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Published: June 03, 2011
Copyrightr 2011 American Chemical Society andDivision of
Chemical Education, Inc. 1133 dx.doi.org/10.1021/ed101090t | J.
Chem. Educ. 2011, 88, 11331136
LABORATORY EXPERIMENT
pubs.acs.org/jchemeduc
Synthesis and Characterization of Self-Assembled LiquidCrystals:
p-Alkoxybenzoic AcidsJana Jensen, Stephan C. Grundy, Stacey Lowery
Bretz, and C. Scott Hartley*
Department of Chemistry and Biochemistry, Miami University,
Oxford, Ohio 45056, United States
bS Supporting Information
Thermotropic liquid crystal phases are condensed phasesexhibited
by some compounds at intermediate temperaturesbetween the liquid
and crystalline solid.1,2 Similar to conven-tional isotropic
liquids, the molecules in a liquid crystal phaseare mobile (i.e.,
diuse). However, similar to crystals, liquidcrystalline materials
exhibit properties arising from long-rangemolecular order. As a
consequence of this combination ofmolecular mobility and
crystal-like properties, liquid crystalsare used in a number of
technological applications, mostprominently as the active
components of the at panel videodisplays of televisions, computers,
and other devices. Liquidcrystals are also used in other
interesting applications, forexample, as ultrahigh-strength
materials including Kevlar andnatural silk.1,3 Despite their
signicance to both current technol-ogies and fundamental science,
liquid crystals are typicallyomitted from the undergraduate
curriculum.4,5 However, theyprovide a straightforward introduction
to organic materialschemistry and self-assembly and are readily
introduced as anactivity at the introductory organic chemistry
level. Here, wepresent a laboratory activity based on the synthesis
and char-acterization of a series of p-alkoxybenzoic acids. The
goal of thisactivity, which can be completed in a single laboratory
session, isto introduce the students to both liquid crystals as a
fundamentalstate of matter and the concepts of self-assembly in
materialsscience.
The oldest class of liquid crystal-forming molecules,
thecalamitics, are dened by their rod-like shape.6 The simplest
liquid crystal phase exhibited by the calamitics is the
nematicphase, which is characterized by long-range orientational
order:on the time-average, the molecular long axes are aligned in
acommon direction dened by the director (n^), as shown inFigure 1.
In the smectic liquid crystal phases, the molecules arefurther
oriented into layers. Shown in Figure 2, the simplestsmectic phases
are the smectic A, in which the molecules areoriented along the
layer normal (z^), and the smectic C, in whichthey are tilted from
the layer normal. The p-alkoxybenzoic acidssynthesized in this
experiment exhibit both the nematic andsmectic C phases, depending
on their structure.
Liquid crystals have been used in physics and physicalchemistry
laboratory courses to demonstrate physical properties,such as order
parameters, electrooptic eects, and optical proper-ties (e.g.,
birefringence, refractive index anisotropy,
selectivereection).5,712 They have also been used to study
otherapplications such as the orientation of dye molecules in
liquidcrystalline solvents7,13,14 and thermochromic thermal
mapping(i.e., mood rings).15,16 Laboratory experiments have
alsofeatured lyotropic (solvent-dependent) liquid crystals.17
Thesynthesis of liquid crystals provides a fundamental
opportunityfor students to gain hands-on experience with organic
materialschemistry. However, there are relatively few published
experi-ments that allow students to explore the synthesis of
liquid
ABSTRACT: Thermotropic liquid crystal phases are ordered uids
found, forsome molecules, at intermediate temperatures between the
crystal and liquidstates. Although technologically important, these
materials typically receive littleattention in the undergraduate
curriculum. Here, we describe a laboratoryactivity for introductory
organic chemistry students on the synthesis andcharacterization of
the p-alkoxybenzoic acids. These compounds, through theformation of
carboxylic acid dimers, exhibit liquid crystal phases common in
rod-like (calamitic) molecules. The students are assigned dierent
alkoxy chainlengths and synthesize the compounds through
microwave-assisted nucleophilicsubstitution. Characterization of
the phase behavior is then carried out bystandard melting point
techniques, dierential scanning calorimetry, or polar-ized optical
microscopy. The results for the class are pooled to allow the
studentsto consider structureproperty eects for the series. This
activity allowsstudents to explore small-molecule synthesis applied
to materials chemistry and concepts of self-assembly: the benzoic
acidsassociate through hydrogen bonding, and the resulting rod-like
dimers further organize into the liquid crystal phases.
