Mesomorphism dependence on positional substitution of same polar group on same phenyl ring in isomeric homologous series

Article abstract of DOI:10.1080/15421406.2014.949597

Novel homologous series 4-(4′-n-alkoxy benzoyloxy) napthyl azo 2″ - nitro benzenes, consisted of eleven members of a series. All the eleven members (methoxy to tetradecyloxy) homologs are only enantiotropically nematogenic without exhibition of any smectogenic character. Transition temperatures and the textures are determined by an optical polarizing microscopy equipped with a heating stage. Textures of a nematic phase are threaded or schlieren. Analytical and spectral data supported the molecular structure of homologs. Transition curves viz., solid-nematic and nematic-isotropic showing phase behavior of the mesophase in a phase diagram behave in normal manner. Odd-even effect and alternation of transition temperatures are exhibited by N-I transition curve. N-I thermal stability is 129.7°C and Nematic mesophase vary from 12 to 64°C. Thus, novel series is entirely nematogenic and middle ordered melting type. Mesomorphic behavior of a novel series are compared with the structurally similar homologous series.

Full text of DOI:10.1080/15421406.2014.949597

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Mesomorphism Dependence on Positional
Substitution of Same Polar Group
on Same Phenyl Ring in Isomeric
Homologous Series
H. R. Prajapatia & A. V. Doshib
a C.U. Shah Science College, Income Tax, Ahmedabad, Gujarat, India
b Matushri Virbaima Mahila Science and Home Science College,
Saurashtra University, Rajkot, Gujarat, India
Published online: 03 Mar 2015.
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To cite this article: H. R. Prajapati & A. V. Doshi (2015) Mesomorphism Dependence on Positional
Substitution of Same Polar Group on Same Phenyl Ring in Isomeric Homologous Series, Molecular
Crystals and Liquid Crystals, 608:1, 62-71, DOI: 10.1080/15421406.2014.949597
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Mol. Cryst. Liq. Cryst., Vol. 608: pp. 62–71, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.949597
Mesomorphism Dependence on Positional
Substitution of Same Polar Group on Same Phenyl
Ring in Isomeric Homologous Series
H. R. PRAJAPATI1 AND A. V. DOSHI2,∗
1C.U. Shah Science College, Income Tax, Ahmedabad, Gujarat, India
2Matushri Virbaima Mahila Science and Home Science College, Saurashtra
University, Rajkot, Gujarat, India
Novel homologous series 4-(4ꢀ-n-alkoxy benzoyloxy) napthyl azo 2ꢀꢀ – nitro benzenes,
consisted of eleven members of a series. All the eleven members (methoxy to tetrade-
cyloxy) homologs are only enantiotropically nematogenic without exhibition of any
smectogenic character. Transition temperatures and the textures are determined by an
optical polarizing microscopy equipped with a heating stage. Textures of a nematic
phase are threaded or schlieren. Analytical and spectral data supported the molecu-
lar structure of homologs. Transition curves viz., solid-nematic and nematic-isotropic
showing phase behavior of the mesophase in a phase diagram behave in normal man-
ner. Odd-even effect and alternation of transition temperatures are exhibited by N-I
transition curve. N-I thermal stability is 129.7◦C and Nematic mesophase vary from 12
to 64◦C. Thus, novel series is entirely nematogenic and middle ordered melting type.
Mesomorphic behavior of a novel series are compared with the structurally similar
homologous series.
Keywords Anisotropic; liquid crystals; monotropy; nematic; smectic
Introduction
Since the birth of an unique liquid crystal state [1, 2] scientist and technologist have showed
their keen interest in synthesizing novel liquid crystalline (LC) materials and exploited them
in number of fields of applications all over the world including India. In India, Bangalore,
Pune, Bomaby, Baroda-M.S.Uni., Punjab, and Assam were contributors to LC research.
USA, UK, China, Pakistan, Korea, Japan, Russia, Germany, etc. were the pioneer con-
tributors to LC study [3]. Saurashtra University-Rajkot-Gujarat has developed interest in
LC study since last 30 years in combination with faculty of technology and engineering,
The M.S. Uni. of Baroda-Vadodara. Dave et al. [4, 5] studied homologous series of novel
Schiff’s bases, carboxy ester, and their binary systems, Lohar et al. studied binary systems
and LC ester homologous series of two phenyl rings [6, 7]. Vora et al. [8–10] studied
cholesteric LC and conventional and unconventional LC, N.H. Shah and Jadav studied
azoester [11] compounds, Doshi et al. studied ester azoester, chalcones, Schiff’s bases
[12–14] in homologous series and binary systems, Prajapati et al. studied dimers of novel
∗Address correspondence to A.V. Doshi, Matushri Virbaima Mahila Science and Home Science
College, Kalavad Road – Rajkot, Saurashtra University, Rajkot 360007, Gujarat, India; E-mail:
arvindbhaidoshi@yahoo.in
62

