Journal of Saudi Chemical Society (2012) 16, 153–159

King Saud University

Journal of Saudi Chemical Society

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Synthesis, characterization and comparative study

of mesomorphic properties of some new

compounds containing both 1,2,4- and

1,3,4-oxadiazole moieties linked in the same molecule

Ivan H.R. Tomi *

Department of Chemistry, College of Science, Al-Mustansiriya University, Baghdad, Iraq

Received 7 July 2010; accepted 13 December 2010Available online 17 December 2010

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KEYWORDS

Liquid crystal;

1,2,4-Oxadiazole;

1,3,4-Oxadiazole;

Mesomorphic properties

Tel.: +964 7901965123.

-mail address: ivanhrtomy@

19-6103 ª 2011 King Saud

sevier B.V. All rights reserve

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i:10.1016/j.jscs.2010.12.002

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osting by E

Abstract One new homologous series of compounds containing 1,2,4- and 1,3,4-oxadiazole rings

in the same molecule was synthesized. 3-(4-Butoxyphenyl)-5-{4-[5-(4-alkoxyphenyl)-1,3,4-oxa-

diazol-2-yl]phenyl}-1,2,4-oxadiazole (VII)1–8 were synthesized by several procedures. This series

has been characterized by FT-IR and 1H NMR spectroscopy. Their liquid crystalline properties

were studied by polarizing optical microscopy (POM) and differential scanning calorimetry

(DSC). This series did not show any liquid crystalline behaviors and only crystal to isotropic liquid

transition was observed.ª 2011 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

The structure/mesomorphic property relationship is the mostattractive study area of a chemist devoted to liquid crystal re-search, and a significant number of papers and monographs

m

y. Production and hosting by

Saud University.

lsevier

dealing with this subject have been published (Zannoni,1979; Kobmehl and Hoppe, 1993).

The synthesis of liquid crystalline compounds with unusualshape has been the goal of many research groups because oftheir importance in LCD technology, sensors and in the devel-

opment of optical devices (Eidenschink, 1989).Over many years a large number of liquid crystalline com-

pounds containing heterocyclic units have been synthesized

(Cristiano et al., 2006; Kauhanka and Kauhanka, 2006). Thisresearch field has grown even more in recent years because ofimprovements in synthetic methodologies. Heterocycles are ofgreat importance as core units in liquid crystals owing to their

ability to impart lateral and/or longitudinal dipoles combinedwith changes in the molecular shape (Gallardo et al., 2002).

Bent shaped molecules came in the focus of liquid crystal

research when Niori et al. (1996) discovered the excep-tional properties of the so-called banana phases. Bent

154 I.H.R. Tomi

‘‘banana-shaped’’ mesomorphic molecules have attracted

rapidly growing interest both among theoreticians and experi-mentalists, mainly because of their ability to originate liquid –crystalline mesophase that exhibits spontaneous polarizationin the absence of chiral groups in their molecular structure

(Matsunaga and Miyamoto, 1993). Among these type ofmesogens, molecules containing five-membered heterocyclessuch as 1,2,4- and 1,3,4-oxadiazoles (Torgova et al., 2003;

Dingemans and Samulski, 2000). Most work has concentratedon the symmetrical 1,3,4-oxadiazoles, and considerably lesswork has been focused on the unsymmetrical 1,2,4-oxadiazoles

(Wen et al., 2005).To our knowledge, there is no study for the preparation and

mesomorphic properties of compounds containing two isomers

of oxadiazoles (1,2,4- and 1,3,4-) units in the same molecule.The goal of this study was the synthesis, characterizationand study of the mesomorphic behavior of unsymmetricalbent-core series of compounds containing both 1,2,4- and

1,3,4-oxadiazole moieties linked in the same molecule.

