Laboratory Experiment pubs.acs.org/jchemeduc
Mechanochemical Synthesis of Two Polymorphs of the Tetrathiafulvalene-Chloranil Charge Transfer Salt: An Experiment for Organic Chemistry Alex Wixtrom,† Jessica Buhler,† and Tarek Abdel-Fattah*,† †
Applied Research Center and Department of Molecular Biology and Chemistry, Christopher Newport University, Newport News, Virginia 23606, United States S Supporting Information *
ABSTRACT: Mechanochemical syntheses avoid or considerably reduce the use of reaction solvents, thus providing green chemistry synthetic alternatives that are both environmentally friendly and economically advantageous. The increased solid-state reactivity generated by mechanical energy imparted to the reactants by grinding or milling can offer alternative synthetic routes, occasionally yielding products (structures or stoichiometries) not obtainable via solution chemistry. Additionally, small volumes of solvents added during grinding can control the polymorphic form of the products. An undergraduate organic chemistry experiment is described involving liquid-assisted grinding (LAG) synthesis of the green and black polymorphs of the tetrathiafulvalene-chloranil (TTF-CA) charge transfer salt and their solid state characterization using FT-IR and melting point analysis.
KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Green Chemistry, IR Spectroscopy, Solid State Chemistry, Synthesis reen (also known as “sustainable”) chemistry represents a modern field of research dedicated to the elaboration of chemical processes that reduce harmful environmental impact.1 This can include the reduction or elimination of waste products, the use of catalysts and nontoxic substances when possible, and the use of less solvent overall. Green chemistry topics are increasingly being included in undergraduate chemistry curricula.2,3 The addition of new laboratory experiments illustrating green chemistry approaches is important for the training of synthetic chemists, who can apply this knowledge to design future chemical processes with reduced environmental impacts. Mechanochemical syntheses are a type of green chemistry approach that typically require significantly less solvent usage (or sometimes none at all) compared to traditional syntheses from solution. Unlike traditional (organic, in particular) experiments, mechanochemical synthetic methods do not generate substantial quantities of waste. Mechanochemical processes are frequently used in industrial applications, such as pharmaceutical production, mineral extraction, metallurgy, and more.4 Methods utilizing mechanochemistry usually involve reactions with very high atom economy, enabling the formation of products (even from very small quantities of reactants) with little or no unnecessary byproducts. The increased adoption of mechanochemical syntheses techniques in place of solutionbased approaches in organic chemistry laboratory experiments can have substantial monetary benefits due to reduced costs of acquiring materials and disposal of waste associated with the
G
© XXXX American Chemical Society and Division of Chemical Education, Inc.
experiment. Syntheses using mechanochemistry also clearly demonstrate useful chemical techniques and processes to students, such as solid state reactivity. This laboratory experiment involves the synthesis of organic charge transfer salts (CTS), a type of compound composed of an electron donor, such as tetrathiafulvalene (TTF), and an electron acceptor, such as a tetrahalo-p-benzoquinone, also known as chloranil (CA) (Figure 1).
Figure 1. Molecular structures of tetrathiafulvalene (TTF) and chloranil (CA) reactants.
The green polymorphic form of tetrathiafulvalene-chloranil (TTF-CA) can be prepared from solution chemistry and has been extensively studied. 4 TTF compounds and their derivatives are used in many applications, such as molecular electronics, supramolecular systems, C60 complexes, nonlinear optical materials, Langmuir−Blodgett films, molecular shuttles,
A
dx.doi.org/10.1021/ed4002267 | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
the reactants, products, and a mixture of the two reactants are recorded. A detailed description of the experimental procedure is provided in the Supporting Information.
semiconducting, conducting and superconducting solids, sensors, conducting polymers, and so forth.5
■
■
PEDAGOGIC GOALS One of the main goals of this experiment is to demonstrate key green chemistry concepts of atom economy and E-factor. Atom economy focuses on the total number of reactant atoms that are actually utilized in the formation of the products instead of being “wasted” in the production of unwanted or unnecessary byproducts. The atom economy of a reaction is defined as a ratio of the molar mass of the reactant atoms utilized in the formation of the desired product in a reaction to the total reactant molar mass (percent atom economy, eq 1)6 atom economy =
HAZARDS Tetrathiafulvalene and chloranil reactants, and the resulting TTF-CA products are possible skin and eye irritants and should not be inhaled or ingested. Acetone is a moderate skin and eye irritant, and should not be inhaled or ingested. Nitrile protective gloves should be worn to avoid skin contact with reactants, and proper eye protection should be worn at all times. After completion of the experiment, any remaining products left on the agate mortar should be wiped off using a few drops of acetone and a tissue wipe. There should be no liquid waste because the 1−2 drops of acetone or water used during synthesis should evaporate during LAG.
