Synthesis and Study of Novel Mesogenic Homologous Series: 4-(4′- n -alkoxy benzoyloxy)- n -propyl Cinnamates

Article abstract of DOI:10.1080/15421406.2015.1036493

A novel homologous series of liquid crystals viz. 4-(4′-n-alkoxy benzoyloxy) n-propyl cinnamates comprising of 11 homologues has been synthesized. It is a middle ordered melting type of series with exhibition of smectic and nematic mesophases. The first to fifth members of the series are non-mesomorphic. Mesomorphic behavior commences from the sixth homologue. The octyloxy (C8) and decyloxy (C10) homologues are polymesomorphic. The smectic-nematic and nematic-isotropic transition curves behave in a normal manner except for the hexadecyl derivative, which slightly deviates from normal behavior (N-I). An odd-even effect was not observed for the N-I or Sm-N transition curves. The texture of nematic phase is threaded or Schlieren and that of the smectic phase is focal conic fan shaped of the smectic-A type. Transition temperatures and textures of mesophases were observed through an optical polarizing microscope equipped with a heating stage. Thermal stability and mesomorphic characteristics are compared with a structurally related series. Analytical and spectral data support the structure of the molecules.

Full text of DOI:10.1080/15421406.2015.1036493

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and Study of Novel Mesogenic
Homologous Series: 4-(4′-n-alkoxy benzoyloxy)-n-
propyl Cinnamates
M. L. Chauhan, A. V. Doshi & P. T. Sheth
To cite this article: M. L. Chauhan, A. V. Doshi & P. T. Sheth (2015) Synthesis and Study of Novel
Mesogenic Homologous Series: 4-(4′-n-alkoxy benzoyloxy)-n-propyl Cinnamates, Molecular
Crystals and Liquid Crystals, 623:1, 157-165, DOI: 10.1080/15421406.2015.1036493
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1036493
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Date: 19 April 2016, At: 22:49

Mol. Cryst. Liq. Cryst., Vol. 623: pp. 157–165, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1036493
Synthesis and Study of Novel Mesogenic
Homologous Series: 4-(4ꢀ-n-alkoxy
benzoyloxy)-n-propyl Cinnamates
M. L. CHAUHAN,1,* A. V. DOSHI,2 AND P. T. SHETH1
1Arts & Science College, Godhra, India
2Matushri Virbaima Mahila Science & Home Science College, Saurastra
University, Rajkot, Gujarat, India
A novel homologous series of liquid crystals viz. 4-(4ꢀ-n-alkoxy benzoyloxy) n-propyl
cinnamates comprising of 11 homologues has been synthesized. It is a middle ordered
melting type of series with exhibition of smectic and nematic mesophases. The first to
fifth members of the series are non-mesomorphic. Mesomorphic behavior commences
from the sixth homologue. The octyloxy (C8) and decyloxy (C10) homologues are poly-
mesomorphic. The smectic–nematic and nematic–isotropic transition curves behave in a
normal manner except for the hexadecyl derivative, which slightly deviates from normal
behavior (N–I). An odd–even effect was not observed for the N–I or Sm–N transition
curves. The texture of nematic phase is threaded or Schlieren and that of the smectic
phase is focal conic fan shaped of the smectic-A type. Transition temperatures and tex-
tures of mesophases were observed through an optical polarizing microscope equipped
with a heating stage. Thermal stability and mesomorphic characteristics are compared
with a structurally related series. Analytical and spectral data support the structure of
the molecules.
Keywords Liquid crystals; mesogen; mesomorphic; nematic; smectic
1. Introduction
Liquid crystalline state of matter is exploited in electronic display devices, medical instru-
ments pharmaceuticals, and medicinal preparations and many other fields of applications.
[1] Our continuous efforts of studying the effects of molecular structure on liquid crystal
behavior continue with the present study, which is aimed at n-propyl cinnamates [2, 3]
with an n-alkoxy (-OR) group at the other end of a molecule. A central bridge of –COO-
and two phenyl rings included, and the left n-alkoxy group is varied in size in order to
investigate the effect of adding sequential –CH2- units on mesomorphic behavior.[4, 5] Our
present study is restricted to synthesize novel substances, their characterization, thermal
transition temperatures, and the discussion on the effect of molecular structure on liquid
crystal behavior of a substance [6–8] in terms of molecular rigidity and flexibility.
∗Address correspondence to M. L. Chauhan, Arts & Science College, Godhra, India. Email:
chauhanmukesh2@yahoo.com
157

