Synthesis and characterization of some novel liquid crystalline compounds of 1,3,4-oxadiazoline and their hydrazone derivatives

Article abstract of DOI:10.1080/15421406.2018.1468277

Three new series of mesogens have been synthesized by fixing 4-methoxy or 4-butoxy substituted phenyl ring in one side and 4-hydroxy, bromo or amino substituted phenyl group in the other side with different central linkage. The molecular structure of these series was confirmed by elemental analysis and FTIR and ¹HNMR spectroscopy. The mesomorphic properties were studied by using differential scanning calorimetry (DSC) and optical polarizing microscopy (OPM). All the compounds of series [I] exhibit enantiotropic smectic B (SmB) phase. All the compounds of series [II] and [III] display SmB and nematic (N) phases. The mesomorphic properties were found to be dependent on the two terminal 4-substituted phenyl groups and the central linkage group.

Full text of DOI:10.1080/15421406.2018.1468277

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and characterization of some novel
liquid crystalline compounds of 1,3,4-oxadiazoline
and their hydrazone derivatives
Jumbad H. Tomma, Thaer S. Ghali & Ammar H. Al-Dujaili
To cite this article: Jumbad H. Tomma, Thaer S. Ghali & Ammar H. Al-Dujaili (2018) Synthesis
and characterization of some novel liquid crystalline compounds of 1,3,4-oxadiazoline and
their hydrazone derivatives, Molecular Crystals and Liquid Crystals, 664:1, 85-94, DOI:
10.1080/15421406.2018.1468277
To link to this article: https://doi.org/10.1080/15421406.2018.1468277
Published online: 21 Jun 2018.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Synthesis and characterization of some novel liquid crystalline
compounds of ,,-oxadiazoline and their hydrazone
derivatives
Jumbad H. Tommaa, Thaer S. Ghalia, and Ammar H. Al-Dujailib
aDepartment of Chemistry, College of Education for Pure Science, Ibn Al-Haytham, University of Baghdad,
Baghdad, Iraq; bHamdi Mango Center for Scientific Research, University of Jordan, Amman, Jordan
ABSTRACT
KEYWORDS
Heterocyclic; hydrazones;
liquid crystal; oxadiazolines;
Schiff bases
Three new series of mesogens have been synthesized by fixing 4-
methoxy or 4-butoxy substituted phenyl ring in one side and 4-hydroxy,
bromo or amino substituted phenyl group in the other side with dif-
ferent central linkage. The molecular structure of these series was con-
1
firmed by elemental analysis and FTIR and HNMR spectroscopy. The
mesomorphic properties were studied by using differential scanning
calorimetry (DSC) and optical polarizing microscopy (OPM). All the com-
pounds of series [I] exhibit enantiotropic smectic B (SmB) phase. All the
compounds of series [II] and [III] display SmB and nematic (N) phases.
The mesomorphic properties were found to be dependent on the two
terminal 4-substituted phenyl groups and the central linkage group.
Introduction
In the past few decades, oxadiazole derivatives have been extensively studied. It is a five mem-
bered heterocyclic that have four isomeric forms [1]. 1,3,4-oxadiazole is more stable one of
them. The 3-acetyl-2,3-dihydro-1,3,4-oxadiazoline have been synthesized by many worker
using different reagent and conditions [2–10]. In two different articles both Yang et al., [11]
and Maghari et al., [12] reported conversion thiosemicarbazide to 2,5-disubstituted-1,3,4-
oxadiazolines by using a suitable coupling reagents. Recently, Alison et al., [13] synthesized
fused 1,3,4-oxadiazolines.
Several heterocyclic rings give liquid crystalline properties, one important member in the
heterocyclic mesogens is 1,3,4-oxadiazole. The 2,5-disubstituted-1,3,4-oxadiazoles have good
thermal stability and rich mesophases [14–20]. Also many series of 1,2,4-oxadiazole have been
synthesized and studied [21–24].
In this work synthesis, characterization and study the mesomorphic behavior of some
3-acetyl-2,3-dihydro-1,3,4-oxadiazoline derivatives initiated by cyclization reaction of the
synthesized Schiff bases with acetic anhydride. Then converted the 1,3,4-oxadiazoline to
a corresponding hydrazones. In the literature survey no evidence reveres to study the
mesomorphic behavior for this type of oxadiazoline derivatives.
CONTACT Ammar H. Al-Dujaili
Jordan, Amman , Jordan.
ah.aldujaili@gmail.com
Hamdi Mango Center for Scientific Research, University of
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