Mesomorphism Dependence on Positional Substitution
63
varieties [15]. The chemists have synthesized novel LC compounds, evaluated their LC
properties and characterized respective molecular structure of novel LC material. Thus,
dependence of mesomorphism and the degree of mesomorphism on the molecular structure
are mainly studied by Gray et al. [16–18]. Physicists and biologist studied physical proper-
ties and biological activity of LC compounds respectively. Technocrats and technologists
applied their knowledge in the scientific pharmaceutical, medicinal and other allied fields
of applications [19–21]. Present investigation is planned to synthesize novel azoester ho-
mologous series consisting of napthyl unit linking a phenyl ring through –COO- and other
phenyl ring through –N N- central bridges to which –OR/n-alkoxy terminal end group
and at the other end ortho substituted –NO2 polar group with respect to –N N- group to be
synthesized and characterized by analytical data. Then mesomorphic characteristics will
be evaluated and compared with structurally isomeric series with –NO2 group substituted
at different positions of same phenyl ring. Thus study is aimed to understand and establish
the effect of molecular structure on mesomorphism as a consequence of molecular rigidity
and flexibility [22–24].
Synthesis
4-hydroxy benzoic acid was alkylated by suitable alkylating agents (R-X) to convert it
into corresponding n-alkoxy benzoic acids by the modified method of Dave and Vora [25].
Azodye 4-hydroxy napthyl 2ꢀ-nitro benzene was prepared by usual established method
(M.P is 134.0◦C & Yield is 73.0%) [26]. 4-n-alkoxy benzoic acids and Azodye were
individually condensed in ice cooled pyridine to get a series of final azoester products [27].
Final products were decomposed, filtered, washed, dried and purified till constant transition
temperature, obtained.
The chemicals 4-hydroxy benzoic acid, alkyl halides [R-X], methanol, Ethanol, KOH,
thionyl chloride pyridine, α – napthol, ortho nitro aniline, HCl, NaNO2, etc. required for
synthesis were used as received excepts solvents which were dried and distilled prior to
use. The synthetic route to the series is mentioned in Scheme 1.
Characterization
Some selected members of a novel series were characterized by elemental analysis, polar-
izing microscopy, mass spectra, Infra red spectra, 1H-NMR spectra, Texture determination
by miscibility method. Thermodynamic quantity enthalpy (δH) and entropy (δS) are qual-
itatively discussed instead of DSC scan. Microanalysis of the compound was performed
on Perkin Elmer PE 2400 CHN analyzer, IR spectra were performed on Perkin Elmer
1
spectrum, and H-NMR spectra were performed on Bruker spectrometer using CDCl3 as
solvent (Table 1).
Analytical Data
IR Spectra in Cm−1: for Ethoxy and Propyloxy derivatives:
Ethoxy: 2920, 2849, 1457 (alkyl group), 1733, 1684, 1201 (ester group), 1559, 1349
(Ar-NO2), 1167 (C-N Str. due to –N N-), 741(o-sub. benzene), and 849 (p-sub benzene).
Propyloxy: 2919, 2879, 1489 (alkyl group), 1733, 1683, 1207 (ester group), 1559,
1349 (Ar-NO2), 1168 (C-N Str. due to –N N-), 748 (o-sub. benzene), and 849 (p-sub
benzene).

