Synthesis and liquid crystal properties of a novel homologous series 4-(4′-n-alkoxy benzoyloxy) benzyl benzoates

Article abstract of DOI:10.1080/15421406.2014.884399

A novel homologous series consisting of 11 homologues is reported. Liquid crystal properties commence from the hexyloxy homologue and continue up to the tetradecyloxy homologue as enantiotropic nematic mesomorphism only. The rest of the homologues (including hexadecyloxy homologue) are not liquid crystalline. None of the homologues show smectogenic mesophase formation. The texture of the nematic mesophase is of a threaded or Schlieren type. The transition temperatures of the homologues were determined by optical polarizing microscopy equipped with a heating stage. Transition curves of a phase diagram behave in normal manner except for the tetradecyloxy homologue which adopts an unexpected trend of propagation. An odd-even effect is absent for nematic-isotropic transition curve. Analytical and spectral data support the molecular structure of the homologues. The average thermal stability of the series is 78.8°C. Mesomorphic/nematogenic phase length varies from 2.0°C to 13.0°C. The liquid crystal behavior of the present series is compared with structurally similar other known series. Thus, the present series is enantiotropically nematogenic only without exhibition of any smectogenic character with a low melting type, whose mesomorphic phase length is relatively short.

Full text of DOI:10.1080/15421406.2014.884399

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Synthesis and Liquid Crystal Properties
of a Novel Homologous Series 4-(4′-n-
Alkoxy benzoyloxy) Benzyl Benzoates
Brijesh H. Patela & A. V. Doshib
a Huntsman, Baroda Textile Effects Pvt. Ltd., Umraya village, Taluka
Padra, Vadodara, Gujarat, India
b Applied Science, Dr K. N. Modi University, Plot No. INS-1, RIICo
Industrial Area Ph-II, Newai, Dist. Tonk, Rajasthan-304 021, India
Published online: 15 Dec 2014.
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To cite this article: Brijesh H. Patel & A. V. Doshi (2014) Synthesis and Liquid Crystal Properties of a
Novel Homologous Series 4-(4′-n-Alkoxy benzoyloxy) Benzyl Benzoates, Molecular Crystals and Liquid
Crystals, 605:1, 61-69, DOI: 10.1080/15421406.2014.884399
To link to this article: http://dx.doi.org/10.1080/15421406.2014.884399
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Mol. Cryst. Liq. Cryst., Vol. 605: pp. 61–69, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.884399
Synthesis and Liquid Crystal Properties of a Novel
Homologous Series 4-(4ꢀ-n-Alkoxy benzoyloxy)
Benzyl Benzoates
BRIJESH H. PATEL1,∗ AND A. V. DOSHI2
1Huntsman, Baroda Textile Effects Pvt. Ltd., Umraya village, Taluka Padra,
Vadodara, Gujarat, India
2Applied Science, Dr K. N. Modi University, Plot No. INS-1, RIICo Industrial
Area Ph-II, Newai, Dist. Tonk, Rajasthan-304 021, India
A novel homologous series consisting of 11 homologues is reported. Liquid crystal
properties commence from the hexyloxy homologue and continue up to the tetradecyloxy
homologue as enantiotropic nematic mesomorphism only. The rest of the homologues
(including hexadecyloxy homologue) are not liquid crystalline. None of the homologues
show smectogenic mesophase formation. The texture of the nematic mesophase is of
a threaded or Schlieren type. The transition temperatures of the homologues were
determined by optical polarizing microscopy equipped with a heating stage. Transition
curves of a phase diagram behave in normal manner except for the tetradecyloxy
homologue which adopts an unexpected trend of propagation. An odd–even effect is
absent for nematic-isotropic transition curve. Analytical and spectral data support the
molecular structure of the homologues. The average thermal stability of the series
is 78.8◦C. Mesomorphic/nematogenic phase length varies from 2.0◦C to 13.0◦C. The
liquid crystal behavior of the present series is compared with structurally similar other
known series. Thus, the present series is enantiotropically nematogenic only without
exhibition of any smectogenic character with a low melting type, whose mesomorphic
phase length is relatively short.
Keywords Enantiotropy; liquid crystal; mesomorphic; smectic; nematic
Introduction
The potential ability of a molecular structure to induce liquid crystal mesophase formation,
depends, upon its internal energy, that is, energy stored by a molecular structure due to
its mass and other characteristics related to molecular rigidity and flexibility [1–3]. The
suitable magnitudes of the combined effects of anisotropic forces of intermolecular end to
end attractions and inter-layered attractions [4] as a consequence of favorable molecular
rigidity and flexibility, is the root cause of inducing liquid crystal behavior in a substance.
Such forces enable a two-dimensional array of molecular ordering or a less ordered orien-
tational organization, causing smectic, and nematic mesophase formation respectively. The
present investigation is planned with a view to understanding and establishing the effect of
molecular structure on liquid crystal behavior by constructing a molecule containing three
∗Address correspondence to Brijesh H. Patel, A-21, Saket Bunglows Opp Yash Complex Gotri
Baroda-390021, Gujarat, India; E-mail: research@dknmu.org
61

