Synthesis and characterization of novel liquid crystalline compounds containing a pyrazole ring

Article abstract of DOI:10.1080/15421406.2011.609043

We report the synthesis and liquid crystalline properties of two new homologous series of calamitic liquid crystals containing a substituted pyrazole ring within the central core of 4-(4-n-alkoxybenzoyl)-5-methyl-2-phenyl-2,4- dihydro-3H-pyrazole-3-one and 4-(4-n-alkoxybenzoyl)-5-methyl-2-(4-methylphenyl)- 2,4-dihydro-3H-pyrazole-3-one and characterization of the compounds done by elemental analysis, FT-IR, ¹H-NMR, and mass spectrometry. Phase transition behavior of these compounds was assessed with the help of polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Both series show liquid crystalline properties from the heptyl homologue. Middle members of Series-A and Series-B exhibit nemetogenic behavior and higher homologues exhibit only smectogenic behavior.

Full text of DOI:10.1080/15421406.2011.609043

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Synthesis and Characterization of Novel
Liquid Crystalline Compounds Containing
a Pyrazole Ring
B. T. Thaker a , D. B. Solanki a , B. S. Patel a , A. D. Vansadia a & Y. T.
Dhimmar a
a Department of Chemistry , Veer Narmad South Gujarat University ,
Surat , Gujarat , India
Published online: 27 Dec 2011.
To cite this article: B. T. Thaker , D. B. Solanki , B. S. Patel , A. D. Vansadia & Y. T. Dhimmar (2012)
Synthesis and Characterization of Novel Liquid Crystalline Compounds Containing a Pyrazole Ring,
Molecular Crystals and Liquid Crystals, 552:1, 134-146, DOI: 10.1080/15421406.2011.609043
To link to this article: http://dx.doi.org/10.1080/15421406.2011.609043
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Mol. Cryst. Liq. Cryst., Vol. 552: pp. 134–146, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.609043
Synthesis and Characterization of Novel Liquid
Crystalline Compounds Containing a Pyrazole Ring
B. T. THAKER,∗ D. B. SOLANKI, B. S. PATEL,
A. D. VANSADIA, AND Y. T. DHIMMAR
Department of Chemistry, Veer Narmad South Gujarat University, Surat,
Gujarat, India
We report the synthesis and liquid crystalline properties of two new homologous series of
calamitic liquid crystals containing a substituted pyrazole ring within the central core of
4-(4-n-alkoxybenzoyl)-5-methyl-2-phenyl-2,4-dihydro-3H-pyrazole-3-one and 4-(4-n-
alkoxybenzoyl)-5-methyl-2-(4-methylphenyl)-2,4-dihydro-3H-pyrazole-3-one and char-
acterization of the compounds done by elemental analysis, FT-IR, 1H-NMR, and mass
spectrometry. Phase transition behavior of these compounds was assessed with the help
of polarizing optical microscopy (POM) and differential scanning calorimetry (DSC).
Both series show liquid crystalline properties from the heptyl homologue. Middle mem-
bers of Series-A and Series-B exhibit nemetogenic behavior and higher homologues
exhibit only smectogenic behavior.
Keywords Calamitic; mesomorphic; nematic; pyrazole; smectic phase
1. Introduction
The detailed study of many mesogenic homologous series has helped to establish some
general rules for the effect of chemical constitution in the nematogenic and smectogenic
compounds [1]. There have been a variety of compounds reported with liquid crystalline
properties, but heterocyclic moieties are less explored compared to homocyclic moieties
[2]. Furthermore, heteroatoms offer the possibility of several modes of co-ordination.
Thermotropic liquid crystals are of great technological importance [3–10]. Many series of
liquid crystalline compounds contain heterocyclic groups have been synthesized because
of their interesting properties [11]. Various mesomorphic heterocyclic compounds have
been reported [12–16]. Heterocyclic compounds provide a great synthetic and structural
versatility due to the number of potential substitution positions.
Terminal substituents and potentially mesogenic systems play an important role. Pyra-
zole has proven to be very efficient as a starting material for various kinds of mesogenic
compounds. Some of these groups are highly polar and others are mildly polar [17]. The five-
membered pyrazole ring has aromatic character [18], planar conformation and a high dipole
moment perpendicular to the principle molecular axis. Indeed, some 3,5-disubstituted pyra-
zoles and 4-substituted pyrazoles have demonstrated their ability to show liquid crystalline
behavior both as themselves [19–23] and with metal co-ordination [24–27]. Calamatic
∗Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad South Gujarat
University, Surat-395007, Gujarat, India. E-mail: btthaker1@yahoo.co.in
134