64
H. R. Prajapati and A. V. Doshi
Scheme 1. Synthetic route of the series.
1H- NMR Spectra in CDCl3 in δ ppm: for Octyloxy and Dodecyl derivatives
Octyloxy: 0.89 (t, 3H, -CH3), 1.27(m, 20H, -CH2), 3.99 (t, 2H,-OCH2), and 6.94-8.90
(m, 14H, Ar-H).
Dodecyl: 0.88(t, 3H, -CH3), 1.27(m, 20H, -CH2), 3.99 (t, 2H,-OCH2), and 6.94-8.90
(m, 14H, Ar-H).
Mass spectra (molecular weight) Butyloxy homolog:
Calculated-469
Experimental-469
Textures by miscibility method
Nematic:
Pentyloxy homolog – Threaded texture
Tetradecyloxy homolog – Schlieren texture
Heptyloxy homolog – Threaded texture
Results and Discussion
Novel homologous series 4-(4ꢀ-n-alkoxy benzoyloxy) napthyl azo-2ꢀꢀ-nitrobenzenes con-
sisted of eleven members. All the members are enantiotropically nematogenic without
exhibition of any smectogenic property. Dimerization of 4-n-alkoxy benzoic acids are dis-
appeared on esterification process. Transition and melting points of homologs (Table 1,
Table 2) as determined by an optical polarizing microscopy, equipped with a heating stage
were plotted versus the number of carbon atoms present in n-alkyl chain of left n-alkoxy

Mesomorphism Dependence on Positional Substitution
65
Table 1. Elemental analysis for methoxy, butoxy, hexyloxy, and decyloxy derivatives
Elements % found (calculated % )
Sr. No.
Molecular formula
C
H
N
1
2
3
4
C24H17N3O5
C27H23N3O5
C29H27N3O5
C33H35N3O5
69.88(69.90)
68.88(69.08)
70.11(70.02)
71.44(71.60)
3.39(3.91)
4.84(4.90)
5.29(5.43)
6.24(6.32)
9.56(9.80)
8.89(8.95)
8.35(8.45)
7.36(7.59)
terminal end group of the molecules of each homolog. The like or related points were linked
to draw the solid-nematic and nematic-isotropic transition curves to form a phase diagram
(Fig. 1) showing the phase behavior of a novel series.
Solid-nematic transition curve initially rises from methoxy to hexyloxy homolog and
then descended up to decyloxy homolog and again ascended to the last tetradecyloxy
homolog derivative with overall descending tendency and adopting partly zigzag path of
propagation. Nematic-Isotropic transition curve initially rises (odd members) and then
descended from second member to the last member of a series with exhibition of odd-
even effect. Thus, both of the transition curves behaved in normal established manner.
N-I transition curves for odd and even members merge into each other at the nonyloxy
homolog and then propagated into a single curve. The mesomorphic (only nematic) phase
length vary between minimum of 12◦C at the pentyloxy homolog and maximum of 64◦C
at the decyloxy homolog. Thermal stability for nematic mesophase is 129.7◦C. Isotropic
temperatures are ranging from 113 to 161◦C. Mesogenic behaviors vary from homolog to
homolog in the presently investigated azoester series.
Azodye 4-hydroxy napthyl azo 2ꢀ-nitro benzene is a nonliquid crystal (NLC) com-
ponent, which on condensation with some nonliquid crystal or liquid crystal n-alkoxy
Table 2. Transition temperatures in ◦C for 4-(4’-n-alkoxy benzoyloxy)-napthyl azo-2”-nitro
benzene
Transition temperatures in ◦C
Compound no.
n-alkyl group Cn H2n+1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
—
—
—
—
—
—
—
—
—
—
—
85.0
104.0
109.0
114.0
118.0
119.0
112.0
96.0
113.0
161.0
134.0
140.0
130.0
132.0
128.0
129.0
125.0
120.0
115.0
10
12
14
61.0
67.0
70.0
10
11

66
H. R. Prajapati and A. V. Doshi
Figure 1. Phase behavior of series.
benzoic acids yielded nematogenic liquid crystal homologs of considerable range of liq-
uid crystallinity, through their corresponding acid chlorides. Linking of two components
through –COO- and –N N- central bridges increases molecular length and breadth. Hence,
molecular polarity and polarizability alter to suitable magnitudes of anisotropic forces of
intermolecular attractions, which facilitate to induce nematogenic mesophase formation
as a consequence of favorable molecular rigidity and flexibility. The disappearance of
dimerization of 4-n-alkoxy benzoic acids is attributed to the breaking of hydrogen bonding
between two molecules of 4-n-alkoxy benzoic acids by esterification process. Exhibition
of odd-even effect and alternation of transition temperatures are attributed to the even and
odd numbered carbon atoms present in n-alkyl chain of left n-alkoxy terminal end group.
The exhibition of nematogenic mesophase formation by the all members of a novel series
is due to the disalignment of the molecules of all homologs at an angle less than ninety
degree with the surface of floating which, maintains the floating of the molecules of each
homolog with statistically parallel orientational order for definite range of temperature un-
der a hot stage polarizing microscope, corresponding to suitable magnitudes of anisotropic

