A novel mesomorphic homologous series of ethylene derivatives α-4-(4-n-alkoxy benzoyloxy) benzoyl β-3″-nitro phenyl ethylenes

Article abstract of DOI:10.1080/15421406.2012.698235

A novel homologous series of 11 ethylene derivatives has been synthesized and evaluated. The methoxy to pentyloxy homologues are nonmesomorphic, whereas the hexyloxy to dodecyloxy derivatives are enantiotropically smectogenic and enantiotropically nematogenic in character. The tetradecyloxy and hexadecyloxy homologues exhibit only nematogenic character without showing of any smectogenic character. An odd-even effect is not observed in either of the transition curves from a phase diagram. The average smectic and nematic thermal stabilities are 126.7C and 148.6C, respectively. The liquid crystal properties of the novel series are compared with other structurally similar compounds. The texture of the nematic phase is a threaded or Schlieren type, and that of smectic phase shows the smectic A type as determined by optical polarizing microscopy. Analytical data support the molecular structures in all cases. The novel series is predominantly nematogenic and partly smectogenic with mesophase temperature ranges between 12C and 63C. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2012.698235

Mol. Cryst. Liq. Cryst., Vol. 569: pp. 64–71, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.698235
A Novel Mesomorphic Homologous Series
of Ethylene Derivatives α-4-(4ꢀ-n-Alkoxy
benzoyloxy) Benzoyl β-3ꢀꢀ-Nitro Phenyl Ethylenes
D. M. SUTHAR1,∗ AND A. V. DOSHI2
1Chemistry Department, K. K. Shah Jarodwala Maninagar Science College, JJT
University Rajasthan, Ahmedabad, Gujarat, India
2JJT University, Jhunjhunu, Rajasthan, India
A novel homologous series of 11 ethylene derivatives has been synthesized and evalu-
ated. The methoxy to pentyloxy homologues are nonmesomorphic, whereas the hexyloxy
to dodecyloxy derivatives are enantiotropically smectogenic and enantiotropically ne-
matogenic in character. The tetradecyloxy and hexadecyloxy homologues exhibit only
nematogenic character without showing of any smectogenic character. An odd–even
effect is not observed in either of the transition curves from a phase diagram. The
average smectic and nematic thermal stabilities are 126.7◦C and 148.6◦C, respectively.
The liquid crystal properties of the novel series are compared with other structurally
similar compounds. The texture of the nematic phase is a threaded or Schlieren type,
and that of smectic phase shows the smectic A type as determined by optical polarizing
microscopy. Analytical data support the molecular structures in all cases. The novel se-
ries is predominantly nematogenic and partly smectogenic with mesophase temperature
ranges between 12◦C and 63◦C.
Keywords Mesomorphism; nematic; smectic
Introduction
Molecular rigidity and flexibility are very important factors controlling the mesomorphic
behavior of a substance. The present investigation involves the synthesis and evaluation
of a benzoester homologous series with three phenyl rings bridged through –COO– and
–CO–CH=CH– as central linking groups within the rigid core. The terminal n-alkoxy chain
and the meta substituted –NO2 group serve as flexible units. Thus, molecular rigidity and
flexibility are varied by introducing central linking groups and varying n-alkoxy terminal
chain. The meta substituted –NO2 group partly broadens the molecule, and linking groups
lengthen the molecular structure. This study will discuss structural property relationships in
terms of the effect of molecular breadth, length, polarity, and polarizability on the magnitude
of anisotropic forces of intermolecular attractions and hence on mesomorphism.
∗Address correspondence to D. M. Suthar, 98, Madhav Park Society Part-4, Opp. Radhe Chamber,
Vastral, Ahmedabad 382418, Gujarat, India. E-mail: dipakmsuthar@gmail.com
64

