Study of the relation between molecular structure and mesomorphic behavior with reference to a novel homologous series: α-3-[4′-n-Alkoxy benzoyloxy] phenyl-β-3″-nitro benzoyl ethylenes

Article abstract of DOI:10.1080/15421406.2012.723961

The synthesis and mesomorphic properties of a novel homologous series is reported. With the exception of the methyl and ethyl homologues, all 11 members of the series exhibit mesomorphism enantiotropically. The propyl to pentyl homologues, and the tetradecyl and hexadecyl homologues are enantiotropic nematic without the exhibition of smectic mesomorphism; while the hexyl to dodecyl homologues exhibit smectic mesomorphism in addition to the nematic phase. The texture of the nematic-mesophase by optical polarizing microscopy is of the threaded or Schlieren type, and that of smectogenic homologues is of the focalconic fan type. A phase diagram of the series shows a typical trend. Analytical and spectral data confirm the molecular structure of the compounds. Transition temperatures and mesomorphic properties are determined by optical polarizing microscopy. Smectic and nematic thermal stabilities are 125°C and 167°C, respectively. The series is predominantly nematogenic and partly smectogenic with middle-order melting type, and the smectic and nematic-mesophase ranges vary from 8°C to 22°C and 9°C to 56°C, respectively. The mesomorphic properties of the novel series are compared with other structurally similar homologous series.

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Study of the Relation Between Molecular
Structure and Mesomorphic Behavior
with Reference to a Novel Homologous
Series: α-3-[4′-n-Alkoxy benzoyloxy]
phenyl-β-3″-Nitro benzoyl ethylenes
R. P. Chaudhari a & A. V. Doshi b
a Department of Chemistry , K. K. Shah Jarodwala Maninagar
Science College , Ahmedabad , Gujarat , India
b Shri Jagdish Prasad Jhabarmal Tibrewala University , Jhunjhunu ,
Rajasthan , India
Published online: 22 Jan 2013.
To cite this article: R. P. Chaudhari & A. V. Doshi (2013) Study of the Relation Between Molecular
Structure and Mesomorphic Behavior with Reference to a Novel Homologous Series: α-3-[4′-n-Alkoxy
benzoyloxy] phenyl-β-3″-Nitro benzoyl ethylenes, Molecular Crystals and Liquid Crystals, 570:1,
109-116, DOI: 10.1080/15421406.2012.723961
To link to this article: http://dx.doi.org/10.1080/15421406.2012.723961
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Mol. Cryst. Liq. Cryst., Vol. 570: pp. 109–116, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.723961
Study of the Relation Between Molecular Structure
and Mesomorphic Behavior with Reference to a
Novel Homologous Series: α-3-[4ꢀ-n-Alkoxy
benzoyloxy] phenyl-β-3ꢀꢀ-Nitro benzoyl ethylenes
R. P. CHAUDHARI1 AND A. V. DOSHI2
1Department of Chemistry, K. K. Shah Jarodwala Maninagar Science College,
Ahmedabad, Gujarat, India
2Shri Jagdish Prasad Jhabarmal Tibrewala University, Jhunjhunu, Rajasthan,
India
The synthesis and mesomorphic properties of a novel homologous series is reported.
With the exception of the methyl and ethyl homologues, all 11 members of the series
exhibit mesomorphism enantiotropically. The propyl to pentyl homologues, and the
tetradecyl and hexadecyl homologues are enantiotropic nematic without the exhibition
of smectic mesomorphism; while the hexyl to dodecyl homologues exhibit smectic me-
somorphism in addition to the nematic phase. The texture of the nematic-mesophase by
optical polarizing microscopy is of the threaded or Schlieren type, and that of smecto-
genic homologues is of the focalconic fan type. A phase diagram of the series shows
a typical trend. Analytical and spectral data confirm the molecular structure of the
compounds. Transition temperatures and mesomorphic properties are determined by
optical polarizing microscopy. Smectic and nematic thermal stabilities are 125◦C and
167◦C, respectively. The series is predominantly nematogenic and partly smectogenic
with middle-order melting type, and the smectic and nematic-mesophase ranges vary
from 8◦C to 22◦C and 9◦C to 56◦C, respectively. The mesomorphic properties of the
novel series are compared with other structurally similar homologous series.
Keywords Enantiotropy; liquid crystals; mesomorphism; nematic; smectic
Introduction
The effect of molecular structure on molecular rigidity and flexibility [1,2] using different
central groups has been widely studied. Lohar et al. [3a] studied the effect of molecular
structure using a –CH CH–CO– central group linking two phenyl rings, but despite a rod-
like shape mesomorphism was not exhibited. Doshi et al. [3b,c] continued in this regard
by linking three phenyl rings through two central groups [3a,b], but also introduced a
lateral substituent and modified the terminal units. This variation of molecular structure
altered the rigidity and flexibility sufficiently to induce mesomorphism [1,2]. The present
series was designed to broaden the molecular width by using two angular linking groups
(–CH CH–CO– and –COO–), and an off-axis terminal nitro group. Thus, the effect of
Address correspondence to R. P. Chaudhari, Department of Chemistry, K. K. Shah Jarodwala
Maninagar Science College, Ahmedabad (380008), Gujarat, India. E-mail: info@kksjmc.com
109