2. Experimental part

2.1. Materials and physical measurements

All starting materials and solvents were purchased from Al-drich and Fluka and used without further purification. Melting

points were determined on Electrothermal capillary apparatusand are uncorrected; FT-IR measurements were recorded onShimadzu model FT-IR-8400S in university of Al-Musta-

nsiriya, department of chemistry, college of science, Baghdad,Iraq. 1H NMR spectra were obtained with Bruker spectrome-ter model ultra shield at 300 MHz in the University of Al-al-bait, Amman, Jordan. The compounds were dissolved in

DMSO-d6 solution with the TMS as internal standard. Theoptical behavior observations were made using OlympusBX40 microscope equipped with a Leitz Laborlux 12 pols

hot stage and PR600 controller. Differential scanning calorim-etry (DSC) measurements were conducted with DSC 25 man-ufactured by Mettler Toledo with Software: STARE version

9.00, Made in Switzerland, Ramp rate: 10 �C per minute undernitrogen atmosphere. Temperature and heat flow calibratedwith standard indium of purity <99.99%. Note: in some 1H

NMR spectra, the peaks at d 2.5 and 3.35 are for the solvent(DMSO-d6) and dissolved water in (DMSO-d6), respectively.

2.2. Preparation methods and physical data of synthesizedcompounds (I–VII1–8)

2.2.1. 4-n-Butoxybenzonitrile (I)This compound was prepared as described by Gallardo andBegnini (1995). A mixture of (10.36 g, 87 mmol) of 4-cyano-

phenol, (4.88 g, 87 mmol) potassium hydroxide and (50 mL)of absolute ethanol was heated at reflux for several minutes.Then (13.95 mL, 130 mmol) n-butyl bromide was added drop-

wise and the reaction mixture was refluxed for 72 h. After cool-ing, the potassium bromide was precipitated, then (50 mL) ofwater and (50 mL) of diethyl ether were added. The mixture

was extracted and the organic layer was collected and washedwith (25 mL) of water and (25 mL) of 10% sodium hydroxidesolution then with (25 mL) of water, dried the organic layer

over anhydrous magnesium sulfate overnight. After filtrationand evaporation of the solvent, the oil residue obtained yielded

(95%); FT-IR (film, cm�1) 2960, 2874 (Csp3–H), 2224 (C„N),

1606, 1508 (Csp2–H), 1259, 1172 (C–O–C).

2.2.2. 4-Butoxybenzamidoxime (II)This compound was prepared as described by Karamyshevaet al. (1994). A mixture (23.5 g, 134 mmol) of nitrile (I),(10.0 g, 144 mmol) of hydroxyl amine hydrochloride in

(200 mL) of ethanol with (50 mL) of water is stirred for 24 hat the boiling temperature, then concentrated the mixture invacuum. The residue was diluted with (500 mL) of cold water,filtered, washed with water and dried. The product is purified

by washing it with n-hexane. Yield (85%); mp 100–102 �C; FT-IR (KBr disk, cm�1) 3441, 3350 (N–H), 3174 (O–H), 1647(C‚N); 1H NMR (DMSO-d6, 300 MHz, d) 0.92 (t, 3H,

CH3), 1.44 (m, 2H, CH3–CH2–CH2–), 1.69 (m, 2H, –CH2–CH2–CH2O–), 3.95 (t, 2H, –CH2O–), 5.70 (s, 2H, NH2), 6.90(d, 2H, 2 arom. H, J = 8.7 Hz), 7.58 (d, 2H, 2 arom. H,

J= 8.7 Hz), 9.43 (s, 1H, OH of the amidoxime group).