Novel Liquid Crystalline Nitrobenzoyl Ethylenes
65
Experimental
Synthesis
4-Hydroxybenzoic acid, n-alkyl halides, thionyl chloride, pyridine, KOH, MeOH, 4-
hydroxy acetophenone, 3-nitro benzaldehyde, and ethanol required for the synthesis were
used as received. 4-Hydroxybenzoic acid was alkylated by appropriate alkylating agents.
The 4-n-alkoxybenzoic acid and the corresponding 4-n-alkoxybenzoyl chloride (A) were
prepared by modified method of Dave and Vora [1]. α-4-Hydroxy benzoyl β-3ꢀ-nitro phenyl
ethylene (B) was prepared by an established method of Doshi and Patel [2c]. Components
(A) and (B) were condensed in dry cold pyridine by an established method [2d]. The
synthetic route to the series is outlined in Scheme 1.
Characterization
Selected representative homologues of novel series were characterized for their molecular
structure by elemental analysis, infrared (IR), and nuclear magnetic resonance (1H NMR)
spectroscopy techniques. Microanalysis was performed using a PerkinElmer PE 2400 C,
H, N analyzer. The IR spectra were recorded on a PerkinElmer spectrum GX, the 1H NMR
spectra were recorded on a Bruker instrument using CDCl3 as the solvent. The mesomorphic
properties were investigated using an optical polarizing microscope equipped with a heating
stage (Table 1).
Analytical Data
Elemental Analysis
Spectral Data.
NMR in ppm for Octyloxy Derivative. 0.880 (t, CH3 of OC8H17 chain), 1.307–1.77
(m) ((CH2)n polymethylene group of OC8H17 chain), 4.00 (t, OCH2 of OC8H17 chain),
6.92 (–CH=CH–) 7.29 (–CH=CH–CO–), 6.75–8.10 (phenyl ring). The NMR supports the
structure.
NMR in ppm for Dodecyloxy Derivative. 0.871 (t, CH3 of OC12H25 chain), 1.25–1.76
(m) ((CH2)n polymethylene group of OC12H25), 4.02 (t, OCH2 of OC12H25), 7.2
(–CH=CH–CO–), 6.9–8.10 (phenyl ring). 7.00 (–CH=CH–). The NMR supports the
structure.
Table 1. Elemental analysis for propyloxy, pentyloxy, tetradecyloxy, and hexadecyloxy
derivatives
Elements % found (% calculated)
Sr. no
Molecular formula
C
H
N
1
2
3
4
C25H21NO6
C27H25NO6
C36H43NO6
C38H47NO6
69.68 (69.60)
71.50 (71.59)
72.90 (73.85)
74.32 (74.39)
4.80 (4.87)
5.55 (5.45)
7.23 (7.35)
7.61 (7.67)
3.31 (3.25)
3.00 (3.05)
2.45 (2.39)
2.35 (2.28)

66
D. M. Suthar and A. V. Doshi

Novel Liquid Crystalline Nitrobenzoyl Ethylenes
67
IR in cm−1 for Hexyloxy Derivative. 720 ((CH2)n polymethylene group of OC6H13
chain), 680,760 (m-substituted phenyl ring), 840 (p-substituted phenyl ring), 1160,
1250,165 (ester group), 1380, 1525 (–NO2 group), 950 (–CH=CH– group). The IR supports
the structure.
IR in cm−1 for Decyloxy Derivative. 720 ((CH2)n polymethylene group of OC12H25
group), 640, 760 (m-substituted phenyl ring), 830 (p-substituted phenyl ring), 1150,1250,
1600, 1660 (ester group), 1440, 1500 (-NO2 group), 950 (–CH=CH– group). The IR
supports the structure. Texture: by miscibility. Method: Hexyloxy homologue shows a
threaded type texture and Tetradecyloxy homologue shows a schlieren type texture.
Results and Discussion
The alkoxybenzoic acids are dimeric through hydrogen bonding. However, hydrogen bond-
ing breaks and disappears on esterification by linking it with α-4-hydroxy benzoyl β-3ꢀ-nitro
phenyl ethylene (B). Dimerized n-alkoxybenzoic acids involve two phenyl rings, but, the
final compound consists of –COO– and –CO–CH=CH– linking groups and three phenyl
rings. Thus, the number of phenyl rings though increases overall by one as compared to
the dimerized n-alkoxybenzoic acid. Overall, the melting point and transition temperatures
of the resultant ester derivatives do not rise proportionately. The esterification process
of n-alkoxybenzoic acids lowers the transition temperatures due to breaking of hydrogen
bonding. The methoxy to pentyloxy homologues are nonliquid crystalline, and sharply
melt at their melting point without showing a liquid crystal mesophase. The hexyloxy
to hexadecyloxy homologues are liquid crystalline and exhibit enantiotropic smectogenic
and nematogenic phases. The tetradecyloxy and hexadecyloxy derivatives exhibit an enan-
tiotropic nematic phase without any smectic character. The transition temperatures are
shown in Table 2, and the phase diagram (Fig. 1) shows the phase behavior of the homo-
logues versus the number of carbon atoms present in n-alkoxy terminal chain. The phase
diagram indicates that the solid–isotropic or solid–mesophase curve rises and falls in a
zigzag manner with an overall falling tendency. The smectic–nematic transition curve ini-
tially falls from the hexyloxy to the octyloxy homologue, and then behaves in the usual
manner by rising up to the dodecyloxy derivative and then smectic mesophase disappears.
Table 2. Transition temperatures in ◦C of series-1
Compound no.
R = CnH2n+1 (n)
SmA
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
8
10
12
14
16
–
–
–
–
–
–
–
–
–
183.0
160.0
169.0
161.0
170.0
165.0
155.0
172.0
158.0
120.0
122.0
–
116.0
99.0
109.0
106.0
–
134.0
112.0
125.0
136.0
105.0
110.0
10
11
–
Note: SmA = smectic A; N = nematic.