158
M. L. Chauhan et al.
2. Experimental
2.1. Synthesis
• p-n-Alkoxy benzoic acids were prepared by the modified method of Vora and Dave
(1970 ) [9], using suitable alkylating agent (R-X)
• p-n-alkoxy benzoyl chlorides were prepared by refluxing corresponding alkoxy
benzoic acids with freshly distilled [10] thionyl chloride.
• p-Hydroxy n-propyl cinnamates was prepared by reacting p-Hydroxy cinnamic acid
with corresponding freshly distilled and dried n-propyl alcohol in presence of con-
centrated sulfuric acid.[11]
• Acid chlorides were directly condensed (Doshi and Lohar 1992, 1993) [12] with the
p-n-hydroxy n-propyl cinnamates dissolved in ice cooled dry pyridine in portions
with stirring reaction mixture. Products were decomposed, filtered, washed, dried
and purified, till the constant transition temperatures obtained. 4-Hydroxy benzoic
acid, alkyl halides (R-X), methanol, KOH, thionyl chloride, 4-Hydroxy cinnamic
acid, n-propyl alcohol, con. H2SO4, pyridine etc required for synthesis were used as
received except solvents which were dried and distilled prior to use. The synthetic
route to the series is shown in Scheme 1.
2.2. Characterization
Representative members of the series were characterized by elemental analysis through
Perkin Elmer PE 2400 CHN analyser, infrared spectra as recorded on a Perkin Elmer
Spectrum GX, 1HNMR spectra on Bruker spectrometer using CDCI3 as solvent and mass
spectra (Table 1). Transition temperature and the liquid crystal properties of textures of
mesophases were determined by a miscibility method through an optical polarizing micro-
scope equipped with a heating stage (Table 2).
3. Results and Discussion
The titled novel homologous series 4-(4ꢀ-n-alkoxy benzoyloxy)-n-propyl cinnamates is me-
somorphic in nature. Mesomorphism is displayed by the members of the series in an enan-
tiotropic manner. The first five members of the homologous series are non-mesomorphic in
nature. The rest of homologues are mesomorphic. The octyloxy and decyloxy homologues
are polymesomorphic, while the remaining mesomorphs are only nematogenic with re-
versible transition temperatures. Mesomorphic transition temperatures are relatively lower
than the corresponding n-alkoxy acids. Transition temperatures are plotted against the num-
ber of carbon atoms in the terminal alkoxy chain. The transition curves (N–I and Sm–N)
are obtained in phase diagram by linking like or related points are shown in Fig. 1. The
transition curves of N–I and Sm–N of the series do not exhibit an odd–even effect. The
solid–mesomorphic transition curve partly follows zigzag path of rising and falling values
and finally descends as series is ascended. The smectic–nematic (Sm–N) transition curve
rises and then on extrapolation [13, 14] merges into nematic–isotropic (N–I) transition
temperature curve at dodecyloxy homologue and then matches with solid–nematic point
of tetradecyloxy homologue. Thus, both transition curves behave in a normal manner. The
N–I transition curve is descended from the hexyloxy to tetradecyloxy homologues as series
is ascended and behave in a normal manner, but the hexadecyloxy homologue behaves in
an abnormal manner, i.e. the N–I transition curve shows an ascending tendency instead

Synthesis and Study of Novel Mesogenic Homologous Series
159
Scheme 1. Synthetic route to the series.
of a descending tendency. The Sm–N transition curve is extrapolated to the dodecyloxy
and tetradecyloxy derivatives by keeping in view of a trend of curvature and normal ex-
pected trend to determine their Sm–N transition temperatures as found absent in reality.
Mesomorphic phase length vary between 12◦C and 44◦C at tetradecyloxy and octyloxy
homologue, respectively. Smectic and nematic mesophase length vary from 12◦C to 18◦C
and 11◦C to 34◦C, respectively. Smectic and nematic thermal stabilities are 107.5 and
121.0, respectively. Smectic and nematic mesophases commences from octyloxy and hexy-
loxy homologues respectively. Smectogenic mesophase formation discontinued from and
beyond dodecyloxy homologue derivative due to unfavorable magnitudes intermolecular
attractions consequence of unsuitable molecular rigidity and flexibility to induce lamellar
packing of molecules in their crystal lattices. Thus, variations in mesomorphic properties
from homologue to homologue in presently investigated series is attributed to the changing
–CH2—unit or units added progressively in n-alkyl chain of the left n-alkoxy terminal end
group which causes variations in molecular length, polarity and polarizability, end to end
and lateral intermolecular attractions. 4-Hydroxy n-propyl cinnamate is nonmesomorphic

