Synthesis and mesomorphism of novel liquid crystalline: P-(p'-n-Alkoxy Benzoyloxy) methyl cinnamates

Article abstract of DOI:10.1080/15421406.2011.591664

A new homologous series p-(p'-n-alkoxy benzoyloxy) methyl cinnamates is synthesized with a view to understand and establish the relation between mesogenic properties and structure of molecules. Ethyl to octyl, decyl, dodecyl, tetradecyl, and hexadecyl homolgues are enantiotropically nematogenic, while the methyl homologue is nonmesogenic. None of the homologues exhibit a smectogenic mesophase. An odd-even effect is observed in the nematic-isotropic transition curve. The average thermal stability is 105.0C and the nematogenic mesophase ranges from 11.0C to 54.0C. Thus, the series is of a middle ordered melting type with moderate nematogenic range. Analytical data confirm the structure of the compounds, and the mesomorphism was identified by optical microscopy. The mesogenic properties are compared with structurally similar compounds. Copyright Taylor &Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.591664

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Synthesis and Mesomorphism of Novel
Liquid Crystalline: p-(p′-n-Alkoxy
Benzoyloxy) Methyl Cinnamates
M. L. Chauhan a , R. N. Pandya a & A. V. Doshi b
a Sheth P. T. Arts and Science College , Godhra, India
b Matushri Virbaima Mahila Science and Home Science College ,
Rajkot, India
Published online: 07 Oct 2011.
To cite this article: M. L. Chauhan , R. N. Pandya & A. V. Doshi (2011) Synthesis and Mesomorphism of
Novel Liquid Crystalline: p-(p′-n-Alkoxy Benzoyloxy) Methyl Cinnamates, Molecular Crystals and Liquid
Crystals, 548:1, 228-234, DOI: 10.1080/15421406.2011.591664
To link to this article: http://dx.doi.org/10.1080/15421406.2011.591664
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Mol. Cryst. Liq. Cryst., Vol. 548: pp. 228–234, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.591664
Synthesis and Mesomorphism of Novel Liquid
Crystalline: p-(pꢀ-n-Alkoxy Benzoyloxy)
Methyl Cinnamates
M. L. CHAUHAN,1,∗ R. N. PANDYA,1 AND A. V. DOSHI2
1Sheth P. T. Arts and Science College, Godhra, India
2Matushri Virbaima Mahila Science and Home Science College, Rajkot, India
A new homologous series p-(pꢀ-n-alkoxy benzoyloxy) methyl cinnamates is synthesized
with a view to understand and establish the relation between mesogenic properties
and structure of molecules. Ethyl to octyl, decyl, dodecyl, tetradecyl, and hexadecyl
homolgues are enantiotropically nematogenic, while the methyl homologue is nonmeso-
genic. None of the homologues exhibit a smectogenic mesophase. An odd–even effect
is observed in the nematic–isotropic transition curve. The average thermal stability
is 105.0◦C and the nematogenic mesophase ranges from 11.0◦C to 54.0◦C. Thus, the
series is of a middle ordered melting type with moderate nematogenic range. Analytical
data confirm the structure of the compounds, and the mesomorphism was identified by
optical microscopy. The mesogenic properties are compared with structurally similar
compounds.
Keywords Liquid crystal; nematic; smectic
Introduction
A number of homologous series with two phenyl rings bridged through a variety of central
linking groups, which causes variation in the molecular rigidity and flexibility, were reported
earlier [1,2]. Such differences in the molecular rigidity cause a difference in the types
and degrees of mesomorphism exhibited by the substance. The ester homologous series
formed by COO and CH CH COO linking groups considerably affect molecular
rigidity. Liquid crystals are important materials in several different fields, particularly
display devices and in the medical fields. Therefore, the present homologous series was
designed and synthesized with a vinylcarboxylate linking group, and its effects on the type
of mesophase and degree of mesomorphism were determined. Thus, the present study is
aimed at developing structure–property relationships in liquid crystals.
Experimental
Characterization
Some representative homologues were selected for full characterization by elemental anal-
1
1
ysis, IR, and HNMR. HNMR was recorded using CDCl3 as solvent. Thermodynamic
∗Address correspondence to M. L. Chauhan, Sheth P. T. Arts and Science College, Godhra
389001, India. E-mail: chauhanmukesh2@yahoo.com
228