68
D. M. Suthar and A. V. Doshi
NO2
RO
COO
CO-CH=CH
240
SOLID-ISOTROPIC OR MESOMORPHIC
SMECTIC - NEMATIC
NEMATIC - ISOTROPIC
220
200
180
160
140
120
100
80
ISOTROPIC
NEMATIC
SMECTIC
SOLID
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
No.of carbon atoms in n-alkyl chain- R
Figure 1. Phase behavior of series.
Therefore, the curve is extrapolated [2c] to the tetradecyloxy homologue following the
trend of the curve. The extrapolated curve merges and coincides to the solid–nematic
transition temperature 120◦C. This clearly illustrates the disappearance of the smectic
phase from and beyond the tetradecyloxy homologue. An odd–even effect is absent for
the smectic–nematic transition curve. The nematic–isotropic transition curve also adopts a
similar pattern of propagation as shown by the smectic–nematic transition as the series is
ascended without showing any odd–even effect, i.e., it initially falls at the octyloxy deriva-
tive then rises and falls from the decyloxy to the hexadecyloxy homologue as the series is

Novel Liquid Crystalline Nitrobenzoyl Ethylenes
69
Table 3. Average thermal stability in ◦C
Series→
1
A
B
Smectic–nematic
Commencement of smectic phase
Nematic–isotropic
126.7 (C6–C12) 141.0 (C5–C10) 106.66 (C10–C14)
C6 C5 C10
148.6 (C6–C16) 156.0 (C5–C16) 119.8 (C8–C16)
C6 C5 C8
Commencement of nematic phase
ascended. The nonmesomorphic behavior of the methoxy to the pentyloxy derivatives is
attributed to their high crystallizing tendency, which arises from unsuitable magnitudes of
intermolecular anisotropic forces, i.e., the molecules of first five homologues are unable to
resist thermal vibrations exposed upon them from the external source of thermodynamic
surroundings. Therefore, neither the statistically parallel orientational order nor the sliding
layered order of the molecules occurs, which results in the absence of either the nematic or
the smectic mesophase, and concerned homologues sharply transform into isotropic state
from solid crystalline state. The hexyloxy to dodecyloxy derivatives of the series exhibit
smectogenic character in addition to nematogenic character reversibly; while the tetradecy-
loxy and the hexadecyloxy derivatives exhibit only nematogenic character. The occurrence
of smectogenic character is attributed to the lamellar packing of molecules in their crystal
lattices, which results in a sliding layered arrangement of molecules from the hexyloxy to
the dodecyloxy homologues. The absence of smectogenic behavior from the tetradecyloxy
and the hexadecyloxy homologues can be due to irregular coiling, bending, and flexing
of longer n-alkyl chain causing variation of molecular rigidity and flexibility maintaining
only statistically parallel orientational order of the molecules without a lamellar packing of
molecules. The exhibition of nematogenic character in addition to smectogenic behavior
by the hexyloxy to dodecyloxy derivatives is attributed to the relatively higher magnitudes
of attractions, which maintains lamellar packing of the molecules as well as stronger end-
to-end attractions. Thus, favorable molecular rigidity and flexibility generates the smectic
and nematic phases.
The mesomorphic properties of the novel series-1 are summarized in Table 3, and are
compared with the structurally similar homologous series A [3] and B [4c] as shown in
Fig. 2.
Figure 2. Structurally similar series.