2.2.3. 3-(4-Butoxyphenyl)-5-(4-methylphenyl)-1,2,4-oxadiazole(III)This compound was synthesized according to the procedurepreviously described by Parra et al. (2005). A mixture of

(2.7 g, 13 mmol) amidoxime (II) and (4.1 g, 25 mmol) of ethyl4-methylbenzoate dissolved in sodium ethoxide solution (pre-pared from dissolving 0.32 g, 14 mmol of sodium metal in(27 mL) of absolute ethanol); then the reaction mixture was

heated under reflux for 48 h. After the mixture was cooled,off white crystals were collected by filtration and dried. Thecrystals were recrystallized from ethanol. Yield (68%); mp

112 �C; FT-IR (KBr disk, cm�1) 2943, 2852 (Csp3–H), 1610(C‚N), 1591 (C‚C), 1248, 1178 (C–O–C), 924 (N–O); 1HNMR (DMSO-d6, 300 MHz, d) 0.95 (t, 3H, CH3), 1.46 (m,

2H, CH3–CH2–CH2–), 1.73 (m, 2H, –CH2–CH2–CH2O–),2.42 (s, 3H, –C6H4–CH3), 4.07 (t, 2H, –CH2O–), 7.12 (d, 2H,2 arom. H of –C6H4– linked to butoxy group, J = 9.0 Hz),7.47 (d, 2H, 2 arom. H of –C6H4– linked to methyl group,

J= 7.8 Hz), 8.01 (d, 2H, 2 arom. H of –C6H4– linked to but-oxy group, J= 9.0 Hz), 8.07 (d, 2H, 2 arom. H of –C6H4–linked to methyl group, J = 8.1 Hz).

2.2.4. 4-[3-(4-Butoxyphenyl)-1,2,4-oxadiazol-5-yl]benzoic acid(IV)(3.33 g, 21 mmol) of potassium permanganate was added por-tionwise to a stirred solution of (0.77 g, 2.5 mmol) of com-pound (III) in (16 mL) pyridine and (11 mL) water at 70 �C.Then the mixture was heated under reflux for 6 h, the solventswere evaporated in vacuum, then (35 mL) of water was addedto the mixture and filtered. The acid IV was precipitated by

acidification of the aqueous solution (2 M HCl). It was filteredunder suction, washed with water and dried at 100 �C. Yield(82%); mp >300 �C; FT-IR (KBr disk, cm�1) 3100–2544 (car-boxylic O–H), 2960, 2874 (Csp3–H), 1697 (C‚O), 1608

(C‚N), 1589 (C‚C), 1249, 1176 (C–O–C), 931 (N–O); 1HNMR (DMSO-d6, 300 MHz, d) 0.96 (t, 3H, CH3), 1.49 (m,2H, CH3–CH2–CH2–), 1.77 (m, 2H, –CH2–CH2–CH2O–),

4.09 (t, 2H, –CH2O–), 7.13 (d, 2H, 2 arom. H of –C6H4– linkedto butoxy group, J= 8.4 Hz), 8.02 (d, 2H, 2 arom. H of –C6H4– linked to butoxy group, J= 8.4 Hz), 8.17 (d, 2H, 2

arom. H of –C6H4– linked to carboxy group, J = 8.1 Hz),8.28 (d, 2H, 2 arom. H of –C6H4– linked to carboxy group,J= 7.8 Hz), 13.4 (s, 1H, very weak and broad COOH).

Synthesis, characterization and comparative study of mesomorphic properties 155

2.2.5. Ethyl 4-[3-(4-butoxyphenyl)-1,2,4-oxadiazol-5-

yl]benzoate (V)As a general method, a solution of (3.2 g, 9.47 mmol) of com-pound (IV) and (0.11 mL) of H2SO4 in (50 mL) absolute etha-

nol was heated under reflux for 72 h. After hot solution wasfiltered and evaporated the solvent, the crude ester was precip-itated. The white crystal of ester was washed with 10% of

NaHCO3 solution and then with water. After drying, we puri-fied it by recrystalization from ethyl acetate. Yield (80%); mp242 �C; FT-IR (KBr disk, cm�1) 2962, 2872 (Csp3–H), 1726

