Synthesis and liquid crystalline properties of novel ethylene derivatives

Article abstract of DOI:10.1080/15421406.2013.781491

A novel homologous series of liquid crystals, viz., α-4-[4′-n- alkoxy cinnamoyloxy] benzoyl-β-4″-chloro phenyl ethylenes, has been synthesized. The series consists of twelve homologs. The methyl to butyl derivatives are not liquid crystalline and the rest of the homologs are enantiotropic liquid crystals. The octyl to dodecyl derivatives of the series are enantiotropically smectogenic in addition to nematogenic in character, but the pentyl, hexyl, heptyl tetradecyl, and hexadecyl homolog derivatives of the series are only nematogenic without the exhibition of a smectic phase. The transition temperatures of the novel substances were determined by optical polarizing microscopy equipped with a heating stage. A plot of transition temperatures versus the number of carbon atoms present in the n-alkoxy terminal chain represents the phase behavior of the series. An odd-even effect is observed for the nematic-isotropic transition curve. The textures of the nematic phase are of a threaded or Schlieren type and those of the smectic A and smectic C phases are typical. Analytical and spectral data agree with the molecular structures. The smectic and nematic thermal stabilities are 132.6°C and 156.7°C, respectively. The smectic phase commences from the octyloxy homolog. Smectogenic phase lengths vary from 26°C to 33°C and the nematogenic phase lengths vary from 6°C to 70°C. The series is predominantly nematogenic and partly smectogenic with considerable mesophase length and a middle-ordered melting type. The liquid crystal properties of the present series are compared with structurally similar homologous series.

Full text of DOI:10.1080/15421406.2013.781491

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Synthesis and Liquid Crystalline
Properties of Novel Ethylene Derivatives
H. N. Chauhan a , R. R. Shah a & A. V. Doshi b
a Department of Chemistry , K. K. Shah Jarodwala Maninagar
Science College , Ahmadabad , Gujarat , India
b Department of Chemistry , Shri Jagdishprasad Jhabarmal Tibrewala
University (JJT Uni.) Jhunjhunu , Rajasthan , India
Published online: 09 Jul 2013.
To cite this article: H. N. Chauhan , R. R. Shah & A. V. Doshi (2013) Synthesis and Liquid Crystalline
Properties of Novel Ethylene Derivatives, Molecular Crystals and Liquid Crystals, 577:1, 36-43, DOI:
10.1080/15421406.2013.781491
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Mol. Cryst. Liq. Cryst., Vol. 577: pp. 36–43, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.781491
Synthesis and Liquid Crystalline Properties
of Novel Ethylene Derivatives
H. N. CHAUHAN,1,∗ R. R. SHAH,1 AND A. V. DOSHI2
1Department of Chemistry, K. K. Shah Jarodwala Maninagar Science College,
Ahmadabad, Gujarat, India
2Department of Chemistry, Shri Jagdishprasad Jhabarmal Tibrewala University
(JJT Uni.) Jhunjhunu, Rajasthan, India
A novel homologous series of liquid crystals, viz., α-4-[4ꢀ-n-alkoxy cinnamoyloxy]
benzoyl -β-4ꢀꢀ-chloro phenyl ethylenes, has been synthesized. The series consists of
twelve homologs. The methyl to butyl derivatives are not liquid crystalline and the rest
of the homologs are enantiotropic liquid crystals. The octyl to dodecyl derivatives of the
series are enantiotropically smectogenic in addition to nematogenic in character, but
the pentyl, hexyl, heptyl tetradecyl, and hexadecyl homolog derivatives of the series are
only nematogenic without the exhibition of a smectic phase. The transition temperatures
of the novel substances were determined by optical polarizing microscopy equipped with
a heating stage. A plot of transition temperatures versus the number of carbon atoms
present in the n-alkoxy terminal chain represents the phase behavior of the series. An
odd-even effect is observed for the nematic-isotropic transition curve. The textures of the
nematic phase are of a threaded or Schlieren type and those of the smectic A and smectic
C phases are typical. Analytical and spectral data agree with the molecular structures.
The smectic and nematic thermal stabilities are 132.6◦C and 156.7◦C, respectively.
The smectic phase commences from the octyloxy homolog. Smectogenic phase lengths
vary from 26◦C to 33◦C and the nematogenic phase lengths vary from 6◦C to 70◦C.
The series is predominantly nematogenic and partly smectogenic with considerable
mesophase length and a middle-ordered melting type. The liquid crystal properties of
the present series are compared with structurally similar homologous series.
Keywords Liquid crystal; smectic; nematic; mesomorphic; thermotropic
Introduction
The unique properties of the liquid crystalline state of a substance is exploited in varying
fields of application in present era. Therefore, it is interesting to study the relationship be-
tween molecular structure and the liquid crystal state of a substance through homologous se-
ries, whose molecular rigidity and flexibility [1–3] dictate the formation of mesomorphism.
Liquid crystal phases are formed as a consequence of suitable magnitudes of anisotropic
intermolecular forces of attraction [4], and as a consequence of favorable molecular rigid-
ity and/or flexibility. The novel series under investigation has a molecular structure based
on three phenyl rings linked through CH CH COO and CO CH CH central
∗Address correspondence to Chauhan, Department of Chemistry, K. K. Shah Jarodwala Mani-
nagar Science College, Ahmadabad, 380008, Gujarat, India. E-mail: shreeyashailee@yahoo.in
36