KEYWORDS: Second-Year Undergraduate, Laboratory Instruction,
Organic Chemistry, Hands-On
Learning/Manipulatives,Calorimetry/Thermochemistry, IR
Spectroscopy, Materials Science, NMR Spectroscopy, Nucleophilic
Substitution, Phases/PhaseTransitions/Diagrams
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1134 dx.doi.org/10.1021/ed101090t |J. Chem. Educ. 2011, 88,
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Journal of Chemical Education LABORATORY EXPERIMENT
crystalline materials. Previous reports have included the
prepara-tion of cholesteryl benzoates, the rst liquid
crystalsdiscovered,6,18,19 and
N-(p-methoxybenzylidene)-p-n-butylani-line (MBBA), the rst liquid
crystal used in displays.4 Althoughboth of these activities employ
synthesis of a single liquidcrystalline compound, our experiment is
unique in two ways:(i) the students synthesize a series of
compounds, allowing themto pool data and explore structureproperty
relationships, and(ii) it emphasizes the concept of self-assembly
in organicmaterials.
Jones rst discovered liquid crystal phases in the
p-alkoxyben-zoic acids in 1929.2023 In developing a teaching
activity, we wereattracted to these molecules for three reasons.
First, they can bereadily synthesized in a single step from
inexpensive, commer-cially available starting materials using
straightforward chemistry(nucleophilic substitution), as shown in
Scheme 1. Conse-quently, this experiment could be used early in the
organiclaboratory curriculum, as both SN2 substitution and
intermole-cular forces have usually been discussed at the midpoint
of therst semester. Second, these molecules provide an ideal
intro-duction to liquid crystals as a form of self-assembly.24 The
liquidcrystal phases formed by p-alkoxybenzoic acids incorporate
twolevels of hierarchical self-assembly. The benzoic acids
dimerizethrough hydrogen bonding to give aggregates with a
morepronounced rod-like shape; these rod-like dimers then
furtherassemble into the liquid crystal phases (Figure 1). Third,
thestudents can synthesize targets with dierent alkoxy chainlengths
(16). Variation of side-chain lengths on the same corestructure is
very common in liquid crystals science as a means tooptimize phase
behavior. The class results can then be pooled toexplore the eect
of molecular structure on bulk properties, suchas phase
transitions.
EXPERIMENTAL DETAILS
The syntheses were conducted in the organic chemistrylaboratory
with second-year chemistry majors. Eleven studentscompleted the
experiment successfully. We adapted the synthesisto make use of a
microwave reactor to shorten the duration of thelab; if a microwave
is not available, the reaction can be performedat reux as in the
original synthesis.21 Students worked in groupsof two or three to
save time using the microwave. In our class,each group was assigned
either pentyl, heptyl, or nonyl bromide.In a typical procedure,
p-hydroxybenzoic acid (10 mmol) wasdissolved in 1 M solution of KOH
in methanol (21 mL). Thebromoalkane (11 mmol) was added using a
syringe and themixture was swirled until well mixed. After
microwave irradiation(20 min at 50% power using a 630 W CEM MDS
2000Microwave Digestion System), the product was transferred toa
ask using a small volume of methanol and acidied with 1 MHCl(aq)
(100 mL). The resulting precipitate was isolated byvacuum ltration,
washing with methanol (100 mL); if a sig-nicant quantity of
additional product precipitated in the ltrate,it was isolated as
well. To remove unreacted starting material, thetotal product was
dissolved in dichloromethane, ltered, andconcentrated under reduced
pressure. The crude products werethen recrystallized from minimum
volumes of hexanes. Studentyields, after recrystallization, ranged
from 13% to 34%. Theproducts were characterized using NMR and IR
spectroscopy,aording spectra typical of these compounds (see the
SupportingInformation). It is noteworthy that IR spectroscopy shows
abroad band in the neighborhood of 30002500 cm1, acharacteristic
signature of the hydrogen-bonded carboxylic aciddimer.25
Characterization of the liquid crystal phases can be done with
avariety of techniques, ranging from simple observation with
astandard melting point apparatus to dierential scanning
calo-rimetry (DSC) and polarized optical microscopy. This
experi-ment does not require sophisticated equipment, as
thetransitions to and from the liquid crystal phases are
readilydiscernible using a standard capillary melting point
apparatus:21
they appear as opaque, opalescent uids. On heating, it
waspossible to observe the more subtle smectic C to
nematictransition, but this can be challenging without prompting
ofthe expected transition temperatures. Students using the
capillarymelting point apparatus usually reported phase transitions
belowthose reported by Gray and Jones.23 Our class also used DSC
tomeasure phase-transition temperatures and enthalpies. Thesmectic
C to nematic phase transitions were readily discernibleby DSC. In
general, the phase transitions of the student productswere within 4
C of the literature values. As is common for liquidcrystals, the
smectic C to nematic and nematic to isotropic liquidtransitions
were characterized by considerably smaller enthalpiesof transition
(ca. 5 J/g) than the crystal to liquid-crystal transi-tions (ca. 50
J/g), reecting the structural similarity of the liquidcrystal and
isotropic liquid phases. A complete list of phase
Figure 1. The nematic phase of calamitic liquid crystals: (left)
a uid inwhich the molecules point in the same direction (n^);
(right) in thecontext of this experiment, the rod-like molecules
are hydrogen-bonded p-alkoxybenzoic acid dimers.