(C‚O), 1610 (C‚N), 1591 (C‚C), 1249, 1172 (C–O–C),904 (N–O); 1H NMR (DMSO-d6, 300 MHz, d) 0.94 (t, 3H,CH3), 1.35 (t, 3H, O–CO–CH2–CH3), 1.45 (m, 2H, CH3–CH2–CH2–), 1.72 (m, 2H, –CH2–CH2–CH2O–), 4.06 (t, 2H,

–CH2O–), 4.37 (q, 2H, –O–CO–CH2CH3), 7.12 (d, 2H, 2 arom.H of –C6H4– linked to butoxy group, J = 6.9 Hz), 8.02 (d, 2H,2 arom. H of –C6H4– linked to butoxy group, J= 6.9 Hz),

8.19 (d, 2H, 2 arom. H of –C6H4– linked to ester group,J = 7.2 Hz), 8.31 (d, 2H, 2 arom. H of –C6H4– linked to estergroup, J = 7.2 Hz).

2.2.6. 4-[3-(4-Butoxyphenyl)-1,2,4-oxadiazol-5-yl]benzohydrazide (VI)Hydrazine hydrate (2.5 g, 50 mmol) was added to a solution ofcompound (V), (3.66 g, 10 mmol) in ethanol (25 mL). Thereaction mixture was heated under reflux for 48 h, concen-

trated in vacuum, the precipitate obtained was filtered, washedwith cold water, dried and recrystallized from ethanol. Yield(79%); mp 220 �C (decom.); FT-IR (KBr disk, cm�1) 3282(N–H), 3205, 3160 (NH2), 2955, 2872 (Csp3–H), 1635

(C‚O), 1612 (C‚N), 1585 (C‚C), 1247, 1172 (C–O–C),902 (N–O); 1H NMR (DMSO-d6, 300 MHz, d) 0.91 (t, 3H,CH3); 1.43 (m, 2H, CH3–CH2–CH2–); 1.74 (m, 2H, –CH2–

CH2–CH2O–); 4.05 (t, 2H, –CH2O–); 4.62 (s, 2H, NH2); 7.13(d, 2H, 2 arom. H of –C6H4– linked to butoxy group,J = 8.1 Hz); 8.02 (d, 2H, 2 arom. H of –C6H4– linked to but-

I

CNC4H9O

V

C4H9ON O

NCOOEt

C4H9ON O

NCOOH

IV

CNHOC4H9Br, KOH, EtOH

Reflux 72 hrs.

KM

EtOH, H

Reflux 72 hrs.

+ NH2-N

Re

R = CnH2n + 1, n = 1-8

Scheme 1 Synthetic route

oxy group, J = 8.4 Hz); 8.06 (d, 2H, 2 arom. H of –C6H4–

linked to hydrazide group, J = 8.1 Hz); 8.24 (d, 2H, 2 arom.H of –C6H4– linked to hydrazide group, J= 8.1 Hz); 10.07(s, 1H, N–H).

2.2.7. 3-(4-Butoxyphenyl)-5-{4-[5-(4-alkoxyphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-1,2,4-oxadiazole (VII)1–8A mixture (0.7 g, 2 mmol) of compound (VI) and (2 mmol) ofappropriate 4-alkoxybenzoic acid in (5 mL) of phosphorousoxychloride was heated gently under reflux for 24 h. The graypaste like residue was poured into ice water and made the mix-

ture basic by adding sodium bicarbonate solution. The result-ing pale gray powder was filtered, dried and recrystallized fromchloroform to give the desired product. Yield (60–65%).

