Synthesis and Liquid Crystal Behavior of a Novel Ester Homologous Series

Article abstract of DOI:10.1080/15421406.2014.884389

A novel ester homologous series of 4-[4-n-alkoxy cinnamoyloxy] benzyl benzoates has been synthesized. The series consists of 12 homologues. Liquid crystal properties commence from the sixth member to the last member of the series with the exhibition of an enantiotropic nematic phase without the exhibition of any smectogenic mesophase. The remaining homologues do not exhibit liquid crystal behavior. The texture of the nematic phase is of the threaded or Schlieren type. The solid-nematic or isotropic transition curve adopts a zigzag path and the nematic-isotropic transition curve steeply rises and then falls in the phase diagram and behaves in normal manner. An odd-even effect is absent in the nematic-isotropic transition curve with an alteration of transition temperatures. The average thermal stability for the nematic mesophase is 153.1°C, and the nematogenic temperature ranges vary from 12°C to 50°C. Analytical data confirm the molecular structures of the homologues. The Liquid Crystal properties and transition temperatures were observed through an optical polarizing microscope, equipped with a heating stage. The mesomorphic characteristics of the novel ester series are compared with other, known and structurally similar series. The novel series is predominantly nematogenic with the absence of any smectogenic character, with relatively short mesophase temperature ranges and of the middle ordered melting type.

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Synthesis and Liquid Crystal Behavior of
a Novel Ester Homologous Series
B. H. Patela & A.V. Doshib
a Huntsman, Baroda Textile Effects Pvt. Ltd, Umraya village, Taluka
Padra, Vadodara, Gujarat, India
b Dr K.N.Modi University, Plot No.INS-1, RIICo Industrial Area Ph-II,
Newai, Dist. Tonk, Rajasthan, India
Published online: 15 Dec 2014.
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To cite this article: B. H. Patel & A.V. Doshi (2014) Synthesis and Liquid Crystal Behavior of
a Novel Ester Homologous Series, Molecular Crystals and Liquid Crystals, 605:1, 23-31, DOI:
10.1080/15421406.2014.884389
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Mol. Cryst. Liq. Cryst., Vol. 605: pp. 23–31, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.884389
Synthesis and Liquid Crystal Behavior of a Novel
Ester Homologous Series
B. H. PATEL1 AND A.V. DOSHI2,∗
1Huntsman, Baroda Textile Effects Pvt. Ltd, Umraya village, Taluka Padra,
Vadodara, Gujarat, India
2Dr K.N.Modi University, Plot No.INS-1, RIICo Industrial Area Ph-II, Newai,
Dist. Tonk, Rajasthan, India
A novel ester homologous series of 4-[4’-n-alkoxy cinnamoyloxy] benzyl benzoates
has been synthesized. The series consists of 12 homologues. Liquid crystal properties
commence from the sixth member to the last member of the series with the exhibition of
an enantiotropic nematic phase without the exhibition of any smectogenic mesophase.
The remaining homologues do not exhibit liquid crystal behavior. The texture of the
nematic phase is of the threaded or Schlieren type. The solid-nematic or isotropic
transition curve adopts a zigzag path and the nematic-isotropic transition curve steeply
rises and then falls in the phase diagram and behaves in normal manner. An odd–even
effect is absent in the nematic-isotropic transition curve with an alteration of transition
temperatures. The average thermal stability for the nematic mesophase is 153.1◦C,
and the nematogenic temperature ranges vary from 12◦C to 50◦C. Analytical data
confirm the molecular structures of the homologues. The Liquid Crystal properties
and transition temperatures were observed through an optical polarizing microscope,
equipped with a heating stage. The mesomorphic characteristics of the novel ester
series are compared with other, known and structurally similar series. The novel series
is predominantly nematogenic with the absence of any smectogenic character, with
relatively short mesophase temperature ranges and of the middle ordered melting type.
Keywords Liquid crystals; mesomorphism; monotropy; nematic; smectic
Introduction
Esters with vinyl carboxy ( CH CH COO ) and/or carboxy ( COO ) functional
groups as a central bridge have been reported previously [1] to aid understanding and to es-
tablish the effect of molecular structure on liquid crystal properties. Such properties include
the temperature of commencement of the smectic or/and the nematic mesophase formation,
the degree of mesomorphism, the group efficiency order on the basis of thermal stabilities
and the early commencement of mesophase formation, and includes a comparative study
with structurally similar series as a consequence of favorable magnitudes of molecular
rigidity and flexibility [2–4]. In order to further enhance understanding of structure prop-
erty relationships, the present investigation is planned with a view to understanding the
dependence of liquid crystal properties in terms of molecular rigidity and flexibility which
has a direct relation with the thermodynamic property such as internal energy (ꢀE) of an
∗Address correspondence to Brijesh H. Patel, A-21 Saket Bunglows Opp Yash Complex Gotri
Baroda, 390021, Gujarat, India. E-mail: research@dknmu.org
23

