view 03, R. Gilliland, Semichem, Inc., Shawner
Mission, KS.
[7] Rastogi, R. P. Singh, N. B., & Singh, R. P. (1977). Ind. J. Chem., 941.
[8] Richards, J. H. & Walker, S. (1961). Trans. Faraday Soc., 57, 406.
[9] JCPDS-International Center of Diffraction Data, 1996.