110
P. Chaudhari and A. V. Doshi
increasing molecular width and generating a nonlinear molecular structure can be correlated
in terms of mesomorphic properties.
Experimental
Synthesis
The 4-Hydroxybenzoic acid was alkylated with a suitable alkyl halide (R–X). The resulting
alkoxy benzoic acids were converted to corresponding acid chlorides (A) using thionyl
chloride by the modified method of Dave and Vora [4]. The 3-Hydroxybenzaldehyde was
reacted with 3-nitroacetophenone to form α-3-hydroxy phenyl β-3ꢀ-nitrobenzoyl ethylene
(B) by an established method [5]. Components (A) and (B) were condensed in dry cold pyri-
dine to form the final products [4,5,6]. All final products of the series were filtered, washed,
dried, and purified until constant transition temperatures were obtained. The synthetic route
to the series is outlined in Scheme 1.
Scheme 1. Synthetic route to the series. where R = CnH2n+1, n = 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16.
Characterization
Some representative selected members of the series were characterized by elemental anal-
ysis (Table 1), infrared (IR), and 1HNMR spectroscopy. Microanalysis was performed on a
Table 1. Elemental analysis for butyloxy, pentyloxy, decyloxy, and dodecyloxy derivatives
Element found (%calculated)
Molecular formula
C
H
N
C26H23NO6
C27H25NO6
C32H35NO6
C34H39NO6
70.35 (70.11)
70.64 (70.60)
72.87 (72.60)
73.81 (73.58)
5.95 (5.17)
5.53 (5.45)
6.88 (6.61)
7.28 (7.05)
3.19 (3.25)
3.08 (3.15)
2.91 (3.05)
2.58 (2.39)

Molecular Structure and Mesomorphic Behavior
111
Perkin–Elmer PE 2400 CHN analyzer. IR spectra were recorded on a Perkin–Elmer spec-
trum GX instrument, and 1HNMR spectra were recorded on a Bruker spectrometer using
CDCl3 as solvent. The textures of the smectic and the nematic phase were determined by a
miscibility method using an optical polarizing microscopy.
Analytical Data
NMR in PPM for octyloxy derivative: 0.89 (–CH3 of OC8H17), 1.29 ((–CH2–)n
of OC8H17), 4.00 (–OCH2 of OC8H17), 7.88–8.10 (aromatic ring) 6.89 and 6.92
(–CH CH–CO–). The data support the molecular structure.
NMR in PPM for tetradecyloxy derivative: 0.87 (–CH3 of OC14H29), 1.44 ((–CH2–)n
of OC14H29), 4.00 (–OCH2of OC14H29), 7.90–8.11 (aromatic ring), 6.89 and 6.92
(–CO–CH CH–). The data support the molecular structure.
IR in cm−1 for propyloxy derivative: 740, 880 (aromatic ring), 1160 (C–O), 1420 (–NO2
group), 1250, 1600, 1680 (–COO-group), 970 (–CH CH–), and 640 (polymethelyne
of C3H7). The data support the molecular structure.
IR in cm−1 for hexyloxy derivative: 840 (aromatic ring), 1170 (C–O), 940 (–CH CH–),
1250, 1600, 1680 (–COO group), 1430 (–NO2 group), 750 (polymethelyne of C6H13).
The data support the molecular structure.
Texture by miscibility method
Result and Discussion
Eleven members of the novel series under discussion were investigated for melting point,
transition temperatures, and mesophase morphology using optical polarizing microscopy.
The propyl to pentyl homologues, and the tetradecyl and hexadecyl derivatives of the series
are enantiotropically nematogenic. The hexyl to dodecyl derivatives are enantiotropic ne-
matic with exhibition of smectogenic character. The transition temperatures are shown in
[Table 2]. A plot of the transition temperatures versus the number of carbon atoms in the ter-
minal alkoxy chain gives rise to a phase diagram (Fig. 1) which consists of solid-isotropic or
solid-mesomorphic, smectic–nematic or smectic-isotropic, and nematic-isotropic transition
curves. The solid-isotropic or solid-mesomorphic transition curve follows a zigzag path of
rising and falling temperatures with an overall descending tendency. The smectic–nematic
transition curve initially rises, passes through maxima at the decyl derivative and then falls
at the dodecyl homologue, and then behaves in the usual expected manner. The curve was
extrapolated [7,5] to the tetradecyl derivative to predict its latent transition temperature
for the smectic phase, which gave a value of 86◦C. However, the actual transition is not
realizable practically due to the high-crystallizing tendency. The smectic-nematic transition
curve behaves in normal manner without any odd–even effect. The nematic-isotropic tran-
sition curve descended as the series is ascended upto the tetradecyl homologue, but it shows
an ascending tendency of 10◦C at the hexadecyl derivative. An odd–even effect is observed
for the nematic-isotropic transition curve. The series is predominantly nematogenic and
partly smectogenic with middle ordered melting type and methyl and ethyl derivatives are
not mesomorphic. The mesomorphic-isotropic transition temperatures for the series vary
between 185◦C and 152◦C.
The methyl and ethyl derivatives of the novel series are not mesomorphic due to
their high-crystallizing tendency, which arises from the shorter and hence stiffer alkoxy
terminal chains. The molecules of mesomorphic the homologues are able to resist thermal
vibrations exposed upon them and disaligned on the plane of a surface at an angle < 90◦. As