Novel Pyrazole Liquid Crystals
135
mesomorphism has also been described in a number of pyrazoles derived from 4-alkoxy
substituted aromatic β-diketones [28, 29]. These compounds exhibit classical nematic and
smectic-A and C mesophases despite the fact that the type of mesophase depends on the
nature and length of the terminal chain. It has been shown that the short chains give rise
to rather narrow mesophase ranges or no mesophases at all; therefore, the intermediate
and long chains are of great interest. In an earlier study, we described the preparation and
mesomorphic study of pyrazolone derivatives as a terminal moiety [30]. Here, we present
the synthesis of new liquid crystalline compounds derived from a 4-n-alkoxy substituted
aromatic β-diketone with pyrazolone derivatives of different chain length and the study of
their thermal behavior. The general structure of the compounds is given in Scheme 1.
CH3
N
X
N
C
O
OR
H
O
Where, X= H or CH3; R= CnH2n+1, n = 2 to 8, 10, 12, 14, 16, 18
Scheme 1.
2. Experimental
2.1 Reagents
For the synthesis of compounds of the homologous series, raw materials were pur-
chased from standard chemical suppliers. 4-Hydroxy benzoic acid and alkyl bromides
were purchased from Venus Chemicals. 5-Methyl-2-phenyl-2,4-dihydro-3H-pyrazole-3-
one and 5-methyl-2-(4-methylphenyl)-2,4-dihydro-3H-pyrazole were obtained from Nutan
Dye Chem, Pandesara, Surat, and used without further purification. Acetone, ethanol,
methanol, 1,4-dioxane, HCl, KOH, Ca(OH)2, NaOH, and thionyl chloride were used after
distillation and purified using the standard method described in literature [31].
2.2 Synthesis
2.2.1. Synthesis of 4-n-alkoxy Benzoic Acids. 4-n-Alkoxy benzoic acids were prepared as
reported by Dave and Vora method [32, 33]. The melting point (m.p.) of each compound
was consistent with the reported value.
2.2.2. Synthesis of 4-n-alkoxy Benzoyl Chlorides. 4-n-Alkoxy benzoyl chlorides were pre-
pared by a reported method [34]. The m.p. of each compound was consistent with the
reported value.
2.2.3. Synthesis of 4-(4-alkoxybenzoyl)-5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one
(Series-A). 5-Methyl-2-phenyl-2,4dihydro-3H-pyrazol-3-one (0.098 mol) was taken in
dioxane (100 ml). A solution was obtained by gentle heating and stirring. Calcium hydrox-
ide (10.0 g, 0.14 mol) was then added. The reaction mixture became paste. The 4-n-alkoxy
benzoyl chloride (0.1 mol) was mixed or dissolved in 50 ml of dioxane, and was added
drop wise. The reaction mixture was then refluxed with stirring for 2 hr; during this period,

