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

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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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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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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: scott.hartley@muohio.edu.

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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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.

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