Properties of Novel Ethylene Derivatives
37
linking groups, two terminal end groups of OR and Cl, hence the structures are linear
with no lateral substituents.
Experimental
Synthesis
4-Hydroxy benzaldehyde was treated with malonic acid in pyridine to prepare 4-hydroxy
cinnamic acid. 4-Hydroxy cinnamic acid was alkylated by suitable alkylating agents [R-
X] and then, the corresponding cis n-alkoxy cinnamic acids were converted to their acid
chlorides [A] using freshly distilled thionyl chloride by modified method of Dave and
Vora [5]. The second reacting component [B], i.e., α-4-hydroxy benzoyl -β-4ꢀ-chloro
phenyl ethylene was prepared by the combination of 4-hydroxy acetophenone with 4-chloro
benzaldehyde by an established method [6]. Components [A] and [B] were condensed in
dry cold pyridine [7]. The Final products of series were individually filtered, washed, dried,
and purified until they gave constant transition temperatures. The chemicals required for
synthesis, viz., 4-hydroxy benzaldehyde, pyridine, malonic acid, alkyl halides, 4-hydroxy
acetophenone, 4-chloro benzaldehyde, MeOH, EtOH, KOH, were used as received except
solvents which were dried and purified before use. The synthetic route to the series is
illustrated below in Scheme 1.
Characterization
Some of the selected homologs of the novel series were characterized for the determination
of molecular structures by elemental analysis, infra red [IR] and 1H NMR (Nuclear Mag-
netic Resonance) techniques. Microanalysis was performed using a Perkin Elmer PE 2400
Scheme 1. Synthetic route to the novel series.

38
H. N. Chauhan et al.
1
CHN analyzer. IR spectra were recorded on a Perkin-Elmer spectrum GX, and H NMR
spectra were recorded on a Bruker instrument using CDCl3 as a solvent. The mesogenic
properties and transition temperatures of the series were investigated using an optical po-
larizing microscope equipped with heating stage. The textures of the smectic and nematic
mesophases were determined by miscibility methods.
Analytical Data
Spectral Data
NMR in ppm for Pentyloxy Derivative. 0.88 ( CH3 of OC5H11 group), 1.20 ( CH2 )n
polymethylene group of OC5H11), 4.02 (triplet) ( OCH2 CH2 of OC5H11), 3.954
( Cl group), 7.77 ( CO CH CH group), 6.87 to 8.08 (p substituted phenyl ring).
The NMR data are consistent with the molecular structure.
NMR in ppm for Dodecyloxy Derivative. 0.87 ( CH3 of OC12H25 group), 1.26
( CH2 )n polymethylene group of OC12H25), 4.01 (triplet) ( OCH2 CH2 of
OC12H25), 4.41 ( Cl group), 7.88 ( CO CH CH group), 6.90 to 8.13 (p substituted
phenyl ring). The NMR data are consistent with the molecular structure.
NMR in ppm for Tetradecyloxy Derivative. 0.87 ( CH3 of OC14H29 group), 1.25
( CH2 )n polymethylene group of OC14H29), 4.01 (triplet) ( OCH2 CH2 of
OC14H29), 4.29 ( Cl group), 7.83 ( CO CH CH group), 6.81 to 8.13 (p substituted
phenyl ring). The NMR data are consistent with the molecular structure.
IR in cm−1 for Propoxy Derivative. 750 polymethylene ( CH2
)
group of OC3H7,
n
Cl group, 885 p substituted phenyl ring, 1160 ( CO of OC3H7 group), 1200, 1260
and 1600 ( COO ester group), 1000 ( CH CH group). The IR data are consistent
with the molecular structure.
IR in cm−1 for Octyloxy Derivative. 770 (polymethylene ( CH2 )n group of OC8H17),
640 Cl group, 845 (p substituted phenyl ring), 1160 ( CO of OC8H17 group), 1260
and 1605 ( COO ester group). The IR data areconsistent with the molecular structure.
IR in cm−1 for Dodecyloxy Derivative. 740 (polymethylene ( CH2 )n group of
OC12H25), 685 Cl group, 840 (p substituted phenyl ring), 1160 ( CO of
OC12H25group), 1210, 1260 & 1600 ( COO ester group. The NMR data are con-
sistent with the molecular structure.
Results and Discussion
Cis 4-n-Alkoxy cinnamic acids and α-4-hydroxy benzoyl β-4ꢀ chloro phenyl ethylene
[B] are nonmesomorphic. However, on condensing the two nonmesomorphic components
through acid chlorides [A], the resultant homologous ethylene derivatives from pentyloxy
to hexadecyloxy give rise to mesomorphic behaviour. The methyl to butyl homologs are
nonmesomorphic. The octyloxy, decyloxy, and dodecyloxy homologs are enantiotropically
smectogenic in addition to nematogenic in character and the rest of the homologs are
enantiotropic nematic only (Table 1). The transition and melting temperatures of all the
homologs were determined on an optical polarizing microscope equipped with a heating