136
B. T. Thaker et al.
the bright yellow complex formed initially turned yellowish brown. The complex was then
decomposed by pouring the reaction mixture into chilled dil. HCl solution (500 ml, 3 N). A
yellowish brown paste separated first, solidified after 2–3 min, was collected over a sintered
glass funnel, and was washed with distilled water until the washings were colorless and
dried in air. The desired compounds obtained were crystallized from ethanol.
Reaction Scheme:
O
O
(i)
RO
HO
[1]
OH
OH
[2]
[3]
O
O
(ii)
RO
HO
[2]
Cl
OH
CH3
CH3
N
O
N
O
(iii)
X
N
X
3
N
+
OR
C
H
O
R = CnH2n+1, n = 2 to 8,10,12,14,16,18
X= H [Series-A];
X= CH3 [Series-B]
Reaction reagents and conditions:
(i)
KOH, RBr, MeOH or EtOH, reflux, 8--10 hr,
SOCl2, reflux, 3 hr,
(ii)
(iii)
Ca(OH)2, 1,4- dioxane, reflux, 3-4 hr.
The purity of the compounds has been checked by TLC. It shows one spot, indicating
a single compound. All the compounds were purified by column chromatography using
silica gel (100–200 mesh) and ethyl acetate: petroleum ether (7:3) solvent system.
Series-B was similarly prepared by the above method. Here, we used 5-methyl-2-(4-
methylphenyl)-2,4-dihydro-3H-pyrazole-3-one instead of 5-methyl-2-phenyl-2,4dihydro-
3H-pyrazole-3-one.
2.3 Characterization
Micro analysis was performed with a Carlo Erba 1108 C, H, N microanalyzer from RSIC
(Regional Sophisticated Instrumentation Centre), CDRI (Central Drugs Research Institute),
Lucknow. Infrared (IR) spectra for all the compounds were obtained using a Perkin–Elmer

Novel Pyrazole Liquid Crystals
137
model Specrum BX, FT-IR Spectrophotometer in KBr disc in the region 4000–400 cm−1
.
1H-NMR spectra were recorded on a BRUKER AVANCE II 400 NMR Spectrometer
in CDCl3 solutions at SAIF (Sophisticated Analytical Instrumentation Facility), Punjab
University, Chandigarh. Mass fragmentation was determined by using GC-MS Mass Spec-
trometer at SAIF (Sophisticated Analytical Instrumentation Facility), IIT Madras, Chennai.
The textures of the mesophase were studied with a Leitz Labourlux polarizing microscope
provided with a Kofter heating stage at Applied Chemistry Department, M S University of
Baroda, Vadodra. Thermal properties of the compounds were investigated by Differential
Scanning Calorimetry (DSC) using a Mettler M-3 thermobalence (Switzerland) with a
microprocessor TA-3000 at a heating rate of 10◦C/min in N2 atmosphere at SICART, V.V.
Nagar.
2.3.1 Series-A, Compound A10, Molecular Formula: C27H34O3N2. Elemental analysis:
calculated C 74.65%; H 7.83%; N 6.45%, found C 74.62%; H 7.80%; N 6.41%.
FT-IR (KBr pellet): 2852, 2919 cm−1 ( CH3 stretching), 1606 cm−1 (C N stretching
of pyrazolone), 1700 cm−1(C O stretching of acyl group), 1672 cm−1(C O stretching of
pyrazolone in keto form), 1579, 1469, 1431 cm−1 (C C ring stretching of pyrazolone),
1306 cm−1(C N stretching of pyrazolone), 1037 and 1254 cm−1(C O C symmetric and
assymetric stretching respectively).
1H-NMR (CDCl3): 0.86–0.89 ppm (t, 3H, CH3 of aliphatic chain), 1.24–1.84 ppm [m,
( CH2)n- of alkoxy chain], 4.00–4.03 ppm (t, 2H, CH2O of alkoxy chain), 3.90 ppm (s,
1H, CH proton of pyrazolone at 4-position), 2.20 ppm (s, 3H, CH3 attached to pyrazolone
ring at 5-position), 6.91–8.05 ppm (m, Ar–H), Mass (GC-MS): Molecular ion peak: m/z
−436 (M + 2)+.
2.3.2 Series-B, Compound B14, Molecular Formula: C32H44O3N2. Elemental analysis:
calculated C 76.19%; H 8.73%; N 5.55%, found C 76.15%; H 8.70%; N 5.52%.
FT-IR (KBr pellet): 2851, 2918 cm−1 ( CH3 stretching), 1606 cm−1 (C N stretching
of pyrazolone), 1700 cm−1 (C O stretching of acyl group), 1671 cm−1 (C O stretching
of pyrazolone in keto form), 1580, 1469, 1437 cm−1 (C C ring stretching of pyrazolone),
1308 cm−1(C N stretching of pyrazolone), 1020 and 1258 cm−-1(C O C symmetric and
assymetric stretching respectively).
1H-NMR (CDCl3): 0.86–0.89 ppm (t, 3H, CH3 of aliphatic chain), 1.26–1.84 ppm
[m, ( CH2)n of alkoxy chain], 4.00–4.03 ppm (t, 2H, CH2O of alkoxy chain), 3.88 ppm
(s, 1H, CH proton of pyrazolone at 4-position), 2.19 ppm (s. 3H, CH3 attached pyrazolone
ring at 5- position), 2.35 ppm (s, 3H, CH3 of substituted phenyl ring at 2-position on
pyrazolone), 6.88–8.05 ppm (m, Ar H), Mass (GC-MS): Molecular ion peak: m/z −504
(M)+.
3. Results and Discussion
In the present study, 12 homologues from the series, 4-(4-n-alkoxybenzoyl)-5-methyl-2-
phenyl-2,4-dihydro-3H-pyrazol-3-ones (Series-A), 4-(4-n-alkoxybenzoyl)-5-methyl-2-(4-
methylphenyl)-2,4-dihydro-3H-pyrazol-3-ones (Series-B) were synthesized as per syn-
thetic routes given in the scheme and their mesomorphic properties are studied. These
compounds have been subjected to elemental analysis. The elemental analysis data agree
with theoretical values as per expected structure. The FT-IR spectra of the representative
compounds (A10, B14) are shown in Fig. 1. 1H-NMR spectra of representative compounds
are shown in Fig. 2. The mass spectra of these compounds are shown in Fig. 3. The m/z