112
P. Chaudhari and A. V. Doshi
Table 2. Transition temperatures of series in ◦C
Transition temperatures in ◦C
Sr. no.
n-alkyl group –CnH2n+1
Sm
N
Isotropic
1
2
3
4
5
6
7
8
1
2
3
4
5
6
8
10
12
14
16
–
–
–
–
–
–
–
213.0
205.0
185.0
175.0
177.0
174.0
170.0
163.0
152.0
151.0
160.0
170.0
166.0
142.0
118.0
124.0
130.0
118.0
132.0
142.0
109.0
116.0
108.0
96.0
–
9
10
11
–
Sm, Smectic; N, Nematic.
α-3-[4ꢀ-n-Alkoxy benzoyloxy] phenyl-β-3ꢀꢀ-Nitro benzoyl ethylenes
a result the molecules are floating either with parallel orientational or a sliding interlayered
arrangement to cause nematic- or smectic-mesophase formation. The propyl to hexadecyl
derivatives exhibit the nematic-mesophase as do the hexyl to dodecyl derivatives, however,
these also exhibit the semctic phase. Such mesomorphism is a result of suitable magnitudes
of anisotropic intermolecular forces of attraction as a consequence of favorable molecular
rigidity and flexibility [1,2] between two temperatures to cause a statistically parallel
orientation of the molecules resulting in a nematic phase. The hexyl to dodecyl derivatives
possess lamellar packing in their crystal lattices, which under the influence of heat, resist
thermal vibrations exposed upon them to maintain a two dimensional array of molecules
with sliding layered molecular arrangement to cause smectogenic-mesophase formation. An
odd–even effect for the smectic-isotropic curve and variations in mesomorphic properties
from homologue to homologue in the same series is observed due to the sequentially added
methylene unit in the alkoxy terminal chain. Both curves for odd and even homologues
merge into each other at the heptyl homologue. The odd–even effect diminishes for higher
homologues, because longer chain may coil, bend, flex, or couple to lie in a line of major axis
of the core [8]. Thus, the higher homologues may show a deviating behavior from normal
behavior because of the molecular rigidity and flexibility [1,2]. The mesomorphic properties
of the novel homologous series-1, are compared with structurally similar homologous
series-X [9] and series-Y [10] in Fig. 2.
The homologous series-1 and series-X are structurally similar with respect to three
phenyl rings and the terminal groups, but they differ in their geometrical shape because
series-1 has the bent core in the center of the molecules. Series-1 and series-Y are isomeric