Properties of Novel Ethylene Derivatives
39
Table 1. Elemental analysis for ethoxy, hexyloxy, and decyloxy derivatives
Elements% found (% Calculated)
No.
Molecular formula
C
H
1
2
3
C26H21O4Cl
C30H29O4Cl
C34H37O4Cl
72.00 (72.10)
73.70 (73.52)
74.38 (74.49)
5.03 (4.86)
5.84 (5.94)
6.62 (6.80)
stage, and are shown in Table 2. A plot of the transition temperatures versus number of
carbon atoms present in n-alkoxy terminal chain shows the phase behavior of the series
(Fig. 1). An odd-even effect is observed for a nematic-isotropic transition curve, but the
effect is missing in the smectic-nematic transition curve. The smectic-nematic transition
curve initially rises, passes through maxima and then descends as series is ascended up
to the dodecyloxy homolog. The same curve is extrapolated [8] following the trend of
the curve up to the tetradecyloxy homolog to predict a latent smectic-nematic transition
temperature [LTT], which merges into the solid-nematic transition temperature 111.0◦C.
Thus, the smectic mesophase formation should occur at 111.0◦C. However, practically it is
a solid-nematic transition temperature, which suggests that the smectic mesophase ceases
to form and only the nematic mesophase forms. The nematic-isotropic transition curve
descends as the series is ascended and behaves in normal manner. The curves for the odd
and the even homologs merge into one nematic-isotropic curve at the eleventh homolog.
Thus, the odd-even effect diminishes as the series is ascended for higher homologs from
and beyond eleventh homolog. The solid mesomorphic or isotropic curve follows a zigzag
path of rising and falling values as the series is ascended with an overall falling tendency.
Thus, all the three transition curves behave in normal manner. The texture of the nematic
Table 2. Transition temperatures of series in ◦C
Transition temperatures in ◦C
Compound no.
n-alkyl group CnH2n+1 (n)
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
108.0
105.0
98.0
—
—
—
—
—
143.0
115.0
123.0
134.0
138.0
128.0
111.0
119.0
215.0
182.0
180.0
185.0
178.0
185.0
155.0
182.0
151.0
144.0
132.0
125.0
10
11
12
—
Sm–smectic; Nm–nematic.

40
H. N. Chauhan et al.
Figure 1. Phase behavior of the series.
mesophase is threaded or Schlieren and that of a smectic mesophase is of the typical A or C
type. The smectic mesophase varies from a minimum of 26◦C at the octyloxy homolog to
a maximum of 33◦C at the decyloxy homolog. The nematic mesophase length varies from
a minimum of 6◦C at the hexadecyloxy homolog and maximum of 70◦C at the hexyloxy
homolog. The present series is predominantly nematogenic and partly smectogenic with a
middle-ordered melting type. The smectic and nematic mesophases commence from the
octyl and pentyl homolog derivatives, respectively.
The pentyl to hexadecyl homologs are mesomorphic due to their suitable magnitudes
of rigidity and flexibility. The octyloxy, decyloxy, and dodecyloxy homologs generate a
lamellar packing of the molecules at a transition temperatue t1, and then adopt a statisti-
cally parallel orientational order from and beyond a higher temperature t2. However, on
further continued heating of the sample, the molecules randomly orient in all possible
directions without any molecular ordering at a higher temperature t3. Thus, the samples
under microscopic examination exhibit and show a smectic mesophase between t1 and t2,
and then a nematic mesophase between t2 and t3, and an isotropic liquid beyond temper-
ature t3. Mesomorphic molecules of homologs other than octyloxy to dodecyloxy exhibit
only nematogenic mesomorphism. The methyl to butyl homologs of the series are unable
to withstand exposed thermal vibrations due to the unsuitable magnitudes of anisotropic
intermolecular forces of attraction as a consequence of unfavorable molecular rigidity and
flexibility [1–4]. The odd-even effect diminishes for higher homologs, because longer n-
alkyl chain from and beyond dodecyloxy homolog may coil, bend, flex or couple to lie