160
M. L. Chauhan et al.
Table 1. Elemental analysis
Calculated %
Observed %
Sr.
No.
R = n-alkyl
Molecular
formula
Chain
C
H
C
H
1
2
3
4
Ethyl
Octyl
C21H22O5
C27H34O5
C33H46O5
C35H50O5
71.18
73.97
75.86
76.36
6.21
7.76
8.81
9.09
71.03
73.83
75.61
76.19
6.29
7.89
8.92
9.01
Tetradecyl
Hexadecyl
IR Data in cm−1 for pentyl homologue.
1HNMR Spectra in ppm for tetradecyloxy homologue.
Homologue
IR Spectra (cm−1
)
3402.2 cm−1 → H- bonded O-H group
2927.7, 2869.9 & 1183.7 cm−1 → alkyl group
Pentyl
1685.7, 1269.1 & 1056.9 cm−1 → Conforms –COO- group
1199.6→ ether group
983.6 cm−1 → –CH CH- group
825.5 cm−1→ p-sub. benzene ring
NMR Spectra (δ ppm)
Homologue
Tetradecyl
0.88 ( -CH3 of –C14H29)
ꢀ
6.908 6.949
8.045 8.063
1.261 (-CH2- of –C14H29)
4.021 (-O-CH2 of –C14H29)
(Two p-sub. benzene ring)
7.263
Note. Textures by miscibility method for Hexyl, Hexadecyl, Dodecyl, Octyl and Decyl derivatives.
Hexyl: Threaded nematic Dodecyl: Threaded nematic.
Hexadecyl: Schlieren nematic Octyl: Smectic – A
Decyl: Smectic –A.
(M.P. 240◦C), however on linking it with n- alkoxy benzoic acid through corresponding
acid chlorides yielded esters with lower transitions, because of the breaking of hydrogen
bonding between two molecules of n-alkoxy benzoic acids through esterification process.
Nonmesomorphicity of methoxy to pentyloxy homologues is attributed to their high crys-
tallizing tendency arising from unfavorable magnitudes of molecular rigidity and flexibility
[6, 7, 8] which results into their inability to resists the exposed thermal vibrations to float
on the surface with two-dimensional array, but their molecules are sharply transformed
into isotopic liquid from crystalline solid state without passing through mesomorphic state.
Thus, molecules of non mesomorphs are randomly oriented in all possible directions with
high order of molecular disorder or high entropy (ꢀS = ꢀH/T). The exhibition of smec-
togenic property by octyloxy and decyloxy homologues is attributed to the presence of
lamellar packing of molecules in their crystal lattice which resisted exposed thermal vibra-
tions and maintained molecular dis alignment at an angle 90◦ or less than 90◦ with the plane
of a floating surface and molecules occupy themselves as sliding layered arrangement in