138
B. T. Thaker et al.
Figure 1. (a) FT-IR for compound A10 (Series-A). (b) FT-IR for compound B14 (Series-B).
ratios obtained from the spectra of a representative sample is matched with the molecular
ion peak.
3.1 Thermal and Phase Behavior
Mesomorphic properties and thermal stability for the new homologous series-A and B
were determined by hot stage polarizing microscopy and DSC and are given in Tables 1
and 2. The plot of the temperature versus number of carbon atoms is given in Fig. 4. Phase

Novel Pyrazole Liquid Crystals
139
Figure 2. (a) 1H NMR for compound A10 (Series-A). (b) 1H NMR for compound B14 (Series-B).
identification was based on the optical textures, and the magnitude of the isotropization on
enthalpies is consistent with the assignment of each mesophase type, using the classification
system reported by Sackmann and Demus [35], Gray and Goodby [36]. The transition
temperatures obtained from polarizing microscopy were compared with those from DSC.
The DSC curves of the different compounds are shown in Fig. 5 and their comparative

140
B. T. Thaker et al.
Figure 3. (a) Mass for compound A10 (Series-A). (b) Mass for compound B14 (Series-B).

Novel Pyrazole Liquid Crystals
141
160
140
120
100
80
Cr-Sm
Cr-N
Sm/N-I
60
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19
Number of carbon atoms in the chain
(a)
160
140
120
100
80
Cr-Sm
Cr-N
Sm/N-I
60
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19
Number of carbon atoms in the chain
(b)
Figure 4. (a) Transition temperature graph for Series-A. (b) Transition temperature graph for Series-
B.