Properties of Novel Ethylene Derivatives
41
Figure 2. Structurally similar series.
with the major axis of the core [4, 9]. The degree of mesomorphism exhibited by the series
is sufficiently high, because the series under discussion has sufficient lateral and terminal
end-to-end attractions by the linear shape with two central groups bearing a conjugating
double bond which strengthens rigidity of a molecule. The variation in mesomorphic prop-
erty from homolog to homolog is attributed to the sequentially added methylene unit of the
n-alkoxy terminal chain. The variation in mesomorphic properties is compared with other
structurally similar homologous series X [10] and Y [11], as shown in Fig. 2.
Figure 2 shows the structurally similar series 1, X, and Y are identical in respect of three
phenyl rings bridged through to central groups CH CH COO and –CO CH CH
with a common left terminal end group OR. However, the uncommon part is a right-
handed terminal end groups para Cl, H, and ortho NO2. Therefore, the molecular
aromaticity and rigidity remains identical, but the molecular flexibility varies due to po-
larity and induced polarizability of laterally and terminally substituted group only on third
phenyl ring, viz., NO2 and Cl, H. Thus, suitable magnitudes of anisotropic forces or
intermolecular adhesion as a consequence of varying molcecular flexibility differ accord-
ingly. Thus, mesomorphic properties and the degree of mesomorphism vary with changing
of the terminal end groups of series 1, X, and Y, respectively. Thermal stability of each
series and commencement of mesophase are given in Table 3.
Table 3 indicates that present series-1 is smectogenic in addition to nematogenic
character, while series X and Y chosen for comparison are nematogenic only without
exhibition of any smectogenic character. The absence of semctogenic character in series
X and Y chosen for comparison indicates the absence of lamellar packing of molecules
in their crystal lattices. The commencement of smectogenic property takes place from the
octyl homolog of series 1, but it does not commence until the last homolog of the series X
and Y. This variation may be attributed to the extent of noncoplanarity [9] caused by the
Table 3. Average thermal stabilities in ◦C
Average transition temperatures in ◦C
Series
Series (1)
Series (X)
—
Series (Y)
—
Smectic-nematic or smectic-isotropic
132.6
(C8 C12)
C8
156.7
(C5 C16)
C5
Commencement of smectic mesophase
Nematic-isotropic
155.4
(C5 C16)
C5
171.5
(C5 C16)
C5
Commencement of nematic mesophase

42
H. N. Chauhan et al.
molecules concerned. The Cl terminal end group is more polar and monoatomic, which
reduces the extent of molecular noncoplanarity than the differing end group H or lateral
group NO2. Thus, suitable and favorable extent of molecular noncoplanarity facilitates
lamellar packing of the molecules from the octyloxy to dodecyloxy homologs of series
1 but it does not facilitate for any homolog of series X and Y. The nematic mesophase
formation commences identically from the pentyl homolog of all the series 1, X, and Y
under comparison. This indicates that suitable magnitudes of anisotropic intermolecular
forces of attraction, as emerging from molecular rigidity and flexibility, operate equally
on end to end attractions, though the thermal stability of series Y is higher among those
under comparison. Suitable magnitudes of anisotropic intermolecular forces of attraction
facilitate a statistically parallel orientational order of the molecules which commences
from the fifth homolog of series 1, X, and Y to generate the nematic mesophase. Thus the
variation of mesomorphic properties for the same homolog from series to series is attributed
to the unchanging terminal and/or lateral end group of each individual homologous series.
The group efficiency order derived on the basis of (a) thermal stability and (b) early
commencement of the mesophase is mentioned below in the conclusion.
Conclusions
(a) Smectic group efficiency order: Cl > H and NO2 [ortho]
(b) Nematic group efficiency order: NO2 [ortho] > Cl >
H
Smectic group efficiency order: Cl > H and NO2 [ortho]
1. Nematic group efficiency order: Cl
H
NO2 [ortho]
2. Variations of mesomorphic properties from homolog to homolog in the same series
depends upon the sequentially added methylene unit.
3. The novel series is predominantly nematogenic and partly smectogenic with middle
ordered melting type.
4. Molecular rigidity and/or flexibility dictate the mesomorphic behavior of a sub-
stance.
5. Molecular structures that generate suitable magnitudes of anisotropic intermolecular
forces of attraction as a consequence of molecular rigidity and flexibility generate
mesomorphic properties in a substance.
Acknowledgments
The authors thank Dr. R. B. Patel, Head of the Department of Chemistry of K. K. Shah
Jarodwala Maninagar Science College, Ahmadabad, for his valuable cooperation and also
Vaibhav Laboratory for analytical services. Authors are thankful to the Head and staff
members of the Chemistry Department, S. P. T. Science College, Godhara, for providing
microscopic facility.
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