62
B. H. Patel and A. V. Doshi
phenyl rings and two central bridges viz. –COO– and –COO–CH2– linking phenyl rings as
a rigid core and an n-alkoxy (–OR) group as a flexible part of the molecule.
Experimental
Synthesis
4-n-Alkoxy benzoic acids were prepared by alkylating dimeric 4-hydroxy benzoic acid
using the suitable alkylating agents through the modified method of Dave and Vora [5].
4-Hydroxy benzyl benzoate was used as received and recrystallized and dried before use.
The m.p. (110◦C) of 4-Hydroxy benzyl benzoate agrees with reported data. Final products
were prepared by condensing 4-n-alkoxy benzoic acids through their corresponding acid
chlorides with 4-hydroxy benzyl benzoate in dry, ice cooled, pyridine [6,7]. Each final
product was individually decomposed, filtered washed, dried, and purified by alcohol until
the constant transition temperatures were obtained. The synthetic route to the series is
shown in scheme 1. The chemicals required for synthesis, 4-hydroxy benzoic acid, alkyl
halides, 4-hydroxy benzyl benzoate, thionyl chloride, MeOH, EtOH, KOH, and pyridine
were used as received, except solvents, which were dried and purified prior to use.
Characterization
Selected members of the present series were characterized by elemental analysis, Infra red
spectroscopy, 1HNMR spectroscopy, and mass spectrometry. These representative members
were characterized by elemental analysis using a Perkin-Elmer PE 2400 CHN analyzer,
infra red spectroscopy were recorded on Perkin-Elmer spectrum GX spectrometer and
1HNMR spectroscopy which were recorded on Bruker spectrometer using DMSO as the
solvent. The liquid crystal behavior, type of texture and transition temperatures of the
novel homologues were recorded and observed through optical polarizing microscopy in
conjunction with a heating stage. The type of nematic mesophase textures was determined
by a miscibility method. Thermodynamic quantities enthalpy change (δH) and entropy
change (δS) are qualitatively discussed.
Analytical Data
1
Spectral Data. HNMR in ppm for the octyloxy derivative: 3.4 (–OCH2 of –OC8H17
group), 4.0 (–COO–CH2–C6H5 group), 6.98 and 7.1 (p-substituted benzene), 7.8
and 8.07 (another p-substituted benzene), and 7.3–7.5 (multiplex mono-substituted
benzene).
1HNMR in ppm for the decyloxy derivative: 3.2 (–OCH2 of –OC10H21 group), 3.9
(–COO–CH2–C6H5 group), 6.7 and 7.03 (p-substituted benzene), 7.7 and 8.0 (another
p-substituted benzene), and 7.2–7.5 (multiplex mono-substituted benzene).
IR in cm−1 for the dodecyloxy derivative: 694 and 759 mono-substituted benzene,
842 p-substituted phenyl ring, 1215 ether linkage of (–O– group), 1730, 1256, and 1163
(–COO– ester group), and 723 (polymethylene of C12H25 group).
IR in cm−1 for the tetradecyloxy derivative: 694 and 760 mono-substituted benzene,
843 p-substituted phenyl ring, 1215 ether linkage of (–O– group), 1729, 1256, and 1163
(–COO– ester group), and 723 (polymethylene of C14H29 group).

Synthesis and Liquid Crystal Properties
63
Scheme 1. Synthetic route to the series.
Mass Spectra. Mass spectra for the pentyloxy derivative:
Molecular formula C26H26O5 Calculated Molecular weight 418. Practical mass ob-
served by Mass spectroscopy 418.
Texture of Nematic phase by miscibility method
Hexyloxy derivative ————–→ Threaded nematic
Octyloxy derivative ————–→ Schlieren nematic
Results and Discussion
The dimerization of 4-hydroxy benzoic acid or n-alkoxy benzoic acids disappears on es-
terification. 4-Hydroxy benzyl benzoate is a nonliquid crystal component, however, liquid
crystal properties are induced from the hexyloxy homologue to the tetradecyloxy ho-
mologue as an enantiotropic nematic phase, without the exhibition of any smectogenic