Synthesis and Study of Novel Mesogenic Homologous Series
161
Table 2. Transition temperatures
Transition temperatures in ◦C
Sr.
n-alkyl
No.
Group
Smectic
Nematic
Isotropic
1
2
3
4
5
6
7
8
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Octyl
Decyl
Dodecyl
C1
C2
C3
C4
C5
C6
C8
C10
C12
C14
C16
—
—
—
—
—
—
—
—
—
—
102.0
102.0
113.0
78.0
92.0
99.0
260.0
265.0
230.0
148.0
113.0
136.0
134.0
124.0
109.0
104.0
119.0
—
90.0
95.0
—
—
—
9
10
11
Tetradecyl
Hexadecyl
floating condition for definite range of temperature and then on continued exposure of ther-
mal vibrations at higher temperature same molecules occupy themselves to float with the
ordered arrangement as statistically parallel orientational order of molecules with threaded
or schlieren texture of nematic mesophase. However on further heating the same from and
beyond isotropic temperature, molecules acquire isotropic state. On cooling the same, the
reversal of mesophase, viz. nematic and then smectic reappears at the same temperature
at which relevant mesophase was appeared on heating. Rest of the nematogenic meso-
morphs, except octyl and decyl homologue derivatives, exhibited only nematic mesophase
formation from and below isotropic temperature, in which dis-aligned molecules arrange
themselves to float on the surface with maintenance of statistically parallel orientational,
two-dimensional order of molecules reversibility by favorable end to end attractions show-
ing only nematogenic mesophase formation. The absence of odd–even effect for Sm–N
and N–I transition curve is due to late (i.e. from hexyloxy homologue) commencement
of mesophase and Nonmesomorphicity exhibited by prior homologues no. 1, 2, 3, 4, 5.
Extrapolation of Sm–N transition curve indicate that, Sm–N transition temperature ranges
into N–I transition temperature 109◦C which conforms and prove the absence of smectic
mesophase in dodecyloxy homologue derivative. On extending the extrapolation of Sm–N
transition curve up to tetradecyloxy homologue, the Sm–N transition temperature matches
exactly at 104◦C which is a solid–nematic transition temperature of a tetradecyloxy homo-
logue derivative. It means that there is no possibility to occur smectic mesophase formation
by tetradecyloxy homologue. Sm–Nm transition curve is not extended to extrapolate up
to hexadecyloxy homologue because N–I transition curve deviates from normal behav-
ior to some extent for hexadecyloxy homologue. The discussion and extrapolation ruled
out for the same (C16). The changing mesomorphic behaviors of presently investigated
novel homologues of present series-1 are compared with the structurally similar known
other homologous series-X [15] and series-Y [16] chosen for comparison as shown in
Fig. 2.
Homologous series (1), (X) and (Y) under comparison contain two phenyl rings and
central bridge –COO– as common identical part as rigid core of the molecules. Left n-
alkoxy group varies from homologue to homologue in the same series but characteristics

162
M. L. Chauhan et al.
Figure 1. Transition curves (N–I and Sm–N) are obtained in phase diagram by linking like or related
points.
are comparable for the same homologue from series to series. Right terminal end groups are
varied from series to series. Left and right terminal end groups of the molecules are acting
as flexible parts of the molecules. Therefore, variations in mesomorphic characteristics
and degree of mesomorphism are attributed to the variations of the terminal end-to-end
attraction due to combine effects of molecular rigidity and flexibility responsible to words
variations of mesomorphic behavior from series to series. Homologous series-1 and series-
X differs by –CH2- unit at the end group for the same homologue of series. Therefore
molecular length of series-X is relatively shorter for same homologue from series-1 to

Synthesis and Study of Novel Mesogenic Homologous Series
163
Figure 2. Structurally similar homologous series.
series-X. Homologous series-1 and series-Y differs with respect to terminal end groups
–CH CH-COO-, and –COO- respectively for the same homologue. Therefore observe
difference in molecular length, length to breadth ratio, molecular polarizability, dispersion
forces, intermolecular end-to-end attraction permanent dipole moment across the long
molecular axis, dipole–dipole interactions etc. contributes to the variation in mesomorphic
behaviors for same homologue from series to series. Table 3 represent thermal stabilities
for commencement of smectic and/or nematic mesophase and exhibition of mesomorphism
of homologues etc. as under.
Table 3 indicates that,
• Smectic thermal stability of present series -1 is greater than the series –X and series
– Y chosen for comparison.
• Nematic mesophase formation is exhibited by the homologous series-1 and series-
X containing –CH CH-COO- terminal end group to which -C3H7 (n) and –C2H5
group bonded.
• Nematic mesophase formation is absent for series-Y containing –COOC3H7 (n) end
group.
• Nematic thermal stability of presently investigated series-1 is greater than the series-
X and series-Y.
• Smectic mesophase commences later (from C8) than the series-X and series-Y (from
–C6 )
• Nematic mesophase commences earlier (from –C4 ) by series-X than present series-1
and it does not commences till the last hexadecyloxy homologue for series-Y.
• Homologous series-1 and series-X containing conjugated double bond as –CH CH-
are predominantly nematogenic and partly smectogenic whereas homologous series-
Y containing –COOC3H7 (n) end group with absence of –CH CH- unit for end
group is smectogenic without exhibition of nematic character.
Table 3. Average thermal stabilities in ◦C
Series
(1)
(X)
(Y)
Smectic–nematic or smectic–isotropic 107.5 (C8–C10)
Commencement of nematic phase C8
Nematic–isotropic or isotropic–nematic 121.0 (C6–C16) 89.0 (C6-C12)
Commencement of smectic phase C6 C6
—
—
82.75 (C6–C12)
C6
—
—