24
B. H. Patel and A. V. Doshi
open system, operated at constant pressure or at atmospheric pressure for thermotropic
liquid crystals. The proposed investigation involves the synthesis of a novel homologous
ester series, that consists of three phenyl rings to be bridged through two central groups
–CH = CH—COO– and –COO–CH2– which act as a rigid core, and the left n-alkoxy
(–OR) terminal end group acts as a flexible part of a molecule, without any substitution as
a lateral and a terminal group at the other end of the molecule.
Experimental
Synthesis
4-Hydroxy cinnamic acid was alkylated by suitable alkylating agents (R-X) to generate the
4-n-alkoxy cinnamic acids. Alkoxy cinnamic acids were then linked with 4-hydroxybenzyl
◦
benzoate (B) (m.p. 110 C) through acid chlorides (A) of alkoxy cinnamic acids using
thionyl chloride [5,6] in dry cold pyridine. 4-Hydroxy benzyl benzoate (B) was used as
received and recrystallized and dried before use. The final products A-B were individually
decomposed, filtered, washed, dried, and purified until constant transition temperatures
were obtained. 4-Hydroxycinnamic acid, 4-hydroxy benzyl benzoate (B), methanol, KOH,
ethanol, pyridine, alkyl halides (R–X), thionyl chloride, etc. required for the synthesis
were used as received except for solvents which were dried and purified prior to use. The
synthetic route to the series is outlined in scheme 1.
Characterization
Some representative members of the novel series were characterized by elemental analysis
on a Perkin-Elmer PE 2400 CHN analyzer. Infra red spectroscopy employed a Perkin-
1
Elmer spectrum GX spectrometer, and HNMR spectroscopy employed a Bruker spec-
trometer using CDCl3 as the solvent. The liquid crystal behavior, type of texture and
transition temperatures of each homologues were recorded and observed through polariz-
ing microscopy in conjunction with a heating stage and temperature controller. The type
of nematic mesophase textures was determined by a miscibility method. Thermodynamic
quantities enthalpy change (ꢀH) and entropy change (ꢀS) are qualitatively discussed.
Analytical Data
1
Spectral Data. HNMR in ppm for the hexadecyloxy derivative: 1.183 ( C15H31 of
C16H33 group), 3.272 ( OCH2 of OC16H33 group), 3.984 ( O–CH2–C6H4 group),
5.297 and 5.250 ( CH CH group), 6.221, 6.261, 6.384, and 6.424 (p-substituted phenyl
ring), 7.370 (mono-substituted phenyl ring), 7.674, 7.745, 7.784, and 7.863 (p-substituted
phenyl ring).
IR in cm−1 for the hexyloxy derivative: 692 and 734 mono-substituted phenyl ring, 827
p-substituted phenyl ring, 1209 ether linkage of ( C6H13O–C6H4), 1718, 1280, and 1139
( COO ester group), 993 (trans CH CH group).
IR in cm−1 for the decyloxy derivative: 694 and 732 mono-substituted phenyl ring, 829
p-substituted phenyl ring, 979 (trans CH CH group), 1676, 1282, and 1172 ( COO
group), 1215 ether linkage of ( C10H21O–C6H4 group).
Mass Spectra. Mass spectra for the propoxy derivative:

Synthesis and Liquid Crystal Behavior
25
CH3OH
Reflux
R-X
CH=CHCOOH
CH=CHCOOH
HO
RO
+
KOH
- HX
-SO2
- HCl
+
SOCl2
Reflux
CH=CHCOCl
RO
A
H2SO4
- H2O
COOCH2
HO
HOCH2
COOH
HO
+
B
dry cold
COOCH2
HO
CH=CHCOCl
RO
+
Py ridine
- HCl
B
A
COOCH2
Final product
CH=CHCOO
RO
Where R = CnH2n+1; n = 1,2,3,4,5,6,7,8,10,12,14,16
Scheme 1. Synthetic route to the series.
Series: 4-[4'-n-Alkoxy Cinnamoyloxy] benzyl benzoates
COO-CH2
CH=CHCOO
RO
Molecular formula C26H24O5 Calculated Molecular weight 416. Practical mass ob-
served by Mass Spectrometry 416.
Texture of Nematic Phase by Miscibility Method.
Octyloxy derivative ————– Threaded nematic
Dodecyloxy derivative ———–Schlieren nematic
Results and Discussion
4-n-Alkoxy cinnamic acids and 4-hydroxy benzyl benzoate are nonliquid crystal compo-
nents; however, the liquid crystal property is induced by linking them by COO CH2–
central bridges through acid chloride formation of the corresponding n-alkoxy acids. The
novel homologous series consists of 12 homologues. The methyl to pentyl homologues
are not liquid crystals, the hexyl to the hexadecyl homologues of the series show liquid
crystal behavior enantiotropically with nematogenic mesophase formation only, without
the formation of any smectogenic mesophase. The transition temperatures of the ho-
mologues (Table 2) as determined from an optical hot stage polarizing microscopy are
plotted versus the number of carbon atoms present in the n-alkyl chain of the left n-
alkoxy group. A phase diagram (Fig. 1) is obtained by joining like or related points,