64
B. H. Patel and A. V. Doshi
Figure 1. Phase behavior of series.
mesophase. The methoxy to pentyloxy and hexadecyloxy homologues are nonliquid crys-
tal. A phase diagram, showing phase behavior (Fig. 1) of the novel series is drawn for the
number of carbon atoms present in n-alkyl chain of left n-alkoxy terminal end group versus
the transition temperatures, as determined by optical polarizing microscopy equipped with
a heating stage. The solid-nematic or isotropic and nematic-isotropic transition curves are
drawn smoothly by joining like or related points. The solid-isotropic or nematic transition
curve adopts a zigzag trend of rising and falling values in a normal manner. The nematic-
isotropic transition curve initially rises from the hexyloxy to the octyloxy homologue and
then descends as the series is ascended up to the dodecyloxy homologue, without exhibition
of an odd-even effect. Then, it abnormally ascends instead of showing descending tendency

Synthesis and Liquid Crystal Properties
65
for the tetradecyloxy homologue only. Thus, the nematic-isotropic transition curve behaves
in a usual established manner for all nematogenic homologues except the tetradecyloxy ho-
mologue. The average nematic thermal stability of the series is relatively low at 78.8◦C and
its nematogenic mesophase length varies between 2.0◦C and 13◦C. Liquid crystal behavior
of the novel series varies from homologue to homologue for changing molecular flexibility
[1–3] by the n-alkyl chain of the left n-alkoxy terminal end group. Thus, the presently
investigated novel homologous series is nematogenic and of a low-melting type, whose,
mesomorphic phase length is relatively short. The nematic-isotropic transition curve is ex-
trapolated [8,9] for the hexadecyloxy homologue in order to predict its ability for a proba◦ble
nematic-isotropic transition temperature, the hypothetical value comes out to be 105 C,
which is above the real or actual melting point. The disappearance of dimerization of the
alkoxy acids is attributed to the breaking of hydrogen bonding between two molecules.
The process of esterification yielded homologues that formed through acid chlorides with
4-hydroxy benzyl benzoate, and resulted in final products, 4-(4ꢀ-n-alkxoy benzoyloxy) ben-
zyl benzoates with relatively lower transition temperatures. Nonmesomorphic behavior of
methoxy to pentyloxy homologues is due to the relatively shorter n-alkyl chain length of
left n-alkoxy group, which induces unfavorable magnitudes of molecular rigidity and/or
flexibility that hinders the suitable magnitudes of anisotropic forces of intermolecular at-
tractions. The hexadecyloxy homologue with longer a n-alkyl chain is susceptible to a high
crystallizing tendency and so is not mesogenic. The molecules of homologues from hexy-
loxy to tetradecyloxy possess suitable magnitudes of anisotropic forces of intermolecular
attractions and so disalign at an angle less than ninety degree resisting exposed thermal
vibrations for definite range of temperature and show a statistically parallel orientational
order of molecules in floating condition with sufficient end to end attractions, favoring
exhibition of nematogenic behavior. The absence of smectogenic behavior of a novel series
is attributed to absence of lamellar packing of molecules in their crystal lattices, which is
hindered by insufficient magnitudes of intermolecular attractions caused by their molec-
ular structures. Absence of an odd–even effect in the nematic-isotropic transition curve
is attributed to (i) the absence of mesophase formation by the odd members of a series
viz. methoxy, propyloxy, pentyloxy, (ii) the exhibition of a nematogenic property by even
numbered higher homologues from hexyloxy to tetradecyloxy, whose longer n-alkyl chain
of left n-alkoxy terminal end group may undergo coiling, bending, flexing, or coupling
with major axis of the core structure to diminish the odd–even effect [4,7]. The observed
variations in nematogenic property in terms of the alternation of transition temperatures
from homologue to homologue is due to the sequential addition of methylene units in the
n-alkyl chain terminal end group. Low-melting behavior, shorter mesophase length and low
thermal stability of homologues indicates relatively inadequate magnitudes of intermolecu-
lar adhering forces to facilitate neither high degree of mesomorphism, nor lamellar packing
of molecules in the crystal lattices, to show smectogenic behavior. The liquid crystalline
behaviors of presently investigated novel homologous series-1 are compared with other
structurally similar known homologous series X [10] and Y [11] as shown below in Fig. 2.
Table 3 represents average thermal stability for smectic and/or nematic mesophases.
The present homologous series-1 is nematogenic and non-smectogenic while, series
X and Y selected for comparison are smectogenic in addition to nematogenic in character.
Average thermal stabilities for the nematic phase increases from the novel series 1 to series
X and Y chosen for comparison.
The smectic mesophase commences earliest from the second member of series Y, late
from the decyloxy homologue of series X and is not exhibited by series 1.