86
J. H. TOMMA ET AL.
Experimental
Materials and techniques
All chemicals were supplied from Merck and Aldrich Chemicals Co. and used as received.
Some representative homologues of the series were selected for characterization of the struc-
tures by elemental analysis, FTIR, and 1H-NMR techniques. Microanalysis was performed on
a Perkin–Elmer PE 2400 CHN analyzer. FTIR spectra were recorded using potassium bromide
disc on a Shimadzo (Ir prestige-21) FTIR spectrophotometer. 1HNMR spectra were obtained
with Bruker spectrophotometer model ultra-shield at 500 MHz using tetramethylsilane as
internal standard and DMSO-d6 as a solvent. Uncorrected melting points were determined
by using hot-stage Gallen Kamp melting point apparatus. Thin layer chromatography (TLC)
was carried out on aluminum sheets. Transition temperature and liquid crystalline textures
were determined using optical polarizing microscope (Olympus BX40)) equipped with a Leitz
Laborlux 12 Pols hot stage and PR600 controller. The transition temperatures and enthalpies
were measured using DSC (STA PT-1000 LINSIS) adopting a scanning rate of 10°C /min
under nitrogen atmosphere. Temperature and heat flow calibrated with standard indium of
purity <99.99%.
Synthesis
Methyl-4-methoxybenzoate [A] and methyl-4-butoxybenzoate [B] were synthesized by clas-
sical esterification method described by Chiang et al., [25]. 4-Methoxybenzohydrazide [C]
and 4-butoxybenzohydrazide [D] were synthesized by method of Reddy et al., [26].
General procedures for synthesis of Schiff bases [I]a-f
A suitable 4-substituted acetophenone (10 mmol) and appropriate hydrazide (10 mmol) and
few drops of glacial acetic acid as catalyst in (10 mL) of an absolute ethanol were refluxed for
(6–7) hours. The precipitate formed during the cooling of mixture it was filtered off, washed
two or three times with cold ethanol. Finally, the solid was recrystallized from an absolute
ethanol.
Analytical data
FTIR Spectra for [I]f
−1
=
FTIR (KBr, υ cm ): 3352-3224 (–NH2, –NH–); 2958-2870 (–C–H, aliphatic); 1639 (C O);
=
=
–
1609 (C N); 1595 (C C); 1255 (C O).
HNMR spectra for [I]b
1HNMR (DMSO-d6), ppm: δ = 11.16 (s, 1H, –NH–); 7.18–7.91 (dd, 8H, Ar-H); 3.73 (s, 3H,
–OCH3) and 2.03 (s, 3H, –CH3).
General procedures for synthesis of -acetyl-,,-oxadiazoline derivatives [II]a-f
A mixture of Schiff bases [I] (10 mmol) and an excess of acetic anhydride (10 mL) was
refluxed for (3–4) hours. The acetic anhydride was distilled off and the residue was poured
onto crushed ice. The solid thus obtained was collected by filtration, washed with water and
recrystallized from an absolute ethanol.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
87
Analytical data
FTIR Spectra for [II]b
−1
=
=
=
FTIR (KBr, υ cm ): 2900–2810 (–C–H, aliphatic); 1662 (C O); 1629 (C N); 1600 (C C);
1253 (C O).
–
HNMR spectra for [II]f
1H-NMR (DMSO-d6), ppm: δ = 9.65 (s, 1H, NH); 8.01-8.04 (dd, J = 8.5 and 8.5 Hz, 4H, Ar-
H); 7.44-7.46 (dd, 4H, Ar-H); 3.28 (s, 2H, –OCH2); 2.60 (s, 3H, CH3CONH–); 2.30 (s, 3H,
=
CH3C O); 2.17 (s, 3H, –CH3 at C5); 2.07 (m, 2H, –CH2–); 1.80-1.84 (m, 2H, –CH2–) and
1.74 (t, 3H, –CH3).
General procedures for synthesis of hydrazones [III]a-f
A mixture of 3-acetyl-1,3,4-oxadiazoline derivatives [II]a-f (10 mmol), phyneyl hydrazine
(10 mmol), absolute ethanol (10 mL) and glacial acetic acid (6 drops) was heated under reflux
for 6 hour. The precipitate formed after evaporation the solvent was washed several times with
diethyl ether. Finally, the solid was recrystallized from an absolute ethanol.
Analytical data
FTIR Spectra for [III]c
FTIR (KBr, υ cm−1): 3286-3232 (–NH2, –NH–); 2940-2860 (–C–H, aliphatic); 1693
=
=
=
–
(NHC O); 1639 (C N); 1597 (C C); 1242 (C O).
HNMR spectra for [III]e
1H-NMR (DMSO-d6), ppm: δ = 9.60 (s, 1H, NH-Ph); 7.02-7.88 (m, 13H, Ar-H); 3.49 (t, 2H,
–OCH2); 2.27 (s, 3H, –CH3); 2.10 (s, 3H, –CH3 at C5); 1.75-1.80 (m, 2H, –CH2–); 1.20-1.24
(m, 2H, –CH2–) and 0.85 (t, 3H, –CH3).
Results and discussion
Synthesis
The synthetic routes for the synthesis of compounds [I-III]a-f are outlined in Scheme 1. All
of the new synthesized compounds gave satisfactory analysis for the proposed structures,
which were confirmed on the basis of their elemental analysis; FTIR and 1H NMR (see ana-
lytical data and Tables 1, 2 and 3). The condensation reaction between hydrazides C and D
with different acetophenones in present of glacial acetic acid lead to formation of hydrazones
compounds [I]a-f in a good yield. The FTIR show the absence of two stretching vibrations of
=
NH2 groups and appearance the stretching vibrations of C N. The acetic anhydride using as
cyclooxidative reagent when reflex with hydrazones [I]a-f and led to the formation of N-acetyl-
2,3-dihydro-1,3,4-oxadiazoline derivatives [II]a-f in about 40–92% yield. The FTIR show the
absence of stretching vibrations of NH groups for all compounds except [II]c and [II]f because
containing NH group for which was formation from the reaction of NH2 group with acetic
anhydride, also the absence of stretching vibrations of OH group in compounds [II]a and [II]d
=
and the appearance of stretching vibration of C O ester group from the reaction of OH with
acetic anhydride. In addition, the 1HNMR exhibited signals for protons of CH3 attached to
=
C O group also show absence the signal related to protons NH. The last step in this work