26
B. H. Patel and A. V. Doshi
Table 1. Elemental analysis for methoxy, ethoxy and propoxy derivatives
Elements % found (% calculated)
Sr. No.
Molecular formula
C
H
1
2
3
C24H20O5
C25H22O5
C26H24O5
74.35 (74.22)
74.51 (74.62)
74.98 (75.00)
5.26 (5.15)
5.31 (5.47)
5.79 (5.76)
showing the phase behavior of the novel series. The solid-isotropic or nematic transition
curve follows a zigzag path of rising and falling values and behaves in normal man-
ner. A nematic to isotropic transition curve steeply rises from hexyloxy homologue to
octyloxy homologue and then descended as series is ascended. An odd–even effect is
260
SOLID-ISOTROPIC / NEMATIC
NEMATIC-ISOTROPIC
220
ISOTRPIC
180
NEMATIC
140
100
60
SOLID
20
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
X axis
No.of carbon atoms in n-alkyl chain-(R)
Figure 1. Phase behavior of series.

Synthesis and Liquid Crystal Behavior
27
Table 2. Transition temperatures of series-1 in ◦C
Transition temperature in ◦C
Compound no.
n-alkyl group (–CnH2n+1) (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
10
11
12
1
2
3
4
5
6
7
8
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
58.0
140.0
162.0
145.0
115.0
126.0
124.0
142.0
136.0
94.0
72.0
111.0
70.0
170.0
189.0
176.0
160.0
153.0
154.0
10
12
14
16
Sm: Smectic
N: Nematic
absent for the nematic-isotropic transition curve. The nematogenic mesophase temperature
ranges vary between 12◦C and 50◦C. The average thermal stability for the nematic phase
is 153.1◦C. Hence, the phase diagram and curves behave in normal manner. Thus, the
novel homologous series is only nematogenic with the absence of smectogenic charac-
ter and is of a middle ordered melting type of considerable temperature range of liquid
crystallinity.
The exhibition of liquid crystal properties by some of the novel molecules formed
from two nonliquid crystal components (A and B) is attributed to the suitable magnitudes
of anisotropic forces of intermolecular attractions as a consequence of the net molecular
rigidity [2–4] and flexibility within the molecules. The methyl to pentyl homologues are
not liquid crystals because these five molecules are unable to resist and withstand thermal
vibrations exposed upon them from an external source of thermodynamic surroundings.
As a result of their inability to resist thermal vibrations, all the molecules of the first five
homologues of the novel series possess high crystallizing tendency and transform directly
into the isotropic liquid without passing through an intermediate liquid crystal state. As the
n-alkyl chain is gradually lengthened the molecular flexibility increases and the mesophase
character strengthens keeping molecular rigidity unchanged. Hence, the capacity of a
molecule to withstand thermal vibrations exposed upon them rises and, the molecules
under exposed thermal vibrations disalign at an angle less than 90◦ on the plane of a surface
within a definite temperature range showing exhibition of liquid crystal properties. The
molecules of the present series from the hexyl to hexadecyl homologues arrange themselves
in statically parallel orientational order of a two dimensional array in a floating condition
[9], as a consequence of suitable magnitudes of end to end intermolecular attractions
causing a threaded or a Schlieren texture of the nematogenic mesophase within a definite
temperature range. On further heating the molecules of the nematogenic sample randomly,
orient in all possible directions with high order of disorder or entropy (ꢀs = ꢀH/T) and
convert to the isotropic state from the anisotropic state in enantiotropic manner. The absence
of smectic character in the homologues of a present series is attributed to the absence of