66
B. H. Patel and A. V. Doshi
Figure 2. Structurally similar homologous series.
The nematic mesophase commences from very first member of the series X and Y
chosen for comparison while, it commences late, from the hexyloxy homologue in series 1.
Homologous series 1 and X are similar with respect to three phenyl rings (two phenyl rings
in series Y) bridged through COO or CH CH COO groups and a left n-alkoxy
terminal end group linked with the first phenyl ring which is also commonly present from
series to series 1 to X to Y for the same homologue. However, the second central bridges
of COO CH2
difference.
,
N N and linking middle and third phenyl rings are a point of
The nematic-isotropic thermal stability progressively increases from series 1 to series
X and Y. Homologous series 1 and Y differ in their terminal end group of methyl or phenyl
(series Y and 1) excluding the difference of second and first central bridges COO CH2
and CH CH COO of series 1 and Y. Both these central bridges are comparable
through the vinyl carboxylate CH CH COO has greater length and causes more
noncoplanarity due to a twist obtained at the oxygen atoms of the vinyl carboxy group,
which bump into the non-bonded adjacent hydrogen atoms of the aromatic phenyl ring
[12]. Moreover, the phenyl ring terminal end group is more polarizable than the methyl
terminal group. The phenyl ring of series 1 increases molecular length and polarizability,
but the central group COO CH2 relatively reduces molecular length, while in case of
series Y the central group CH CH COO increases molecular length but at the same
time the methyl terminal group reduces molecular length and molecular polarizability. On
account of these differences, the smectic-nematic (absent) and nematic-isotropic thermal
stabilities of series 1 are lower than for series Y.
On comparing homologous series 1 and X, both series resemble each other in all
respects, except the second central bridge which is N N in case of series X and
Table 1. Elemental analysis for methoxy, ethoxy and propoxy derivatives
Elements % found (% calculated)
Sr. no.
Molecular formula
C
H
1
2
3
C22H18O5
C23H20O5
C24H22O5
72.69 (72.92)
73.12 (73.40)
73.65 (73.84)
4.73 (4.97)
5.15 (5.31)
5.48 (5.64)

Synthesis and Liquid Crystal Properties
67
Table 2. Transition temperatures of series in ◦C
Transition temperature in ◦C
n-alkyl group
Compound no.
( CnH2n+1) (n)
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
1
2
3
4
5
6
8
10
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
64.0
79.0
68.0
64.0
78.0
—
138.0
82.0
105.0
89.0
78.0
66.0
92.0
73.0
72.0
91.0
80.0
9
10
11
12
14
16
Sm: Smectic
Nm: Nematic
COO CH2 in case of presently investigated series 1 and series Y. Both these central
bridges are comparable. The oxygen atom of the central carboxy group in the molecules of
series 1 will be bumping into the non bonded sides of the adjacent hydrogens of the aromatic
phenyl ring which will cause considerable strain on the molecule [12]. Consequently, the
twist around C O bond will occur, forcing the phenyl ring out of the plane of a molecule.
Thus, the coplanarity of a molecule is reduced to some extent making phenyl ring thick. The
linear central bridge N N certainly endows the molecules with coplanarity. On account
of this difference, the smectic-nematic and nematic-isotropic thermal stabilities of series 1
are in general far lower than the corresponding thermal stabilities of the homologous series
X. The variations in liquid crystal properties from series to series for the same homologue is
attributed to the differing part or differing structural configuration of the molecules and the
variations from homologue to homologue in a same series is attributed to the sequentially
added methylene units.
1. Conclusions
2. The smectic and nematic group efficiency order derived on the basis of (a) thermal
stability and (b) on the basis of early commencement of mesophase are as under.
Table 3. Average thermal stability in ◦C
Series→
smectic-nematic
or smectic-
Series 1
—
Series X
Series Y
114.0 (C10–C16) C10
120.75 (C7–C12) C2
isotropic
commencement
of smectic phase
nematic-isotropic
commencement
of smectic phase
78.8 (C6 – C14) C6
138.54 (C1–C8) C1
119.3 (C5–C6) C1

68
B. H. Patel and A. V. Doshi
>
>
a. Smectic: CH CH COO
N N
COO CH2
COO CH2
COO CH2
COO CH2
>
>
Nematic: N N
b. Smectic: CH CH—COO
Nematic: CH CH COO
CH CH—COO
>
>
N N
N N
>
=
3. Normally ester series are smectogenic, but the presence of CH CH unit along
with COO unit, induces nematic tendency more in comparison with smectogenic
tendency.
4. The novel homologous series of present investigation is nematogenic without exhi-
bition of the smectic phase with low temperature transition temperatures, thermal
stability and poor degree of mesomorphism.
5. Variations in mesomorphic properties are directly related to the molecular structure
of as substance.
6. Suitable magnitudes of anisotropic forces as a consequence of favorable molec-
ular rigidity and/or flexibility operate or governs the possibility of liquid crystal
formation tendency and the degree of liquid crystallinity.
7. The substitution of functional group or groups of proper polarity at lateral or
terminal position or positions can yield, liquid crystal compounds of rich degree of
mesomorphism.
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
Analytical Instrumentation Facility, Punjab University, Chandigarh for analytical services.
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