Data for 3-(4-butoxyphenyl)-5-{4-[5-(4-methoxyphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-1,2,4-oxadiazole (VII)1. FT-IR(KBr disk, cm�1) 2960, 2872 (Csp3–H), 1612 (C‚N of 1,2,4-

oxadiazole ring), 1595 (C‚N of 1,3,4-oxadiazole ring), 1580(C‚C), 1251, 1174 (C–O–C), 906 (N–O); 1H NMR (DMSO-d6, 300 MHz, d) 1.05 (t, 3H, CH3); 1.43 (m, 2H, CH3–CH2–CH2–); 1.70 (m, 2H, –CH2–CH2–CH2O–); 3.85 (s, 3H,

CH3O); 4.05 (t, 2H, –CH2O–); 7.15 (d, 2H, 2 arom. H of –C6H4– linked to butoxy group, J = 8.1 Hz); 8.10 (d, 2H, 2arom. H of –C6H4– linked to butoxy group, J= 8.7 Hz);

8.17 (d, 2H, 2 arom. H of –C6H4– linked to methoxy group,J= 8.4 Hz); 8.29 (d, 2H, 2 arom. H of –C6H4– linked to meth-oxy group, J = 8.4 Hz); 8.39 (s, 4H, 4 arom H of –C6H4– be-

tween 1,2,4- and 1,3,4-oxadiazole rings).

3. Results and discussion

3.1. Synthesis

The synthesis of the series (VII)1–8 compounds is outlined inScheme 1.

III

CH3-Ph-COOEtEtONa

CC4H9ON OH

NH2

C4H9ON O

NCH3

II

C4H9ON O

N

HN NH2

O

C4H9ON O

N

N N

O OR

VI

NH2OH.HCl, NaOH, EtOH

Reflux 24 hrs.

Reflux 48 hrs.

nO4, Pyridine, H2O

Reflux 6 hrs.

H2.H2O, EtOH

flux 48 hrs.

RO-Ph-COOH, POCl3Reflux 24 hrs.

VII1-8

of compounds VII1–8.

CC4H9ON OH

NH2

EtO

CC4H9ON O

NH2

CO

CH3

CC4H9ON O

NH2

CH3CO

OEt

- -

+

CC4H9ON O

C CH3

O-

NH H

Proton transfer

CC4H9ON O

C CH3

OH

NH

+

- H2OC4H9O CH3

ON

N

Scheme 2 The mechanism steps of formation of compound III.

156 I.H.R. Tomi

All the newly synthesized compounds gave satisfactory

analysis for the proposed structures, which were confirmedon the basis of their spectral (FT-IR and 1H NMR) data.

The bis-oxadiazoles compounds (VII)1–8 were synthesizedstarting with an alkylation reaction of 4-hydroxybenzonitrile

with 1-bromobutane in ethanolic solution of potassiumhydroxide to give the 4-butoxybenzonitrile (I). The reactionof compound (I) with hydroxylamine hydrochloride yielded

the corresponding amidoxime (II). This compound (II) was re-acted with ethyl 4-methylbenzoate in sodium ethoxide solu-tion, similar to the method reported by Bartulin, leading to

the formation of 1,2,4-oxadiazole compound (III) (Bartulinet al., 1992). The suggested mechanism of this reaction maybe outlined in Scheme 2.

The acid compound of 1,2,4-oxadiazole (IV) was obtained

by the standard procedures of oxidation reaction of compound(III) by potassium permanganate in pyridine and water.

Then, esterification of the acid compound (IV) by general

method yielded the ester compound of 1,2,4-oxadiazole (V).The reaction between the ester (V) and hydrazine hydrate80% in ethanol leading to formation of the hydrazide of

1,2,4-oxadiazole (VI). The condensation reaction of hydrazide(VI) with appropriate 4-alkoxybenzoic acid in phosphorous-oxy chloride leads to the bis-(1,2,4- and 1,3,4-)oxadiazole com-

pounds (VII)1–8. The mechanism steps of the ring closure of1,3,4-oxadiazole were described by us (Tomi et al., 2011).

Fig. 1 shows the 1H NMR spectra of compounds (II–VII1),respectively, Table 1 shows the melting points and FT-IR re-

sults of all compounds of series (VII)1–8.

3.2. Mesomorphic properties

The mesomorphic studies of bis-heterocyclic derivatives as po-tential materials are very limited. Little known examples of bis-

heterocyclic compounds exhibiting mesomorphic propertieshave been reported.