88
J. H. TOMMA ET AL.
O
O
(i)
RO
RO
OCH3
OH
R= CH , (A); C H , (B)
3
4
9
(ii)
O
O
RO
+
X
HN
NH2
H3C
R= CH , (C); C H , (D)
3
4 9
(iii)
O
CH3
RO
HN
N
C
X
[I]a-f
Compound, R, X: [I]a, -CH , -OH; [I]b, -CH , -Br; [I]c, -CH , -NH ; [I]d, -C H , -OH; [I]e, -C H , -Br; [I]f, -C H , -NH
3
3
3
2
4
9
4
9
4
9
2
(iv)
O
N
CH3
N
H
N
RO
X
+
H2N
O
CH3
[II]a-f
Compound, R, X: [II]a, -CH , -OH; [II]b, -CH , -Br; [II]c, -CH , -NH ; [II]d, -C H , -OH; [II]e, -C H , -Br; [II]f, -C H , -NH
3
3
3
2
4
9
4
9
4
9
2
(V)
N
N
CH3
N
H
CH3
N
X
O
[III]a-f
RO
Compound, R, X: [III]a, -CH , -OH; [III]b, -CH , -Br; [III]c, -CH , -NH ; [III]d, -C H , -OH; [III]e, -C H , -Br; [III]f, -C H , -NH
2
3
3
3
2
4
9
4
9
4
9
Scheme . Synthetic route for compounds [I]a-f, [II]a-f and [III]a-f, reagents and conditions: (i) Methanol,
HSO, (ii) hydrazine hydrate %, ethanol, (iii) glacial acetic acid, ethanol, (iv) acetic anhydride, (v) glacial
acetic acid, ethanol.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
89
Table . Elemental analysis for [I]c and [I]e compounds.
Analysis % found (calcd.)
Compound
Molecular formula
M.Wt (g/mol)
C
H
N
[I]c
[I]e
CHON