28
B. H. Patel and A. V. Doshi
COO-CH2
CH=CHCOO
CH=CHCOO
CH=CHCOO
RO
Series 1
Series X
COO-CH2-CH3
RO
COO-CH2-C2H5
RO
Series Y
Figure 2. Structurally similar homologous series.
lamellar packing of molecules in their crystal lattices [10] due to inadequate intermolecular
attractions, for the reason of absence of flexible terminal end group or laterally substituted
group on right side of the molecules. Thus, lamellar packing of molecules in the crystal
lattices and the smectogenic mesophase formations is hindered because of unsuitable
magnitudes of intermolecular forces of adhesion. The absence of an odd–even effect in
the nematic-isotropic transition curve is attributed to the absence of liquid crystallinity
in the odd members of the alkyl chain. A longer n-alkyl chain may coil, flex, bend, or
couple to lie with the major axis of the core without showing an odd–even effect from
and beyond the hexyloxy homologue which are of an even numbered chain length. The
nematogenic phase temperature length is a minimum of 12 ◦C at the hexyloxy homologue
and a maximum of 50 ◦C at the octyloxy homologue, which is attributed to the steep and
drastic rise of nematic-isotropic transition curve for a sudden rising tendency of molecular
flexibility in combination with its molecular rigidity. Thereafter, molecular rigidity and
flexibility gradually diminish on progressive addition of methylene units. The references
[1–4] indicating a descending tendency of a transition curve as the series is ascended. The
average thermal stability 157.1 ◦C of the series indicates a heat-resisting tendency as related
to the enthalpy change (ꢀH) of the homologues, which is responsible for liquid crystal
mesophase formation. Thus, the novel series is only nematogenic without the exhibition of
any smectic property even in the monotropic condition, and is of a middle ordered melting
type. The liquid crystal properties and behavior of presently investigated novel series-1 are
compared with structurally similar other known homologous series-X [11] and series-Y
[12] as shown below in Fig. 2.
As can be seen from Fig.-2 homologous series-1, series-X, and series-Y are structurally
identical with respect to two phenyl rings bridged through a –CH = CH—COO– central
group and a left n-alkoxy (–OR) terminal end group, with the only difference being what is
attached to the –COO–CH2– group. Therefore, variation in mesomorphic or liquid crystal
behavior of series-1, series-X, and series-Y can be attributed to the differing part of the
molecules of series-1, series-X, and series-Y by –C6H5, –CH3, and –C2H5 respectively.
Table 3 shows the average thermal stabilities and commencement of the smectic and the
nematic phase as well as the homologues showing type of mesophase or mesophases as
mentioned below.
Table 3 summarizes the following significant points.

Synthesis and Liquid Crystal Behavior
29
Table 3. Average thermal stability in ◦C
Series→
Series-1
—
Series-X
Series-Y
smectic-nematic or smectic-isotropic
commencement of smectic phase
120.75
118.0
(C7—C12)
C2
(C6—C12)
C2
nematic-isotropic commencement of
nematic phase
153.1
119.7
115.7
(C6–C16)
C6
(C5–C6)
C1
(C3–C6)
C1
• The presently investigated novel series is only nematogenic while series-X and
series-Y chosen for comparison are nematogenic in addition to smectogenic in
character.
• The nematic-isotropic thermal stability of the novel series-1 is the highest (153.1 ◦C)
and decreases in passing from series-1 to series-X to series-Y.
• Series-X and series-Y exhibit smectogenic behavior with very similar average ther-
mal stability.
• The smectic mesophase commences from the second member of a series in case of
series-X and series-Y, while it does not occur at all in series-1.
• The nematic mesophase commences from the first member of series-X and series-Y,
while it commences from the sixth member of the present novel series-1.
The presently investigated novel series-1, and the comparison series-X and series-Y are
all structurally comparable with the structure (Z) of 4-(4ꢀ-n-alkoxy cinnamoyloxy) methyl
benzoates [1(c)(iii)] as shown in Fig. 3.
Homologous series-Z [1(c)(iii)] is analogous to presently investigated series-1, and
series-X, series-Y chosen for comparison. They all resemble to each other for the unchang-
ing molecular part, but differ with respect to the moiety that is linked to the –COO–CH2–
unit, which is a nonpolar benzene ring in case of the novel series-1 and a polar methyl
(–CH3) and ethyl (–C2H5) group in case of series-X and series-Y respectively. Thus, vari-
ations in mesomorphic (liquid crystal) behavior and the degree of mesomorphism can be
correlated or linked with the differing molecular part, nonpolar benzene ring, and polar
n-alkyl (methyl and ethyl) groups linked to terminal –COO–CH2– group. The changing
molecular part causes variations in molecular rigidity and/or flexibility, that is, the ter-
minal phenyl ring in series-1 increases molecular length but decreases molecular polarity
and flexibility, (or increases molecular rigidity) whereas, the methyl and ethyl groups in-
crease molecular polarity in comparison with series-1. Therefore, variations occur in the
molecules of series-1, series-X, and series-Y in terms of length to breadth ratio, the ratio
of the molecular polarity to polarizability, molecular rigidity, and flexibility as well as
COO-CH2-H
Figure 3. General structure for series-Z.
CH=CHCOO
RO
Series Z