Novel Liquid Crystalline Benzoyloxy Methyl Cinnamates
229
treatment related to thermal stability is qualitatively discussed by enthalpy (ꢀH) entropy
(ꢀS) concept. Mesomorphic properties were investigated by optical polarizing microscopy.
Synthesis
4-Hydroxy cinnamic acid, methanol, pyridine, thionyl chloride, 4-hydroxy benzoic acid,
and n-alkyl halides were used as received. The synthetic route to the present series is
reported here by selecting the octyloxy homologue of the series as outlined in Scheme 1;
other homologues were similarly prepared.
Scheme 1.
4-Hydroxy methyl cinnamate was synthesized by using a known method [4]. 4-n-
Alkoxy benzoic acids and 4-alkoxy benzoyl chlorides were synthesized by the modified
method of Dave and Vora [3]. The 11 esters of the series were synthesized by condensing
equimolar amounts of 4-n-alkoxy benzoyl chlorides with p-hydroxy methyl cinnamate
(A) in pyridine. Final products were recrystallized from alcohol until constant transition
temperatures were obtained [7,9,10].
Analytical Data for the Octyl Homologue
H NMR Spectra δ (ppm). 0.890 ( CH3 of C8H17), 1.308 ( CH2 of C8H17), 1.772
( OCH2 CH2 of C8H17), 3.816 ( O CH3), 4.021 ( O CH2- of O C8H17), 6.460
& 7.260 [ O C6H4-CH CH CO ], 6.905 & 8.026 (p-sub. benzene ring).
IR Spectra (cm−1). 2854.5 cm−1 – alkyl group, 1685.7 & 1168.8 cm−1
–
COO group
and ether group, 844.8 cm−1 – p-sub. benzene ring, 721.3 cm−1 – polymethylene of C8H17,
698.2 cm−1 – cis – CH CH group (see Table 2).
Results and Discussion
The titled homologous series consists of 12 homologues. Eleven homologues (ethyl to octyl,
decyl, dodecyl, tetradecyl, and hexadecyl) are liquid crystalline while the methyl homologue
is nonmesogenic. All the liquid crystalline homologues are enantiotropic nematogenic in
character with threaded type of texture. No smectic mesophase is exhibited by any of the
homologues. Transition temperatures and melting temperatures were determined by optical

230
M. L. Chauhan et al.
Table 1. Elemental analysis for hexyloxy and octyloxy derivatives
Molecular formula
Calculated%
C
Observed%
H
C23H26O5
C25H30O5
72.08 (72.25)
74.92 (75.0)
6.80 (6.80)
7.56 (7.50)
polarizing microscopy, and the results are recorded in Table 2. Transition temperatures are
plotted versus the number of carbon atoms in n-alkyl chain of the left n-alkoxy terminal
end group. A phase diagram (Fig. 1) is obtained by joining like or related points. Careful
observation of the phase diagram indicates that the solid–isotropic or solid–mesomorphic
transition curve falls steeply from the methyl to propyl homologue and then follows a partly
zigzag path of rising and falling as series is ascended. The nematic–isotropic transition curve
behaves in a normal manner with an odd–even effect observed initially, but has disappeared
at the seventh homologue. The nematic mesophase temperature range stretches from 11◦
to 54◦ at ethyl and pentyl derivatives of the series, respectively. Thus, the mesophase range
is of a moderate type. The mesomorphism of the titled homologous series – 1 is compared
with the other structurally similar homologous series A (11), B (6), and C (5), respectively.
4-n-Alkoxy benzoic acids are dimeric in structure linked through hydrogen bonding.
On esterification by 4-hydroxy methyl cinnamate, the hydrogen bonding is not present
and the resultant esters are obtained with lower transition temperatures with exhibition
of mesogenic character. The methyl homologue is nonmesogenic and melts sharply from
solid crystalline state to isotropic liquid. Exhibition of nematogenic character by ethyl to
hexadecyl homologues is attributed to the anisotropic intermolecular forces of attraction of
suitable magnitude arising from aligned molecules at an angle less than 90◦ on the plane
of surface resisting thermal vibrations imposed upon them. This situation does not restrict
statistically parallel orientational order of molecules, maintaining two-dimensional array
in floating condition causing occurrence and appearance of nematic mesophase within
definite range of temperature. Absence of smectic mesophase is attributed to the absence
of lamellar packing of molecules in their crystal lattices. The odd–even effect in the
Table 2. Transition temperatures in ◦C
Sr. No.
R = n-alkyl group
Smectic
Nematic
Isotropic
1.
2.
3.
4.
5.
6.
7.
8.
9.
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
—
—
—
—
—
—
—
—
—
—
—
—
—
143.0
107.0
108.0
118.0
98.0
110.0
102.0
105.0
103.0
109.0
99.0
96.0
68.0
78.0
62.0
58.0
62.0
66.0
65.0
83.0
79.0
79.0
10.
11.
12.
96.0