Molecular Structure and Mesomorphic Behavior
113
Figure 1. Phase behavior of series.
and identical in the linking groups and terminal groups, but they differ in their molecular
geometrical shape and linearity because of the bent-core nature of series-1. Therefore, the
difference in mesomorphic properties and degree of mesomorphism are correlated with the
differing structural features of series-1, series-X, and series-Y under comparison. Thermal
stabilities and the commencement of mesophase formation are outlined in Table 3.
The smectic and nematic thermal stabilities of series-1 and series-Y differ due to
their molecular shape difference and magnitudes of molecular width. The intermolecular
closeness varies, though the molecular length is almost equivalent. Therefore, length to
breadth ratio and the ratio of the molecular polarity to polarizability varies accordingly.
The difference in intermolecular closeness of series-1 and series-Y causes a difference in the
magnitudes of molecular polarizability and intermolecular distance. The net effect due to
polarizability exceeds the effect due to intermolecular width. Hence, the net magnitudes of

114
P. Chaudhari and A. V. Doshi
Figure 2. Structurally similar/isomeric series.
anisotropic forces of attraction of series-1 is more than for series-Y. Therefore, the smectic
and nematic thermal stabilities (Table 3) of series-1 are greater than series-Y. The smectic
thermal stability of series-1 is less than for series-X, but the nematic thermal stability of
series-1 is greater thanfor series-X. The molecules of series-Xduetotwocentral groups with
conjugated double bonds have higher molecular rigidity. Moreover, the –CH CH–COO–
central group increases the molecular length compared with the –COO– central group. Thus,
the overall magnitudes of anisotropic forces of adhesion are enhanced for series-1. Hence,
smectic thermal stability of series-1 is lower than series-X. The commencement of the smec-
tic phase takes place for series-1, series-X, and series-Y from fifth/sixth members of the se-
ries. This indicates that the extent of noncoplanarity caused by the molecules of series-1 and
series-Y are equivalent. This variation causes early commencement of smectic phase as the
–COO– central group preserves molecular linearity more than the –CH CH–COO– group.
Hence, the nematic-mesophase formation of series-1 and series-Y takes place from C3 and
C2 carbon while it commences from fifth carbon of series-X. Thus, the difference in meso-
morphic properties from series to series for the same homologue is attributed to the molec-
ular geometrical shape in the isomeric series. However, the differing central group, molec-
ular width, and hence the molecular polarity and polarizability can also vary the degree of
mesomorphism.
Table 3. Relative average thermal stability in ◦C
Average transition temperatures in ◦C
Series
Series (1)
Series (X)
Series (Y)
Smectic-nematic
125.0
(C6–C12)
C6
167.4
(C3–C16)
C3
154.6
(C5–C8)
C5
163.8
(C5–C16)
C5
121.2
(C6–C14)
C6
160.0
(C2–C16)
C2
Commencement of smectic phase
Nematic-isotropic
Commencement of nematic phase

Molecular Structure and Mesomorphic Behavior
115
Conclusion
The novel presently investigated and discussed homologous series has the following points.
(1) Predominantly nematogenic and partly smectogenic with middle-ordered melting
type.
(2) The group efficiency order derived on the basis of thermal stability for smectic and
nematic.
With common presence of central group –CH CH–CO– is as under
Smectic: Series-X
With
Series-1
With
Series-Y
With
−CH CH−COO− > −COO− > −COO−
Linear shape Nonlinear shape
Linear shape
Series-Y
>With
Nematic: Series-1
Series-X
With
>With
−COO−
−CH CH−COO− −COO−
(3) Group efficiency order derived on the basis of early commencement of smectic-
and nematic phase.
Smectic: Series-X
With
Series-1
>With
Series-Y
=With
−CH CH−COO− −COO− −COO−
Nematic: Series-1
Series-X Series-Y
>With >With
−COO− −CH CH−COO−
With
−COO−
(4) The variation in mesomorphic properties from series to series for the same homo-
logue depends upon the molecular shape, size, molecular width, and polarizability
if terminal-end groups are common of fixed polarity, substituted at identical posi-
tion.
(5) Magnitudes of molecular rigidity and flexibility depend upon shape, size, and
molecular width when other factors affecting intermolecular adhesion forces are
identical.
Acknowledgments
Authors thank to authorities of K.K. Shah Jarodwala Maninagar Science College and S.P.T.
Arts & Science College, Godhara for their valuable cooperation in present work. Authors
are also thankful to Vaibhav laboratory for their analytical services. Thanks are due to
the Ankita Doshi, Department of Biochemistry, M.S. University of Baroda, Vadodara for
valuable support in the work.
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and accepted for publication in the issue of June-2012.