Mesomorphism Dependence on Positional Substitution
67
forces of intermolecular adhesion, as a consequence of molecular rigidity and flexibility or
internal energy stored by a homolog concerned. Thus, the molecules of all the homologs of
a present series are capable enough to resist exposed thermal vibrations for definite range
of temperatures, depending upon their thermal withstanding capacity, related to thermal
stability or molecular rigidity/flexibility. Absence of smectogenic character in a series indi-
cates absence of lamellar packing of molecules in their crystal lattices due to the inadequate
quantum of energy associated to build up two- or three-dimensional net work, which can
float on the surface as sliding layers under the influence of exposed thermal vibrations.
Thus, facilitating of lamellar packing of molecules is hindered by unsuitable magnitudes
of anisotropic forces of intermolecular attractions, which causes disappearance of smectic
mesophase formation till the last tetradecyloxy homolog. From and above the isotropic
temperatures of each homolog of a series, the mesophase ceases to appear during micro-
scopic observation, because the molecules are randomly oriented in all possible directions
with high order of disorder or high entropy(δS=δH/T). However, on cooling the same, the
nematogenic mesophase formation reappears from and below isotropic temperature of a
homolog concerned, till a sample of a substance undergoes solidification. Disappearance of
odd-even effect from and beyond nonyloxy homolog is attributed to the coiling, bending,
flexing, or coupling of n-alkyl chain with the major axis of a core structure. The mesogenic
behaviors from homolog to homolog in the presently investigated novel azoester series are
due to the sequential addition of methylene unit or units which, causes gradual variations in
molecular rigidity and/or flexibility. Thus, the present series is entirely nematogenic whose
range of liquid crystallinity is considerably good and of middle ordered melting type. Some
liquid crystal behaviors of presently investigated homologous series 1 are compared with
the structurally similar or ISOMERIC known homologous series X [28] and Y [29] as
under in Fig. 2.
Homologous series 1, X and Y are isomeric to each other. All the three series under
comparison are perfectly identical with respect to their rigid core consisted of two phenyl
rings, one napthyl ring bonded through -COO- and –N N- central bridges. The flexible part
of all the comparative series are -OR and –NO2 (R = CnH2n+1). Flexible group-OR varies
with number of carbon atoms present in n-alkyl chain (n = 1,2,3, . . . ) which contributes
Figure 2. Structurally similar homologous series.

68
H. R. Prajapati and A. V. Doshi
to the total molecular flexibility varying from homolog to homolog in the same series but,
its contribution to flexibility remains identical for the same homolog from series to series.
The flexibility contribution by polar –NO2 group remains identical for the same series but it
varies from series to series for the same homolog due to changing positional substitutional
variation(ortho, meta, para to –N N-) of –NO2 polar group.
Thus, total molecular flexibility plus molecular rigidity constituting suitable magni-
tudes of anisotropic forces of intermolecular end to end attractions depending upon inter-
molecular closeness, molecular polarity, polarizability, and positional substitution of –NO2
etc., vary from homolog to homolog in the same series as well as from series to series for the
same homolog. Hence, a phenomenon of mesomorphism and the degree of mesomorphism
being susceptible and sensitive to molecular structure or molecular rigidity-flexibility will
differ with differing positions of –NO2 at ortho, meta, or para position only in presently
investigated novel series 1 and the series X and Y chosen for comparison respectively.
Following Table 3 represents some mesogenic characteristics for the series 1, X and Y as
under.
Table 3 indicates that the homologous series 1, X and Y under comparison are:
• Only nematogenic without exhibition of any smectogenic character.
• Nematogenic mesophase commences from very first member of each series but
smectogenic mesophase does not commence till the last member of any of the series
under comparison.
• Thermal stability for nematic, gradually increases by 6.48◦C from ortho nitro to
meta nitro, i.e., from series 1 to X, but, it, drastically increases 57.22◦C in case of
para substituted –NO2 isomeric series Y.
• The degree of mesomorphism for all the three homologous series 1, X and Y under
comparison are varied as maximum in case of series Y and minimum in case of
series X whereas it is of intermittent valued for series 1 between the values of series
X and Y.
An exhibition of mesomorphism by any molecule requires the suitable magnitudes of
anisotropic forces of an intermolecular attraction as a consequence of favorable molecular
rigidity and flexibility, as well as appropriate intermolecular closeness. Homologous series
1, X and Y possess one of the fused phenyl ring of napthyl unit as common lateral
substitution. Homologous series 1 and X possess –NO2 group occupying its position of
substitution at ortho and meta places to –N N- central bridge and also acts as lateral
substitution. However –NO2 group occupied at para position in series Y does not act
as lateral substitution but acts as a terminal end group. Therefore, fused phenyl ring of
napthyl unit is being commonly considered as lateral substitution to all the series under
comparison which contributes to molecular polarizability factor in equivalent manner.
Table 3. Average thermal stabilities in ◦C
Series →
1
X
Y
Smectic-nematic
or isotropic
Commencement of smectic mesophase
Nematic-isotropic
—
129.7
—
136.18
—
193.4
commencement of nematic phase
(C1-C14) C1
(C1-C14) C1
(C1-C14) C1

