Synthesis and Characterization of a Novel Azoester Homologous Series

Article abstract of DOI:10.1080/15421406.2014.915622

An azoester homologous series of 4-(4'-n-alkoxybenzoyloxy) 3-methylphenyl azo-2''-chloro-3''-methyl benzenes consisting of 12 homologues was synthesized. The methoxy to pentyloxy homologues are nonmesogenic, the hexyloxy to dodecyloxy homologues are monotropic nematic and the tetradecyloxy to hexadecyloxy homologues are enantiotropic nematic. The textures of the nemetic phase are of the threaded or schlieren type. Transition curves showing phase behavior in a phase diagram behave in normal manner, with the exception of the last enantiotropic homologue. An odd-even effect is observed for the nematic-isotropic transition curve. Thermal phase transitions were observed through an optical polarizing microscope equipped with a heating stage. Analytical and spectral data support the molecular structures of the homologue molecules. Smectogenic character is totally absent from all the novel homologues. The average thermal stability for the nematic phase is 121.5 °C. Mesogenic properties of the present novel series are compared with other structurally similar known series.

Full text of DOI:10.1080/15421406.2014.915622

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Synthesis and Characterization of a
Novel Azoester Homologous Series
a
a
a
V. C. Kotadiya , M. D. Khunt & U. C. Bhoya
a
Department of Chemistry, Saurashtra University, Rajkot, Gujarat,
India
Published online: 15 Jan 2015.
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To cite this article: V. C. Kotadiya, M. D. Khunt & U. C. Bhoya (2015) Synthesis and Characterization
of a Novel Azoester Homologous Series, Molecular Crystals and Liquid Crystals, 606:1, 47-55, DOI:
10.1080/15421406.2014.915622
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Mol. Cryst. Liq. Cryst., Vol. 606: pp. 47–55, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.915622
Synthesis and Characterization of a Novel Azoester
Homologous Series
∗
V. C. KOTADIYA, M. D. KHUNT, AND U. C. BHOYA
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ꢀ
ꢀꢀ
An azoester homologous series of 4-(4 -n-alkoxybenzoyloxy) 3-methylphenyl azo-2 -
ꢀ
ꢀ
chloro-3 -methyl benzenes consisting of 12 homologues was synthesized. The methoxy
to pentyloxy homologues are nonmesogenic, the hexyloxy to dodecyloxy homologues
are monotropic nematic and the tetradecyloxy to hexadecyloxy homologues are enan-
tiotropic nematic. The textures of the nemetic phase are of the threaded or schlieren
type. Transition curves showing phase behavior in a phase diagram behave in normal
manner, with the exception of the last enantiotropic homologue. An odd–even effect
is observed for the nematic-isotropic transition curve. Thermal phase transitions were
observed through an optical polarizing microscope equipped with a heating stage. Ana-
lytical and spectral data support the molecular structures of the homologue molecules.
Smectogenic character is totally absent from all the novel homologues. The average
◦
thermal stability for the nematic phase is 121.5 C. Mesogenic properties of the present
novel series are compared with other structurally similar known series.
Keywords Liquid crystal; mesogens; monotropy; nematic; smectic
Introduction
Substances with long linear narrow lath like shapes and appropriately substituted polar end
groups or/and laterally substituted group or groups have [1] the potential to exhibit meso-
genic behavior as a consequence of favorable molecular rigidity and flexibility [2–4]. The
present investigation is aimed at understanding and establishing the influence of molecular
structure on mesogenic behavior of a substance by constructing a molecular architecture,
which consists of three phenyl rings bonded through COO and N N central bridges
as the rigid core, and the left n-alkoxy terminal end group and laterally substituted methyl
and chloro units to act as the flexible section of the azoester [5] molecule.
Experimental
Synthesis
Dimerized 4-n-alkoxy benzoic acids were synthesized by reacting suitable alkyl halides
(R–X) with 4-hydroxy benzoic acid in methanol by a modified method of Dave and Vora [6].
∗
Address correspondence to U.C. Bhoya, Department of Chemistry, Saurashtra University,
Rajkot- 360005, Gujarat, India. E-mail: drucbhoya@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
47