164
M. L. Chauhan et al.
Looking to the above comparative points, the differences are mainly associated with
the presence and the absence of –CH CH- unit at the end group, including effect due to
linking of –C3H7 (n) and –C2H5 bonded to –COO- group. Decreasing order of thermal
stability for present series-1 and series-X is attributed to the decreasing transitions as the
chain length decreased from –C3H7 (n) to –C2H5 at the right handed terminal end group.
Thus, decreasing tendency of suitable magnitudes of anisotropic forces of intermolecular
attractions is very well supported by present study. Now on comparing thermal stabilities
which is higher of present series-1 and lower for the series-Y which containing common
–C3H7 (n) group bonded to –CH CH-COO- and –COO- respectively at the end of the
molecules. In all other respects except the terminally situated end group –CH CH-COO-
in case of a homologous series-1 and –COO- in case of series-Y are closely similar to
each other. Both these end groups are comparable though the vinyl carboxylate –CH CH-
COO- has greater length and causes more noncoplanarity due to a twist obtained as the
oxygen atoms of the vinyl carboxy group bump into the non-bonded adjacent hydrogen
atom of the phenyl ring. On account of these difference, the smectic thermal stabilities of
homologous series-1 is in general way higher than series-Y (or smectic thermal stability of
Y is lower than series-1). Exhibition of nematic property by series-1 in addition to smectic
property is also attributes to added –CH CH- unit which increases intermolecular end-
to-end attractions as a consequence of higher molecular rigidity of series -1 as compared
to series-Y. The extent of noncoplanarity caused by the molecules differs from series to
series due to differing –CH CH-COO- and –COO- as well as –C2H5 and –C3H7 (n) group
which facilitate early or late commencement of mesophase or mesophases. Thus, all the
homologous series-1, series-X, and series-Y under comparison though possess linear lath
like geometrical shape, but their mesomorphic behavior vary with the differing end groups
–CH CH-COOC3H7 (n), –CH CH-COOC2H5 and –COOC3H7 (n) from series to series
for the same homologue.
4. Conclusions
• Variations in mesomorphic properties from homologue to homologue in the same
series is attributed to the sequentially and progressively added methylene unit at the
n-alkyl chain of the left n-alkoxy terminal end group.
• Variations in mesomorphic property from series to series for the same homologue
is attributed to the changing right handed end group. viz. –CH CH-COOC3H7 (n),
-CH CH-COOC2H5, and –COOC3H7 (n).
• Presently investigated novel series is a predominantly nematogenic and partly smec-
togenic with middle ordered melting type and mesomorphic range.
• Group efficiency order derived on the basis of (i) thermal stability and (ii) the early
commencement of smectic and nematic phase is as under.
(i) Smectic:
-CH CH-COOC3H7(n) > –COOC3H7 (n) > -CH CH- COOC2H5
Nematic:
-CH CH-COOC3H7(n) > -CH CH- COOC2H5 > –COOC3H7 (n)
(ii) Smectic:
–COOC3H7 (n) = -CH CH- COOC2H5 > -CH CH-COOC3H7(n)
Nematic:
-CH CH- COOC2H5 > -CH CH-COOC3H7(n) > –COOC3H7 (n)

Synthesis and Study of Novel Mesogenic Homologous Series
165
• –CH CH-COO- group is preferably nematogenic while -COO- group is preferably
smectogenic.
• Phenomenon of mesomorphism depends upon favorable molecular rigidity and flex-
ibility and very sensitive and susceptible to the suitable magnitudes of anisotropic
forces of intermolecular adhesion as a consequence of molecular rigidity and flexi-
bility.
Acknowledgments
The authors are thankful to the management of M. V. M. Science and Home Science College,
Rajkot and Dr. N. D. Jadav (Retd. Prof.) of Faculty of Technology and Engineering. The
M.S. University of Baroda, Vadodara, for their valuable co-operation in the work. The
authors are also thankful to scientists of National Salt and Marine Research Institute,
Bhavnagar for their analytical services.
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