mass of atoms in desired product mass of atoms in reactants × 100%
■
(1)
DISCUSSION Multiple trials of this experiment were performed in three independent study format laboratory courses and twice with a full class of over 20 undergraduate students. The experiment required a single laboratory period of 4 h to complete; however, modifications to accommodate shorter or longer laboratory periods can also be made (see the instructor notes in the Supporting Information). There was a clearly visible color difference between the two reactants and the two product polymorphs (Figure 2). Prior to further analysis using melting point or FT-IR, this visual difference was confirmed.
Green chemistry reactions are designed to maximize atom economy, avoiding excess production of wasteful byproducts. The reactions performed in this experiment have a 100% atom economy because all of the atoms of the reactants are used in the formation of the products, and no byproducts are formed. Another metric used to evaluate reaction efficiency relevant to this experiment is the E-factor, defined as a ratio of the mass of the reaction waste to the mass of the desired products formed (E-factor, eq 2)7 E‐factor =
mass of reaction waste × 100% mass of desired product(s)
(2)
Because no waste is produced in this experiment, the E-factor is zero. Another important concept this experiment demonstrates is the formation of product polymorphs and how the crystal packing of solid materials can influence their physical and chemical properties. Organic polymorphs are identical in chemical composition but differ in crystal structure. The differences between the “stacking” of TTF units in the crystal structures of the green and black forms, together with the charge transferred between donor and acceptor, determine the different properties of the materials. In this experiment, two different polymorphs of the TTF-CA charge transfer salt are selectively obtained using mechanochemical syntheses. The solvent used for liquid-assisted grinding (LAG) directs the formation of each specific polymorph, despite the same two reactants (and the same quantity of each) being used. This experiment also demonstrates the use of solid state techniques for the characterization of the properties of materials, such as FT-IR spectroscopy, illustrating their scope for polymorphism studies.
Figure 2. (A) Brown polymorph of TTF reactant, (B) chloranil reactant, and (C) green and (D) black TTF-CA polymorph products.
Student results for melting point analysis of the TTF and CA reactants and the two TTF-CA polymorph products are shown in Table 1. Neither of the two polymorphs of the mechanochemically synthesized products melted; instead, each decomposed prior to melting. Both the green and black polymorphs changed to a burgundy color at ∼130 °C prior to decomposition.
■
EXPERIMENTAL PROCEDURE Each student (or group of 2−4 students) performs two LAG synthesis methods to prepare two polymorphs of the TTF-CA charge transfer salt from TTF and CA solid reactants. Two polymorphs of TTF are known, brown (triclinic) and orange (monoclinic);8 the brown polymorph is used for this experiment. The green TTF-CA polymorph is prepared by manual grinding in an agate mortar and pestle TTF (50.0 mg) and an equimolar amount of CA with acetone (200 μL) for 20 min. The black TTF-CA polymorph is prepared in an identical manner using distilled water in place of acetone. Melting points are taken of the reactants and products, and FT-IR spectra of
Table 1. Student Results Showing the Melting Temperatures of the Reactants and Products Compound Tetrathiafulvalene Chloranil Green TTF-CA polymorph Black TTF-CA polymorph a
B
Temperature (°C)a 120−131 289−296 210−218 218−220
(melting point) (melting point) (decomposition) (decomposition)
The ranges are the averages of three melting point trials. dx.doi.org/10.1021/ed4002267 | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Figure 3. Student FT-IR spectra of the black and green TTF-CA polymorphs and the TTF/CA reactant mixture.
quantitative methods for the characterization of the properties of solid materials, including FT-IR spectral and melting point analysis, which are valuable tools for the identification of organic compounds. Students were also familiarized with the concept of organic polymorphism and its consequences in the properties of solids, along with the importance of understanding the structures and bonding in materials in order to interpret their physical and chemical properties. Student laboratory reports and question responses indicated the successful achievement of the pedagogic goals of the experiment. The concepts presented in this experiment are ultimately useful in the design of new materials with targeted properties for particular applications.