48
V. C. Kotadiya et al.
Scheme 1. Synthetic route to the series.
ꢀ ꢀ
-Hydroxy 3-methyl phenyl azo-2 -chloro-3 -methyl benzene as the azo dye was prepared
4
by the diazotization of 4-hydroxy 3-methyl benzene with 2-chloro-3-methyl aniline by the
usual established method [7]. 4-n-Alkoxy benzoic acid and the azo dye were condensed
in dry cold pyridine [8, 9]. The final product azoesters were individually decomposed,
filtered, washed, dried, and crystallized until constant transition temperature obtained. The
synthetic route to the series is shown in Scheme. 1.
4-Hydroxy benzoic acid, alkyl halides, thionyl chloride, methanol, ortho-cresol, 2-
chloro-3-methyl aniline, pyridine, ethanol, KOH, NaOH, and glacial acetic acid required
for synthesis were used as received, except solvents which were purified and dried prior
to use.

Novel Azoester Liquid Crystals
49
Table 1. Elemental analysis for (1) Methyloxy (2) Butyloxy (3) Heptyloxy derivatives.
Elements % found Elements % calculated
Sr. no.
Molecular formula
C
H
N
C
H
N
1
2
3
C22H19ClN2O3
C25H25ClN2O3
C28H31ClN2O3
66.85
68.80
65.65
4.80
5.82
6.40
7.15
6.40
5.92
66.92
68.72
70.21
4.85
5.77
6.52
7.09
6.41
5.85
Characterization
The structures of compounds of this series were identified using various analytical tech-
1
niques such as HNMR, mass spectrometry, IR spectroscopy, and Elemental analysis. All
analytical data supported the novel structures. Elemental analysis data was obtained using
a EuroEA Elemental Analyser, and as shown in Table 1 are in good agreement with the
theoretical values. IR data were determined using a SIMADZU-FTIR-8400 spectrometer.
1
HNMR spectra were recorded using a BRUKER spectrometer instrument in DMSO-d6
solvent. The molecular ion peak obtained from the experimental EI-MS data matched with
the molecular mass using a SHIMADZU-GC-MS, Model No.QP-2010 spectrometer.
Elemental Analysis
Spectral Data
NMR in ppm for the Ethyloxy Derivative. 1.36–1.39 Triplet, (3H, CH3 of OC2H5 group),
2.29 Singlet (3H CH3 of phenyl ring ortho to COO group), 2.73 singlet, ( CH3 of
phenyl ring meta to N N group), 4.17 Triplet ( OCH2 of OC2H5), 7.13–;8.14 (10
Aromatic proton in substituted phenyl ring) (Table 1). The NMR data support the molecular
structure.
NMR in ppm for the Octyloxy Derivative. 0.85–0.88 Triplet, (3H, CH3 of OC8H17
group), 1.23–1.77 ( CH2 )6 group of OC8H17, 2.28 Singlet(3H CH3 of phenyl ring ortho
to COO group), 2.71–2.76 multiplet, ( CH3 of phenyl ring meta to N N group),
4.09–4.12 Triplet ( OCH2 of OC8H17), 7.13–8.18 (10 Aromatic proton in substituted
phenyl ring). The NMR data support the molecular structure.
NMR in ppm for the Dodecyloxy Derivative. 0.86, Triplet, (3H, CH3 of OC12H25 group),
1.26–1.45 ( CH2 )9 group of OC12H25, 2.28 Singlet (3H CH3 of phenyl ring ortho
to COO group), 2.67 multiplet, ( CH3 of phenyl ring meta to N N group), 4.11
Triplet, ( OCH2 of OC12H25), 7.13–8.15 (10 Aromatic proton in substituted phenyl ring).
The NMR data support the molecular structure.
−1
IR in cm for the Propyloxy Derivative. 628 and 763 ( CH2 )n group of OC3H7), 810
1,2 disubstituted phenyl ring), 848 (p-substituted phenyl ring), 1165 (C O of OC3H7),
(
1
(
7
236, 1263&1643 ( COO ester group), 1602 ( N N ), 2870 and 2933 (C H Str. of
CH2 )n group), 3076 (C H Str. of phenyl ring), 1728 (>C O group), 692 (C Cl),
17&848 (para, ortho and meta substituted phenyl ring). The IR data support the molecular
structure.