Novel Liquid Crystalline Benzoyloxy Methyl Cinnamates
231
Figure 1. p-(pꢀ-n-alkoxy benzoyloxy) methyl cinnamates.
nematic–isotropic transition curve observed up to seventh homologue is due to sequential
addition of methylene unit at the flexible left n-alkoxy terminal end group. In case of higher
homologues, the longer left n-alkyl chain of n-alkoxy group may be coiled or coupled to lie
in line with the major axis of core. Thus, end-to-end contact would then ultimately be the
same for odd- and even-numbered homologues beyond the seventh homologue [5,6,11].
The mesogenic characteristics of titled homologous series 1 are compared with struc-
turally similar homologous series as under (Fig. 2).
The average thermal stability and commencements of smectic/nematic mesophase are
given in Table 3 as under.
Table 3 indicates that homologous series 1 and B are entirely nematogenic, series C is
entirely smectogenic, and series A is partly smectogenic and partly nematogenic. In all of
the homologous series under comparison, two phenyl rings linked through central bridge
and left n-alkoxy terminal are the common identical features. Only terminal end groups
Table 3. Average thermal stability
Series
(1)
(A)
(B)
(C)
—
Nematic–isotropic
105.0
(C2–C16)
C2
138.25
(C3–C6)
C3
145.88
(C3–C16)
C3
Commencement of
nematic mesophase
Smectic–isotropic or
Smectic–nematic
Commencement of
smectic mesophase
—
—
—
135.9
(C7–C12)
C4
—
118.75
(C6–C12)
C5

232
M. L. Chauhan et al.
Figure 2. Phase diagram.
differ. Therefore, neglecting the effect of the central group, the degree of liquid crystallinity
and thermal stability, etc., vary with the magnitude of polarity and polarizability of the
terminal end groups for same homologue from series to series, and sequential addition of
methylene unit in left n-alkoxy terminal end group causes variation in length to breadth

Novel Liquid Crystalline Benzoyloxy Methyl Cinnamates
233
ratio which results in a variation in mesogenic properties from homologue to homologue
in the same series. Variation in length to breadth ratio can vary between COO and
CH CH COO by the difference CH CH , and hence, intermolecular forces of
attraction undergo variation [12–14].
The commencement of the smectic mesophase does not occur until the last hex-
adecyl homologue of the series 1 and B. The early commencement of the smectic
mesophase is related with less noncoplanarity of a molecule. Central bridge vinyl car-
boxylate CH CH COO has relatively greater length than the central bridge COO
unit. Longer length of central bridge causes more noncoplanarity due to a twist attained as
the oxygen atoms of the vinyl carboxy group bump into the nonbonded adjacent hydrogen
atoms of the aromatic phenyl rings in series A and B. Similarly, the oxygen atom of the
central carboxy COO group in the molecules of series 1 and C will be bumping into
the nonbonded sides of the adjacent hydrogens of aromatic ring which causes consider-
able strain on the molecule. Consequently, a twist around the C O bond occurs, forcing
the phenyl ring out of the plane of the molecule. Thus, the coplanarity of the molecule
is reduced to some extent making a molecule thick, and contributing to the variation in
commencement of smectic phase. Thus, smectic mesophase commences from butyl and
pentyl homologue in series A and C, respectively, while the smectic phase does not occur
or commence till the last homologue (hexadecyl) of the series 1 and B [15–17].
Conclusion
The present study of homologous series p-(-pꢀ-n-alkoxy benzoyloxy) methyl cinnamates
leads to the following conclusions.
i. The homologous series with a vinyl cinnamate terminal end group induces nemato-
genic character more favorably as compared to smectogenic character as induced
by the benzoate terminal end group, irrespective of any central bridge.
ii. The homologous series with a CH CH COO as central and terminal group
(series-B) is only nematogenic while a series with a COO as central and terminal
end group is only smectogenic (series-C).
iii. The present homologous series (series-1) with a CH CH COO terminal end
group is entirely nematogenic.
iv. The vinyl carboxylate group containing a CH CH unit of conjugated double
bond stabilizes the rigid core of a molecule and consequently, statistically paral-
lel orientations of molecules are relatively favored more than the sliding layered
arrangement of molecules in floating condition. Also, the increase in rigidity si-
multaneously increases the molecular thermal stability of the mesophase. The vinyl
carboxylate unit in a molecule hinders lamellar packing of molecules in crystal
lattices. Thus, the present investigation supports earlier research [8] and raises
credibility to the conclusions.
Acknowledgment
The authors thank Shri Jalaram Uchcha Kelavani Mandal, Saurashtra University, Rajkot and
the management of SPT Arts and Science College, Godhra for their valuable cooperation
in carrying out this research work. The authors are also thankful to CSMRI Bhavnagar for
analysis of samples.

234
M. L. Chauhan et al.
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