Scheme 1. Synthesis of Liquid Crystals from p-Alkoxy-benzoic
Acid
Figure 2. The (A) smectic A and (B) smectic C phases of a
calamiticliquid crystal.
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1135 dx.doi.org/10.1021/ed101090t |J. Chem. Educ. 2011, 88,
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transitions and enthalpies is included in the Supporting
Informa-tion. As groups of students had been assigned dierent
alkylchain lengths, the results were pooled to illustrate
structureproperty eects in this system. In very broad terms,
increasingthe alkyl chain length decreases the clearing point
(nematic toisotropic transition temperature), reecting the less
ecientnematic packing as the conformational exibility of the
chainsincreases. The other obvious trend was promotion of the
smecticC phase at long chain lengths, whichmay be ascribed to
increasedmicrophase segregation between the rigid benzoic acid
dimercores and the exible side chains.
One of the most attractive aspects of liquid crystals research
isthat, similar to crystals, liquid crystals are birefringent
(i.e., liquidcrystals, as anisotropic materials, have dierent
refractive indicesdepending on the polarization of incident light).
Consequently,thin lms of liquid crystals give highly colored
textures whenviewed through crossed polarizers, whereas isotropic
phasesappear black. Although not required for this experiment,
theconstruction of polarizing microscopes out of easily
accessibleitems has been previously described: regular microscopes
can beused along with polarizing lm,4 a hot air gun can be used
tocontrol temperature of the materials, or a convenient
variabletemperature stage can be constructed as described by Verbit
andHalbert.26 In the case of the p-alkoxybenzoic acids, the
nematicand smectic C phases displayed characteristic so-called
schlierentextures, as shown in Figure 3 (also shown, for
comparison, is thecrystal phase). Although a detailed discussion of
the molecularorigins of liquid crystal textures is beyond the scope
of thisactivity, we point out that the wavy black lines of the
schlierentexture correspond to regions in which the long axes of
themolecules are lined up parallel or perpendicular to the
polarizersof the microscope. Thus, the textures are a direct
reection of thebulk molecular order of a liquid crystalline
material.
HAZARDS
The 1-bromoalkanes are considered irritants and may beharmful if
ingested or inhaled. p-Hydroxybenzoic acid is alsoan irritant.
Sodium hydroxide is caustic and hydrochloric acid iscorrosive.
Methanol is highly ammable and toxic. Hexane isdangerous for the
environment, harmful, and highly ammable.All chemicals in the
experiment should be handled with gogglesand protective gloves. The
synthesis is best done in a fume hood,and waste should be disposed
of according to standard procedures.
STUDENT DISCUSSIONS
Five discussion questions and answers are provided in
theinstructors manual included in the Supporting Information.
Thestudents answers indicated comprehension of most of the
liquid
crystal topics introduced. They were able to note that
liquidcrystal to isotropic transition enthalpies are typically
smaller thancrystal to liquid crystal enthalpies, including a
discussion of therelationship between transition temperatures and
structures. Thestudents were also able to distinguish the dierent
liquid crystalphase textures produced by a polarizing microscope.
Somestudents demonstrated a thoughtful understanding of the
alkyla-tion reaction, explaining that the reaction occurs with
highchemoselectivity due to the higher nucleophilicity of the
morebasic phenoxide group; one such student explained that
thereaction only happens at hydroxy [sic] group because a
depro-tonated hydroxy is much more reactive than COOH due to
theelectron withdrawing properties of the carbonyl group. A
fewstudents misunderstood the use of hydrochloric acid and
thoughtit was used to remove excess starting material. Also,
whereas moststudents grasped the relationship between structure and
trends inphase transition temperatures, a few students
misinterpreted howhydrogen bonding aected these trends. For
example, studentssuccessfully noted that the initial melting points
are higher for 1and 2. However, some students incorrectly reasoned
that thisoccurred because shorter chains are more apt to form
H-bondsand at 78, the steric factors make hydrogen bonds less
eective.