Mesomorphism Dependence on Positional Substitution
69
However, ortho- and meta-substituted –NO2 group in series 1 and X adds some contribution
to the molecular polarizability factor. But at the same –NO2 substituted at para position in
series Y relatively increases molecular length and decreases intermolecular distance. Hence,
increase of intermolecular distance (or breadth) in case of series 1 and X can reduce the
intermolecular attractions on one hand and the intermolecular attractions enhanced on other
hand by increasing molecular polarizability. Thus, two opposing effects are operated at a
same time for a same reason. The net magnitudes of intermolecular attractions will depend
upon the predominancy of any one of the forces out of two opposing forces operating at a
time for the same reason. Hence, the effect to the intermolecular attractions due to lateral
substitution to the molecules under comparison is almost the equivalent irrespective of
ortho or meta substituted -NO2 molecules, but it largely affect to the molecules of para
substituted -NO2 of series Y, because, para substituted -NO2 increases molecular length,
length to breadth ratio and end to end intermolecular attractions comparatively more than
ortho and meta substituted -NO2 of series 1 and X. Thus increased intermolecular distance
by lateral substitution predominates in by decreasing intermolecular attractions of series 1
and X and while it predominates in by increasing end to end attractions of series Y. Hence
the thermal stability for nematic is the highest and drastically raised to 193.4 for series Y
due to decreased intermolecular distance, but the same are lower 136.18 or 129.7 for series
X and 1 due to combined effect of increased intermolecular distance and poor polarizability
factor. Thus, nematic mesophase is thermally facilitated to stabilize relatively more in case
of series Y than the series 1 and X under comparison. The commencement of nematogenic
mesophase formation from very first member of each series 1, X and Y is obvious due
to favorable molecular rigidity contributed by napthyl unit and flexibility contribution by
highly polar –NO2 and –OR groups. The extent of molecular noncoplanarity in case of all
the series under comparison hinders the formation of lamellar packing of molecules in their
crystal lattices, which does not allow to induce smectogenic character till the last homolog
member of each series under comparative study.
Conclusions
• The group efficiency order for nematic derived on the basis of (i) thermal stability
and (ii) the early commencement of nematic mesophase are as under.
(i) Nematic:
para − NO2> meta − NO2 > ortho − NO2
(ii) Nematic:
ortho − NO2 = meta − NO2 = para − NO2
• A phenomenon of mesomorphism and the degree of mesomorphism in isomeric
homologous series, bearing, same flexible polar groups, same central bridges and
the same number of aromatic phenyl rings, depend upon the position (ortho, meta,
para) of substitution of same polar group on the same phenyl ring.
• Transition temperature alternates with overall descending order.
• Homologous series with napthyl unit are generally nematogenic.
• Presently investigated novel homologous series is entirely nematogenic with rela-
tively middle ordered melting type and of considerable rich degree of mesomor-
phism.

70
H. R. Prajapati and A. V. Doshi
• A phenomenon of mesomorphism and the degree of mesomorphism are very sensi-
tive and susceptible to the favorable magnitudes of molecular rigidity and/or flexi-
bility or molecular structure.
Hence present investigation supported early views about mesomorphism and raised
the credibility to the conclusions drawn earlier [18].
Acknowledgments
Authors acknowledge thanks to the Principal and the Management of M.V.M. Science and
Home Science College, Rajkot for their valuable support and cooperation, as and when
needed. Thanks are due to the CSMCRI Bhavnagar for analytical services and The M.S.
Uni. of Baroda, Faculty of Tech. and Engg., Vadodara for providing microscopic facility
by applied chemistry department.
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