50
V. C. Kotadiya et al.
−1
IR in cm for Tetradecyloxy Derivative. 650 and 771,( CH2 )n group of OC14H29, 810
1, 2 disubstituted phenyl ring), 850 (p-substituted phenyl ring), 1166 (C O of OC14H29),
(
1
(
7
257, 1280&1643,( COO ester group), 1604 ( N N ), 2848 and 2916 (C H Str. of
CH2 )n group), 3068 ( C H Str. of phenyl ring), 1730 (>C O group), 694 (C Cl),
20 and 844 (para, ortho, and meta substituted phenyl ring). The IR data support the
molecular structure.
−1
IR in cm for the Hexadecyloxy Derivative. 719 and 617,( CH2 )n group of OC16H33,
20 (1,2 disubstituted phenyl ring), 848 (p-substituted phenyl ring), 1166 (C O
of OC16H33), 1253, 1288 and 1643 ( COO ester group), 1604 ( N N ), 2848 and
918 (C H Str. of ( CH2 )n group), 3061 (C H Str. of phenyl ring), 1730 (>C O
8
2
group), 646 (C Cl), 769, 848 and 692 (para, ortho, and meta substituted phenyl ring). The
IR data support the molecular structure.
+
Mass Spectra of the Pentyloxy Derivative. m/z (rel.int%): 450(M)
+
Mass Spectra of the Hexyloxy Derivative. m/z (rel.int%): 464 (M)
+
Mass Spectra of the Decyloxy Derivative. m/z (rel.int%): 521(M)
Results and Discussion
4
-n-Alkoxy benzoic acids are dimeric, which on condensation with 4-hydroxy-3-methyl
ꢀ
ꢀ
ꢀ
azo 2 -chloro-3 -methyl benzene yielded 4-(4 -n-alkoxy benzoyloxy)-3-methyl phenyl azo-
ꢀ
ꢀ
ꢀꢀ
2
-chloro-3 -methyl benzene homologue derivatives through the appropriate acid chlo-
rides. The transition temperatures were determined by an optical polarizing microscope
equipped with a heating stage (Table 2). The homologous series consists of 12 homologues
◦
Table 2. Transition Temperatures in C
◦
Transition temperature in C
Compound no.
n-alkyl CnH2n+1 chain (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
(74.0)
(72.0)
(79.0)
(83.0)
(71.0)
96.0
94.0
146.0
126.0
113.0
105.0
101.0
96.0
90.0
84.0
86.0
83.0
10
11
12
118.0
125.0
(
) indicate monotropy.
Sm – Smectic, N – Nematic.

Novel Azoester Liquid Crystals
51
Figure 1. Phase behavior of series.
whose mesogenic property commences from the hexyloxy homologue and continued as a
monotropic nematic phase up to the dodecyloxy homologue. Then, enantiotropic nematic
mesophase formation is exhibited by the tetradecyloxy and hexadecyloxy homologues.
None of the homologues exhibit smectogenic behavior. The methoxy to pentyloxy homo-
logues are nonmesogenic. The textures of the nematic mesophase are of the threaded or
schlieren type. Transition temperatures of homologues were plotted against the number of
carbon atoms present in n-alkyl chain of a left n-alkoxy terminal end group. A phase diagram
[Fig. 1] is obtained by smooth linking of like or related points forming the solid-isotropic
or nematic and nematic-isotropic (isotropic-nematic) transition curves. An odd–even effect
is observed for the nematic-isotropic transition curve. The mesogenic behavior of the novel
series varies from homologue to homologue. The average nematic-isotropic thermal sta-
◦
bility for the novel series is 121.5 C. Thus, the present novel series is mainly monotropic
nematogenic without exhibition of smectogenic character. The solid-isotropic or nematic
transition curve continuously descends up to the octyloxy homologue without adopting a
zigzag path of rising and falling values. However, a zigzag path of rising and falling is
adopted from and beyond the octyloxy homologue. The nematic-isotropic (or vice versa)
transition curve initially rises and then descends for the monotropic homologues up to the
dodecyloxy homologues, but, then it suddenly rises for the enantiotropic tetradecyloxy and
hexadecyloxy homologues. Analytical and spectral data support the molecular structures.
ꢀ ꢀ
-Hydroxy –2-methyl phenyl azo 2 -chloro-3 -methyl benzene azodye is a non-
4
mesogenic, but on extending the molecular length of the 4-n-alkoxy benzoic acids through
their corresponding acid chlorides, the length to breadth ratio is enhanced to yield azoester
products with relatively lower transition temperatures, due to the breaking of the hydro-
gen bonding between two molecules of the n-alkoxy acids. The suitable magnitudes of
anisotropic forces of intermolecular attractions commences from the hexyloxy homologue
on cooling the isotropic melt which shows nematic phase below the melting point irre-
versibly. The homologues from hexyl to dodecyl of the novel series arrange themselves
to disalign perpendicular or at an angle less than ninety degree on the plane of a floating