CONCLUSION
The microwave-assisted synthesis and characterization of
p-alkoxybenzoic acids is well suited for the introduction of
organicmaterials chemistry in the undergraduate laboratory. The
targetcompounds are readily prepared by alkylation of
p-hydroxyben-zoic acid with microwave irradiation. Observation with
a stan-dard melting point apparatus, as well as dierential
scanningcalorimetry and polarized optical microscopy, can be used
tostudy the phase behavior. Collaborative eorts by student
groupspermits pooling of data to observe structureproperty
relation-ships: the eect of varying the molecular structure on the
phasebehavior of a series of compounds.
ASSOCIATED CONTENT
bS Supporting InformationStudent handouts; instructor notes,
including phase transi-
tions and enthalpies; CAS registry numbers of
chemicals;manufacturers of equipment; NMR and IR spectra; and
polariz-ing microscope photos. This material is available via the
Internetat http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author*E-mail: [email protected].
Figure 3. Polarized optical micrographs of the schlieren
textures of (A) the nematic and (B) the smectic C phases of
student-synthesized p-heptyloxybenzoic acid. Also shown is the
crystal phase (C). Note that the spots interrupting the texture
likely represent small quantities of impurities inthe sample,
possibly 4-hydroxybenzoic acid.
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1136 dx.doi.org/10.1021/ed101090t |J. Chem. Educ. 2011, 88,
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ACKNOWLEDGMENT
The authors wish to thank the National Science Foundation(award
#0733642) and Miami University for nancial support.We are grateful
to the students and Ben Gung. We also thankJason Crase for aid in
optimizing the procedures used.
REFERENCES
(1) Brown, G. H. J. Chem. Educ. 1983, 60, 900905.(2) Collings,
P. J.; Hird, M. Introduction to Liquid Crystals; Taylor &
Francis Ltd.: London, 1997.(3) Kerkam, K.; Viney, C.; Kaplan,
D.; Lombardi, S. Nature 1991,
349, 596598.(4) Liberko, C. A.; Shearer, J. J. Chem. Educ. 2000,
77, 12041205.(5) Waclawik, E. R.; Ford, M. J.; Hale, P. S.;
Shapter, J. G.; Voelcker,
N. H. J. Chem. Educ. 2004, 81, 854858.(6) Reinitzer, F. Monatsh.
Chem. 1888, 9, 421441.(7) Demirbas, E.; Devonshire, R. J. Chem.
Educ. 1996, 73, 586589.(8) DuPre, D. B.; Chapoy, L. L. J. Chem.
Educ. 1979, 56, 759761.(9) Lalanne, J. R.; Hare, F. J. Chem. Educ.
1976, 53, 793795.(10) Jeppesen, M. A.; Hughes, W. T. Am. J. Phys.
1970, 38, 199201.(11) Olah, A.; Doane, J. W. Am. J. Phys. 1977, 45,
485488.(12) Van Hecke, G. R.; Karukstis, K. K.; Li, H. H.;
Hendargo, H. C.;
Cosand, A. J.; Fox, M. M. J. Chem. Educ. 2005, 82, 13491354.(13)
Sadlej-Sosnowska, N. J. Chem. Educ. 1980, 57, 223224.(14) Wilson,
R. M.; Gardner, E. J.; Squire, R. H. J. Chem. Educ. 1973,
50, 9498.(15) Fergason, J. L. Am. J. Phys. 1970, 38, 425428.(16)
Lisensky, G. C.; Horoszewski, D.; Gentry, K.; Zenner, G. M.;
Crone, W. C. Sci. Teach. 2006, 73, 3035.(17) Friberg, S. E.;
Bendiksen, B. J. Chem. Educ. 1979, 56, 553555.(18) Patch, G.; Hope,
G. A. J. Chem. Educ. 1985, 62, 454455.(19) Verbit, L. J. Chem.
Educ. 1972, 49, 3639.(20) Bradeld, A. E.; Jones, B. J. Chem. Soc.
1929, 26602661.(21) Jones, B. J. Chem. Soc. 1935, 1874.(22)
Bennett, G. M.; Jones, B. J. Chem. Soc. 1939, 420425.(23) Gray, G.
W.; Jones, B. J. Chem. Soc. 1953, 41794180.(24) Kato, T.;
Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed.
2006, 45, 3868.(25) Silverstein, R. M.; Webster, F. X.; Kiemle,
D. J. Spectrometric
Identication of Organic Compounds, 7th ed.; John Wiley &
Sons, Inc.:Hoboken, NJ, 2005.
(26) Verbit, L.; Halbert, T. R. J. Chem. Educ. 1971, 48,
773774.