A new series of compounds containing bis-heterocyclic

(thiadiazole-triazole) as the core group were prepared andstudied the liquid crystalline properties by Tomma et al.(2009). The first compound of this series (see structures shownbelow) exhibited an enantiotropic nematic mesophase, and

that with propyloxy terminal substituent shows one enantio-

tropic nematic phase on heating and a monotropic smectic A(Sm A) phase on cooling, while the last two homologues exhi-bit a monotropic Sm A phase. In contrast, the compound withan ethoxy terminal substituent does not show any liquid crys-

talline properties.

H3CONN

NH

SN N

S N CH OCnH2n+1

n = 1 - 5

CH2

Also, the new derivatives containing bis-heterocyclic (oxa-diazole-tetrazole) (see structures shown below) were synthe-sized and studied the liquid crystalline properties by Dos

santos et al. (2008).

C8H17OON

NCO

ON N

NN

CnH2n+1

n = 6 , 8 , 10 , 12

All synthesized compounds exhibited liquid crystallinebehavior and showed enantiotropic smectic C, smectic A and

nematic phase.Ricardo et al. (2009) synthesized new light-emitting liquid

crystals containing (1,2,4- and 1,3,4-oxadiazole) (see structures

shown below).

ON

N N N

OOC10H21

n = 1, 6 , 8 , 10 , 12

CnH2n+1O

It is very attractive to the chemist to use the five memberedheterocycles of oxadiazolic type as a part of a liquid crystallinemolecule in designing new mesogens, due to the possibility of

obtaining a large variety of products. This capability is pro-

Figure 1 1H NMR spectra of compounds (II–VII1), respectively.

Synthesis, characterization and comparative study of mesomorphic properties 157

vided by the isomerism of the heteroatoms position in the cycle

and the substituent position concerning the symmetricalheterocycle.

A symmetry of 1,2,4-oxadiazole compounds involves threedistinct aspects:

1. Asymmetrical distribution of electronic density in the het-

erocycle due to the presence of three heteroatoms with dif-ferent electro-negativity (Chiou and Shine, 1989) (thedipole moment of this type of compounds is 1.5–1.6 D).

Table 1 Melting points and FT-IR results of compounds VII1–8.

Comp. Mp (�C) tCsp3–H tC‚N 1,2,4-oxadiazole tC‚N 1,3,4-oxadiazole tC‚C tC–O–C sym. and asym. tN–O

VII1 >300 2960, 2872 1612 1595 1580 1251, 1174 906

VII2 291–293 2956, 2866 1609 1599 1583 1253, 1175 902

VII3 286–288 2962, 2871 1614 1601 1585 1259, 1171 909

VII4 285–286 2929, 2856 1619 1604 1590 1242, 1165 904

VII5 280–283 2926, 2861 1610 1602 1591 1254, 1177 905

VII6 279–281 2928, 2855 1622 1602 1588 1247, 1169 902

VII7 274–276 2927, 2869 1618 1597 1581 1255, 1173 906

VII8 272–273 2931, 2857 1620 1603 1586 1261, 1167 901

158 I.H.R. Tomi

2. A distortion of the linearity of the molecule due to thebonding angle in the five-membered cycle.

3. The different types of substituents at C-3 and C-5 atoms.

All compounds described above show mesomorphic prop-

erties. In contrast to this, the compounds of series VII1–8 here-in do not show mesomorphic behavior and only Cr–Itransitions are observed when investigated using differentialscanning calorimetry (DSC) and hot-stage polarizing optical

microscopy (POM). Fig. 2 shows the DSC thermo-grams ofcompounds VII4 and VII6 as representative examples of seriesVII1–8.

The same results were obtained with the analogous 1,3,4-oxadiazole derivatives containing a symmetrical rigid core, re-ported by Dimitrowa et al. (1980). The authors assumed that

the bend associated with the exocyclic bonds in the 2- and 5-positions of the 1,3,4-oxadiazole unit was too severe to achievethe requisite ordered packing in mesophases.