70
D. M. Suthar and A. V. Doshi
Although, series 1, A and B are structurally similar, they differ from their linking
groups and the nitro group. Therefore, their rigidity and/or flexibility [5–7] are different,
and hence the variation in the mesomorphic properties and degree of mesomorphism
accordingly. Series 1 and A are identical with respect to aromaticity due to three phenyl
rings, the terminal alkoxy chain and the nitro group, but they differ with respect to a linking
group. Therefore, the molecular length due to the –CH=CH– unit of series A is longer than
that of series 1. Hence, the length to breadth ratios vary accordingly causing a difference
of molecular rigidity and flexibility, and hence the magnitude of anisotropic forces of
intermolecular attractions. Series 1 and B differ only by the absence of the nitro group in
series B. Therefore, they differ in molecular breadth, and hence the difference is reflected
in variations in molecular polarity and polarizability. The comparison of mesomorphic
behavior is summarized in Table 3.
Table 3 indicates that smectic and nematic thermal stability for series 1 is lower than
that of series A. The molecules of series 1 and A are identical in all other respects except
the central bridge. Both these central bridges are comparable, though the –COO– has
shorter length than the –CH=CH–COO– linking group. Therefore, greater length of the
vinyl carboxylate –CH=CH–COO– causes more noncoplanarity due to a twist obtained
as the oxygen atoms of vinyl carboxy group impinge on the adjacent hydrogen atoms of
the aromatic rings. Moreover, increased molecular length increases length to breadth ratio,
enhancing the end-to-end intermolecular anisotropic forces of attractions [8]. On account of
these differences, the smectic–nematic and nematic–isotropic thermal stabilities are lower
for series 1 than for the corresponding homologous series A. Lamellar packing of molecules
commences from sixth and fifth member of the series 1 and A, respectively. Molecules of
series 1 and B differ in isomeric central group –CO–CH=CH– and –CH=CH–CO– and the
laterally substituted –NO2 group in place of –H on the third phenyl ring. The shifting of
the >C=O group causes an increase in thermal stabilities. Moreover, the –NO2 group is
more polar than –H due to its more electron withdrawing tendency than –H. Thus, the end-
to-end attractions cause a rise in the nematic isotropic thermal stability. The mesomorphic
phase range varies from 12◦C at the hexadecyloxy derivative to a maximum of 63◦C at the
decyloxy derivative. The smectogenic and nematogenic phase range varies from 13◦C to
30◦C and 12◦C to 47◦C, respectively. Thus, the novel series is predominantly nematogenic
and partly smectogenic.
Conclusion
(1) The novel series is predominantly nematogenic and partly smectogenic with middle
range melting points.
(2) Molecular rigidity and flexibility with reference to width of molecules are the
deciding factor for mesophase formation.
(3) Variation of mesomorphic properties from homologue to homologue in the same
series and series to series for same homologue depends upon varying the polarity
of the n-alkoxy terminal chain.
(4) Broadening of molecules increases the molecular polarizability as a result of
changing molecular rigidity and flexibility. Thus, degree of mesomorphisam varies
accordingly.
Acknowledgments
The authors acknowledge thanks to Dr. R. R. Shah and Dr. R. B. Patel, the Principal
and Head, Chemistry Department of K. K. Shah Jarodwala Maninagar Science College,

Novel Liquid Crystalline Nitrobenzoyl Ethylenes
71
Ahmedabad for their valued co-operation, interest, and encouragement in present work.
Authors also thank to Vaibhav laboratory and authorities of S.P.T. Arts and Science College,
Godhra for their valuable co-operation.
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