.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
CHONBr
Table . Elemental analysis for [II]c and [II]d compounds.
Analysis % found (calcd.)
H
Compound
Molecular formula
M.Wt (g/mol)
C
N
[II]c
[II]d
CHON
CHON


.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
is the reaction of compounds [II]a-f with phenyl hydrazine in the acidic medium to form the
desired hydrazone derivatives [III]a-f with about 35–75% yield. The FTIR show the absence
=
of stretching vibration of C O groups and appearance both stretching vibrations of NH and
1
=
C N group. The HNMR exhibited signals of protons NH attached to phenyl group for all
compounds that good evidence for the formation of hydrazones.
Mesomorphic behaviors
The thermal, mesomorphic properties and corresponding enthalpy changes of all compounds
in the three series [I]a-f, [II]a-f and [III]a-f were studied by OPM and (DSC). Generally, the
thermal behavior obtained by DSC is consistent with the data observed by OPM, where it
was possible to identify the mesomorphic character of the mesophases. Phase identification
was based on the optical textures, and the magnitude of isotropization on enthalpies is con-
sistent with the assignment of each mesophase type, using the classification systems reported
by Sackmann and Demus and Richter [27, 28] and Gray and Goodby [29].
Mesomorphic behavior of compounds [I]a-f
The data obtained from the OPM and DSC analyses for all compounds in series [I]a-f are
summarized in Table 4. All the compounds of this series display an enantiotropic smectic B
(SmB) phase. Mesophases were identified by liquid crystalline texture analysis. Under POM,
a mosaic texture of a SmB phase was observed during the heating and cooling cycle. Repre-
sentative examples of the SmB textures are shown in Fig. 1. Figure 1 shows photomicrograph
=
of a mosaic texture, which is very characteristic of SmB phase. The C N (known as imine)
is linkage group used for connecting the rigid core groups, though it provides a stepped core
structure. Besides to retain of the linearity for molecule, hence providing higher stability and
Table . Elemental analysis for [III]b and [III]d compounds.
Analysis % found (calcd.)
Compound
Molecular formula
M.Wt (g/mol)
C
H
N
[III]b
[III]d
CHONBr
CHON


.(.)
.(.)
.(.)
.(.)
.(.)
.(.)

90
J. H. TOMMA ET AL.
Table . Transition temperatures and associated enthalpy changes for series [I].
Compound
Transition temperatures (°C)(ꢀH, kJ/mol)
[I]a
[I]b
[I]c
[I]d
[I]e
[I]f
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr = Crystal; SmB = smectic B; I = Isotropic transition.
Table . Transition temperatures and associated enthalpy changes for series [II].
Compound
Transition temperatures (°C) (ꢀH, kJ/mol)
[II]a
[II]b
[II]c
[II]d
[II]e
[II]f
Cr  (.) N  (.) I
Cr  (.) N  (.) I
Cr  (.) N  (.) I
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr = Crystal; N = Nematic; SmB = smectic B; I = Isotropic Transition.
enabling formation of smectic mesophase [30]. In addition the lengthening of alkoxy chain
in the terminal compounds [I]d, [I]e and [I]f led to reduce melting point of these compounds
and increasing the range of mesophases thermal stability.
Mesomorphic properties of compounds [II]a-f
The transition temperatures, enthalpy changes, and phase sequences for the compounds of
series [II]a-f are summarized in Table 5. Compounds [II]a, [II]b and [II]c displayed enan-
tiotropic nematic phase from textural observation under OPM and this in these showed a
Figure . Mesophase texture of the SmB phase obtained on cooling. A mosaic texture ( × magnification)
for the compound [I]e at °C.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
91
Figure . Mesophase texture of the nematic phase obtained on cooling. A thread-like texture ( × mag-
nification) for the compound [II]b at °C.
characteristic thread-like texture. The broadening of these molecules by the formation of
1,3,4-oxadiazoline ring reduces the lateral molecular interaction and increase the terminal
interaction which is favor with the formation of nematic phase. In contrast, an increase in
alkoxy terminal length in compounds [II]d, [II]e and [II]f is often helpful to enhance the lat-
eral interaction and to facilitate the formation of the smectic phase [31]. On the other hand,
the conjugated bonds between the aromatic ring with 1,3,4-oxadiazoline ring enhanced and
increase the rigidity of the molecule, increasing lateral forces because increasing polarity of the
molecule that effected on type and thermal stability of the mesomorphic phase. Thread-like
texture has been observed as a representative texture of nematic phase (Fig. 2). For compound
[II]e as an example, a mosaic texture was observed at 123°C as an optical evidence for the SmB
phase (see Fig. 1).
Mesomorphic behaviors of compounds [III] a-f
The phase transition temperatures and corresponding enthalpy changes of the compounds in
series [III] are given in Table 6. Compounds [III]a, [III]b, [III]c and [III]e are enantiotropic
smectogens, while an enantiotropic nematic phase was present with the compounds [III]d
and [III]f of this series. Reference to the geometry of compounds [III]a, [III]b and [III]c and
compounds [III]d, [III]e and [III]f show that they have similar structure except in their alkoxy
terminal substituent. The compounds [III]a, [III]b and [III]c has methoxy group, whereas the
compounds [III]d, [III]e and [III]f has n-butoxy group at that terminus. Compounds [III]d,
Table . Transition temperatures and associated enthalpy changes of series [III].
Compound
Transition temperatures (°C) (ꢀH, kJ/mol)
[III]a
[III]b
[III]c
[III]d
[III]e
[III]f
Cr  (.) SmB >  I
Cr  (.) SmB  (.) I
Cr  (.) SmB  (.) I
Cr  (.) N  (.) I
Cr  (.) SmB  (.) I
Cr  (.) N  (.) I
Cr = Crystal; N = Nematic; SmA = smectic A; SmC = smectic C; I = Isotropic Transition.