From previous studies reported by Torgova et al., it isknown that 2,5-disubstituted 1,3,4-oxadiazole and 3,5-disub-stituted 1,2,4-oxadiazole derivatives have an exocyclic bondangle of 134� and 140�, respectively (Torgova et al., 2003),

indicating that the combination of the two isomers 1,2,4-and 1,3,4-oxadiazole in central rigid core in the same moleculeproduces a great distortion of the linearity of the molecules.

This deviation from the typical rod-like mesogen symmetry to-wards a non-linear shape could explain the suppression of

Figure 2 DSC thermo-grams of compou

mesomorphic behavior of compounds in series (VII)1–8.Fig. 3 shows the most stable conformation of compound

(VII)8 as representative example of series (VII)1–8.Moreover, it can be seen that the mesophase behaviors are

greatly affected by the length of the terminal chains (Songnanet al., 2008). Compounds of series (VII)1–8 are non-mesomor-

phic perhaps due to the short alkoxy terminal groups whichcause the higher melting points and thus suppress the liquidcrystal phases. We expected that if we synthesized the same

compounds with longer alkoxy terminal groups (n> 12)showed the mesomorphic properties.

In order to obtain structural information, I recommend

something like ‘‘semi-empirical calculations at AM1 level,implemented on the GAUSSIAN 94 W program’’ were per-formed. The derivative with a octoxy terminal chain (VII)8was used as molecular model. The linearity of this compoundis affected by the relative orientation of the alkoxy chain. Thusthe conformation diagram of compound (VII)8 is not linear,which could explain the non-mesomorphic behavior. See Fig. 3.

We can observe from the DSC thermo-grams of com-pounds (VII)4 and (VII)6, Fig. 2 and the conformation dia-gram of compound (VII)8, Fig. 3 that the mesophase

formation was formed to be dependent on the number of flex-ible side chains attached to the core group; however all com-pounds (VII)1–8 formed only crystalline phases regardless of

the carbon chain length. Crystal to isotropic transition at

nds (VII)4 and (VII)6 during heating.

Figure 3 Diagram of the most stable conformation of the

compound (VII)8.

Synthesis, characterization and comparative study of mesomorphic properties 159

285 �C and 280 �C was observed for compounds (VII)4 and(VII)6, respectively. The molecular structures of compounds

(VII)1–8 indicated that the rod-like in molecular shape (similarto the reported compounds), its much shorter side chain andmore distortion from the linearity of the molecules may lead

to the lack of observed mesomorphism.

4. Conclusion

A series of novel bis-heterocyclic compounds containing 1,2,4-and 1,3,4-oxadiazole in same molecules (VII)1–8 were synthe-sized. All of these compounds did not show any liquid crystal

properties but simply go to the isotropic melt because the 2,5-disubstituted 1,3,4-oxadiazole and 3,5-disubstituted 1,2,4-oxa-diazole derivatives have an exocyclic bond angle of 134� and

140�, respectively, indicating that the combination of the twoisomers 1,2,4- and 1,3,4-oxadiazole in central rigid core inthe same molecule produces a great distortion of the linearity

of the molecules. Also the short alkoxy terminal groups whichcause the higher melting points and thus suppress the liquidcrystal phases.

Acknowledgements

I thank Mr. Mohanad H.M. Masad (Al al-Bayt University,Jordan) for helpful about doing the 1H NMR spectra, andMrs. Zainab K.M. Jawad (Al-Mustansiriya University, Col-

lege of Science, Department of Chemistry) about doing theFT-IR spectra. Also we are very grateful to Dr. Esam A. Al-Karim and Dr. Amar H. Al-Dujaili (Baghdad University, Col-

lege of education Ibn Al-Haitham, Department of Chemistry)for performing the optical behavior observations and DSCthermograms, respectively.

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