30
B. H. Patel and A. V. Doshi
molecular shape, size and polarity [1]. Thus, the suitable magnitudes of anisotropic forces
of intermolecular attractions varies from series to series due to the changing molecular part
of series-1, series-X, and series-Y compared with the –H of the analogous series-Z.
The molecules of series-1, series-X, and series-Y disalign at an angle less than ninety
degrees and show liquid crystalline behavior. However, the molecules of series-1 are un-
able to form lamellar packing of molecules in their crystal lattices, while the molecules of
series-X and series-Y form lamellar packing of molecules in their crystal lattices due to the
presence of polar –CH3 and –C2H5 group at the end. Thus, increased molecular polarity
and flexibility favors the formation of smectic mesophase, because of suitable magnitudes
of anisotropic forces of adhesion, responsible to maintain sliding layered molecular ordered
arrangement in floating condition; in case of series-X and series-Y. But, same does not hap-
pen in case of series-1. Thus, smectogenic behavior is absent for the presently investigated
novel series-1. However, disalignment of molecules at an angle less than 90◦ with adequate
anisotropic forces of intermolecular end to end attractions which withstand exposed ther-
mal vibrations to float the molecules in a two dimensional array with statistically parallel
orientational order on a surface to show only nematogenic mesophase formation, but for-
mation of the smectogenic mesophase is hindered for the presently investigated series-1,
yet not hindered for series-X and series-Y. The higher value of nematic thermal stability
of series-1 is attributed to the absence of the smectic phase and relatively higher molecular
rigidity caused by the third phenyl ring. The absence of lamellar packing of molecules
of series-1 is due to the presence of a nonpolar phenyl ring at the molecular end, which
reduces molecular flexibility and increases molecular rigidity. Thus, nematic mesophase
stabilizes more at the cost of losing smectic mesophase formation. Hence, the highest ne-
matic thermal stability (153.1 ◦C) among the series-1, series-X, and series-Y is observed.
The variations in mesomorphic behavior from homologue to homologue in the same series
is attributed to the sequentially and progressively added methylene unit in n-alkyl chain [1].
Similarly, variations in mesomorphic behavior and the degree of mesomorphisim for the
same homologue from series to series is attributed to the changing molecular part, keeping
the rest of the molecular part unchanged [1–4]. Early or late commencement of the smectic
mesophase depends upon the extent of noncopolarity caused by a molecule [8,9], that is,
the molecules of series-X and series-Y are equally coplanar. Hence, the smectic mesophase
commences from the second member of each series-X and series-Y, but it does not appear
at all in series-1.
Conclusions
The presently investigated novel homologous series-1 of 4-[4ꢀ-n-alkoxy cinnamoyloxy]
benzyl benzoates is summed up as follows.
• Nematogenic only without exhibition of any smectogenic property.
• Reduction in molecular flexibility or enhancement of molecular rigidity decreases
the possibility of smectic mesophase formation.
• Suitable magnitudes of molecular rigidity and flexibility play an important role in
inducing liquid crystal properties of a substance.
• Group efficiency order derived on the basis of thermal stabilities for smectic and
nematic are as under.
Smectic: –CH3 > C2H5 > –C6H5
Nematic: –C6H5 > –CH3 > –C2H5

Synthesis and Liquid Crystal Behavior
31
• Group efficiency order derived on the basis of early commencement of mesophase
is as under.
Mesophase (smectic and/or nematic): –CH3 = –C2H5 > –C6H5
• Variations in mesomorphic property from series to series for the same homologue
and from homologue to homologue in the same series are due to the changing part
of a molecule at right handed terminal end group and sequentially added methylene
unit in the left n-alkoxy terminal end group respectively.
• Series-1 is a middle ordered melting type with relatively high thermal stability for
the nematic mesophase.
Acknowledgment
Authors acknowledge thanks to the Green Circle Inc. Lab for research facilities services as
and when needed. Authors also thank DR. N.N.Vyas, DR Vipul Patel, and DR M.L.Chauhan
for their valuable helping hand and microscopic facility. Thanks are due to the Sophisticated
Instrumentation Center for Applied Research and Testing, Vallabh Vidhyanagar, Gujarat
for analytical services.
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