142
B. T. Thaker et al.
Table 1. Transition temperature data of series-A
Transition temperature in ◦C
Code no.
Sm
N
I
A2
A3
A4
A5
A6
A7
A8
A10
A12
A14
A16
A18
—
—
—
—
—
—
—
—
69
66
65
59
—
—
—
—
—
73
76
83
93
—
—
—
146
141
135
129
125
123
120
90
110
75
97
95
parameters are given in Table 3. The phase transitions obtained from DSC almost match
those from optical polarizing microscope. In order to understand better why the pyrazolone
derivatives are mesogenic with alkoxy terminal phenyl ring, we must remember that the
most favorable geometry for a molecule showing calamitic mesophases is linear. All of
the molecules described here have a nonlinear core, the deviation from linearity being
variable depending on the type of compound [37]. In the present work compound 1 to
6 of both the series are not mesomorphic. This can be accounted for by considering the
short terminal chain, which does not provide the linearity and flexibility to the molecular
core [38]. In series-A, compounds A7, A8, A10 show an enantiotropic nematic phase. The
smectic mesophase starts from compound A12; this compound shows both smectic and
Table 2. Transition temperature data of series-B
Transition temperature in ◦C
Code no.
Sm
N
I
B2
B3
B4
B5
B6
B7
B8
B10
B12
B14
B16
B18
—
—
—
—
—
—
—
—
—
72
61
58
—
—
—
—
—
67
87
90
85
82
—
—
152
146
142
137
135
130
110
107
105
126
100
80

Novel Pyrazole Liquid Crystals
143
Figure 5. (a) DSC curve of compound A10 (Series-A). (b) DSC curve of compound B14 (Series-B).
nematic mesophases. It shows a smectic A phase and thread like nematic mesophase.
After, the A12 compound, compounds A14 to A18 only show a smectic C mesophase, which
is due to increasing the length of terminal alkoxy chain. The increase in the length of
alkoxy terminal chain disrupts the nematic mesomorphic stability [39]. Compounds A14
to A18 only show a smectic mesophase, which indicates that pyrazolone compounds tend
toward higher levels of structural order, perhaps due to the higher molecular symmetry and

144
B. T. Thaker et al.
Table 3. Transition temperature and DSC data of Series-A and B
Peak temp.
Code no.
A10
Transition
(Microscopic temp.) ◦C
ꢀH Jg−1
ꢀS Jg−1 k−1
Cr–N
N–I
83.29 (83)
90.69 (90)
65.00 (66)
74.80 (75)
73.38 (72)
82.83 (82)
126 (126)
32.997
14.788
—
72.006
−17.377
−26.121
—
0.0926
0.0406
—
0.2070
−0.2367
−0.3153
—
A14
B14
Cr–Sm
Sm–I
Cr–Sm
Sm–N
N–I
B16
Cr–Sm
Sm–I
61.23 (61)
100.10 (100)
26.084
48.992
0.0780
0.1313
increased molecular rigidity. In series-B, compounds B7 to B12 show a nematic phase while
B14 shows a smectic as well as a nematic phase. B16 and B18 show only a smectic phase.
The mesomorphic phase stability is greater in series-A than in series-B, in which CH3
group at para-position on the 1-phenyl ring (which is attached to pyrazoline ring). This is
because alkyl substitution gives more stability than H and also the alkyl group is more
conducive to mesomorphism. This stability order may be related to a difference in linearity
and subsequent packing of the molecules in the crystal lattice. The group efficiency derived
by Dave and Dewar [40, 41] is in the decreasing order of group polarizability.
The two homologous series made it possible to observe the effects of structural changes
on mesomorphic behavior in a novel structural system.
4. Conclusion
Pyrazolone heterocyclic rings with a terminal alkoxy phenyl ring unit appear to increase
molecular rigidity. In the entire series compounds having alkoxy chain C1 to C7 do not
exhibit liquid crystallinity; this is because the short carbon chain is not conducive to
mesomorphism, which is exhibited only in long carbon chain C8 to C18 compounds. The
presence of lone pairs of electrons on the two nitrogen atoms of the pyrazolone ring
enhances the polarization of the molecules giving more conjugation, which increase the
dipole moment of the compound. This fact would explain the high fusion and transition
temperatures, but as the length of the alkoxy terminal chain is increased, the transition
temperatures are decreased.
Acknowledgment
We thank CDRI, Lucknow, SAIF, Punjab University, Chandigarh, and SAIF, IIT Madras,
Chennai for providing the facility of Elemental analysis, 1H-NMR and Mass spectra exper-
iments. We are also thankful to Applied Chemistry Department, Faculty of technology &
engineering, MS University of Baroda, for providing the polarizing microscopy studies.
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