52
V. C. Kotadiya et al.
Figure 2. Structurally similar homologous series.
surfaces, resisting thermal vibration at the monotropic temperature under the influence of
thermal energy and then immediately transform into solid state. Thus, statically parallel
orientational order of molecules requires suitable magnitudes of anisotropic forces of end
to end attractions to maintain the nematic mesophase for few degrees of temperature differ-
ence. However, the mesophase of monotropic homologues is difficult to detect accurately
due to the high crystallizing tendency. Enantiotropic nemetogenic mesophase formation
is exhibited by the tetradecyloxy and hexadecyloxy homologues and is attributed to their
longer n-alkoxy chain of a terminal end group which has strengthened the intermolecular
anisotropic forces to maintain a statistically parallel orientational order of molecules in a
floating condition in a reversible manner for detectable accurate range of temperature. The
absence of a smectogenic phase in the novel series indicates absence of lamellar packing
of molecules. The lateral chloro and methyl groups increase the molecular width and in-
termolecular distance, which causes a reduction in the intermolecular attractions, however,
the intermolecular attraction is enhanced by increased polarizability of the molecule by the
three lateral almost equally polar functional groups. Therefore, the resultant net effective
intermolecular attractions depend upon the dominant effect from the two opposing effects.
Hence, absence of smectogenic property, poor range of nemetogenic phase length, late com-
mencement of the mesogenic phase (from sixth homologue), exhibition of nemetogenic
phase in monotropic manner and low melting type behavior, suggests that the first effect
of reduced intermolecular attractions by increasing molecular width surpasses the second
effect related to molecular polarizability. The absence of a mesophase in the methoxy to
pentyloxy homologues is attributed to their high crystallizing tendency arising from unsuit-
able magnitudes of anisotropic forces of intermolecular end to end attractions. Variations
in mesogenic behavior from homologue to homologue in the novel series is attributed to
sequentially added methylene unit to the left n-alkoxy terminal end group. The mesogenic
behaviors of the present series-1 are compared with the structurally similar other known
series X [10] and Y [11]; as shown in Fig. 2.
The presently investigated homologus series-1 and series-X and series-Y selected for
comparison from series to series are identical with respect to left n-alkoxy terminal end