92
J. H. TOMMA ET AL.
Figure . Mesophase texture of compounds [III]b and [III]d: (a) A polygonal-like texture of SmB phase of
compounds [III]b at °C. (b) A nematic droplet texture of compounds [III]d at °C. (c) A schlieren texture
of nematic phase of compound [III]d at °C ( × magnification).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
93
and [III]f showed nematic phase, this may be due to the fact that in those two compounds, the
alkyl chain length is longer by three carbon atoms as compared to the compounds [III]a and
[III]c. It is known [32] that increase of methylene spacer in alkoxy group increases the end-to-
end cohesive forces. This in turn results in lowering of lateral molecular interaction. However,
the smectogenic character of compound [III]e could be due to the fact that bromo terminal
group enhances smectogenic tendency in a mesogenic bent molecules, due to enhances lat-
eral attractions as compared with terminal attractions [32] as in case of hydroxyl and amino
terminal substituent in compounds [III]d, and [III]f. These results illustrate the importance
of the influence of terminal chain length on the appearance of smectic order.
The optical photomicrographs of [III]b and [III]d are depicted in Fig. 3 as a representative
illustration. The texture of compound [III]b under OPM showed a series of dark-lines, when
further cooled, the bands expanded, met and eventually coalesce to produce a polygonal-
like texture (Fig. 3(a)) [33, 34]. This phase is identified as a SmB phase. Upon cooling the
isotropic liquid of [III]d, nematic droplets (Fig. 3(b)) appeared and coalesced to form the
typical schlieren texture with disclination lines (Fig. 3(c)).
Conclusions
The liquid crystalline compounds containing heterocyclic groups have been synthesized
due to their potentially wide range of applications, such as in the optical, electrical and
biological medical fields. Hence, in this article three series having the same rigid core con-
nected by three different central linkages have been synthesized and characterized and their
mesomorphic behavior was studied by DSC and OPM. The first series with azomethine
linkage group, enantiotropic phases are observed, and there is a strong tendency to form
purely enantiotropic SmB phase for all members. The first three members, namely with
4-methoxyphenyl group of the second series with N-acetyl-2,3-dihydro-1,3,4-oxadiazoline
central linkage display enantiotropic nematic phase. While the members with 4-butoxyphenyl
group showed enantiotropic SmB phase. The last series in this work are synthesized by the
reaction of the second series compounds with phenyl hydrazine in the acidic medium to
form compounds with phenyl hydrazones central linkage group. All of the compounds of
this series exhibit an enantiotropic SmB phase, with the exception of the two compounds
with terminal 4-butoxyphenyl group and terminal 4-hydroxyphenyl or 4-aminophenyl group
in other side, respectively. These two compounds show enantiotropic nematic phase. The
mesomorphic properties were found to be dependent on the two terminal 4-substituted
phenyl groups and the central linkage group.
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