Novel Azoester Liquid Crystals
53
◦
Table 3. Average thermal stability in C
Series →
[1]
[X]
[Y]
smectic – nematic
or
smectic- isotropic
or
1.0 to 2.0
few degrees
(C5 C16)
—
—
—
125.57
[C6 C16]
isotropic-smectic
commencement of smectic phase
nematic –isotropic
or
—
121.5
[C14 C16]
C5
—
isotropic-Nematic
commencement of
nematic phase
C6
—
C6
group (-OR), and three phenyl rings bonded through –COO– and –N N– central bridges.
However, they differ with respect to the lateral and/or terminal end groups, which of course
causes differences in liquid crystal behavior and the degree of mesomorphism as a result
of varying molecular rigidity and flexibility [2–4]. Thus, variations in phase behavior or
mesogenic properties from series to series can be correlated with the molecular structure
of a substance. Table 3 shows the average thermal stabilities for the series 1, X, and Y.
Table 3 indicates that (i) the present novel homologous series 1 is, predominantly
monotropic nematic and partly enantiotropically nematogenic only, whereas, series X is
monotropically smectogenic only, and homologous series Y is enantiotropically nemato-
genic without exhibition of any smectogenic character; (ii) the nematogenic mesophase
commences from the hexyloxy homologue equally in series 1 and Y, without the exhi-
bition of smectogenic character, however, the smectogenic mesophase commences from
the pentyloxy homologue without exhibition of nemetogenic character in series X. Thus,
the initial four or five members of each series (Fig. 2) under comparison are nonmeso-
genic; and mesophase exhibition commences from the fifth to sixth member of the series.
Homologous series 1 and X are only nematogenic or only smectogenic respectively. Ho-
mologous series-1 has three laterally substituted groups with absence of right sided terminal
end group; whereas homologous series X bears two laterally substituted groups and one
terminally substituted right handed end group. Moreover, the OCH3 is more polar than
CH3 substituted to the middle phenyl ring. On account of these differences the molecular
polarity and polarizability differs for the series-1 and X. Accountability of such differences
makes possible to induce inter layered lamellar packing of molecules with favorable ex-
tent of molecular co-planarity including end to end anisotropic forces of intermolecular
attractions to induced smectogenic character monotropically below the melting point from
the pentyloxy homologue, without the exhibition of a nematogenic mesophase in series-X.
However, in case of series-1, only parallel orientational order of molecules by end to end
attractions are maintained in floating condition without formation of lamellar packing of
molecules. On comparing the homologous series 1 and series Y, they are perfectly identical
including CH3 group at middle phenyl ring and ;Cl group bonded to the ortho position
to N N central group with rest of the unchanged molecular part. Thus, series 1 and Y
differ only by CH3 and Cl as laterally substituted flexible groups. Therefore, whatever
the difference observed for mesogenic properties between the series 1 and Y is attributed

54
V. C. Kotadiya et al.
to the positional deference of CH3 and Cl and to their individual group polarity differ-
ence. The individual group polarity of CH3 and Cl are almost equal [1] for the nematic
mesophase formation. Hence, the mesogenic difference between series 1 and Y is attributed
to the position of lateral substitution at the third phenyl ring of CH3 and Cl for series
1
and Y. Thus, the novel aeries 1 and series Y exhibit only a nemetogenic mesophase, but
◦
◦
their average thermal stabilities are 121.5 C and 125.57 C with equal commencement of
nematic mesophase formation from the hexyloxy homologue. The positional difference of
CH3 and Cl also created differing magnitudes of anisotropic forces of intermolecular
end to end attractions to stabilize the statistically parallel orientational order of molecule
to cause nematogenic mesophase. Variations in mesogenic behavior from series to series
for the same homologue is attributed to the varying ratio of molecular polarity to polar-
izability, varying the length to breadth ratio etc. as a consequence of individual effective
molecular rigidity and flexibility [2–4] of the novel series 1 and the series X and Y chosen
for comparison.
Conclusions
•
•
The group efficiency order derived on the basis of
(i) Thermal stability and
(ii) The commencement of mesophase formations
are as under for smectic and nematic mesophases
(
i) Smectic: series X > series Y = series 1
Nematic: series Y > series 1 > series X
(
ii) Smectic: series X > series Y = series 1
Nematic: series 1 = series Y > series X
•
•
Variations in mesogenic properties from homologue to homologue in the same series
are due to the varying methylene units at the left n-alkoxy terminal end group.
Variations in mesogenic properties for the same homologue, from series to series
is due to changing molecular rigidity and flexibility by either changing ortho, meta
and pera position of same functional group or different functional group or groups
of dissimilar polarities keeping rest of the molecular part intact.
Mesogenic behavior of a substance is the most sensitive and susceptible to its
molecular structure.
•
•
Present novel series is nematogenic without exhibition of smectogenic character low
melting behavior.
Acknowledgments
Authors would like to thank to the Department of Chemistry (DST-FIST Funded & UGC-
SAP Sponsored) Saurashtra University, Rajkot, for giving this type of opportunity. Authors
are also thankful to Dr. A. V. Doshi, Ex. Principal M.V.M. Science and Home Science
College Rajkot, for given reliable guidance when needed. Also thanks are due to the
National Facility for Drug Discovery through New Chemical Entities (NCE’s), for provides
analytical data.

Novel Azoester Liquid Crystals
55
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