Melting point analysis was useful for differentiating the mechanochemically synthesized TTF-CA products from the reactants; though, due to the fact that both polymorphs decomposed prior to melting at similar temperature ranges (only ∼10 °C difference), further analysis using other solid state techniques, such as FT-IR spectroscopy, was required to confirm the successful production of two distinct products. Xray powder diffraction (XRPD) is an alternative method for definitive confirmation of two unique polymorphs; however, this method is time-consuming (∼90 min per sample) and requires expensive instrumentation to complete, which a typical undergraduate laboratory would not have access to. For the purposes of this experiment, visual observation of a color difference, melting point, and FT-IR spectroscopy were sufficient to demonstrate the relevant concepts. The FT-IR spectra obtained by students showed a clear distinction between the two synthesized TTF-CA polymorphs (Figure 3). The FT-IR spectrum of the mixture of reactants was also clearly differentiable from the two products (Figure 3). Repeated trials indicated that the peak positions for each product and the reactant mixture remained in the same locations, although depending on the quantity used, the intensity of the peaks differed, whereas the relative intensities between peaks remained constant. The differences observed in the FT-IR spectra were due to the changes in symmetry between the two products and the reactants because no new functional groups were formed.9 This demonstrated the successful synthesis of two unique products, each produced using a different solvent for LAG.4 The specific effects of the changes in symmetry on the FT-IR spectra have been explained in detail.10−12
■
ASSOCIATED CONTENT
S Supporting Information *
The student handout with experimental procedures and questions; notes for the instructor including hazards, authorobtained FT-IR spectra, and modifications to adjust the total duration of the experiment. This material is available via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors gratefully acknowledge Silvina Pagola for her contributions to the development and procedure of this research.
■
CONCLUSION The mechanochemical syntheses performed effectively demonstrated the successful formation of two polymorphic forms of the TTF-CA charge transfer salt, highlighted some of the benefits of green chemistry, and introduced green chemistry concepts such as atom economy and E-factor. In addition to providing the ability to control the desired product polymorph using LAG, this synthetic method produced essentially no waste. The lack of polluting waste and the minimal solvent use demonstrates a green chemistry synthetic approach, another important concept for students to discover. During the experiment, students practiced data collection and analysis of
■
REFERENCES
(1) Ritter, S. K. Green Chemistry. Chem. Eng. News 2001, 79 (29), 27−34. (2) Andraos, J.; Dicks, A. P. Green chemistry teaching in higher education: a review of effective practices. Chem. Educ. Res. Pract. 2012, 13, 69−79. (3) Eilks, I.; Rauch, F. Sustainable development and green chemistry in chemistry education. Chem. Educ. Res. Pract. 2012, 13, 57−58. (4) Benjamin, S.; Pagola, S.; Huba, Z.; Carpenter, E.; Abdel-Fattah, T. Solvent-drop assisted mechanochemical synthesis of the black and
C
dx.doi.org/10.1021/ed4002267 | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
green polymorphs of the tetrathiafulvalene-chloranil charge transfer salt. Synth. Met. 2011, 161, 996−1000. (5) Singleton, J. Why do Physicists Love Charge-Transfer Salts? J. Solid State Chem. 2002, 168, 675−689. (6) Baird, C.; Cann, M. Environmental Chemistry, 5th ed.; W. H. Freeman and Company: New York, 2012. (7) Cheney, M. L.; Zaworotko, M. J.; Beaton, S.; Singer, R. D. Cocrystal Controlled Solid-State Synthesis. A Green Chemistry Experiment for Undergraduate Organic Chemistry. J. Chem. Educ. 2008, 85, 1649−1651. (8) Ellern, A.; Bernstein, J.; Becker, J. Y.; Zamir, S.; Shahal, L.; Cohen, S. A New Polymorphic Modification of Tetrathiafulvalene. Crystal Structure, Lattice Energy and Intermolecular Interactions. Chem. Mater. 1994, 6, 1378−1385. (9) Trask, A. V.; Jones, W. Crystal Engineering of Organic Cocrystals by the Solid-State Grinding Approach. Top. Curr. Chem. 2005, 254, 41−70. (10) Girlando, A.; Marzola, F.; Pecile, C.; Torrance, J. B. Vibrational spectroscopy of mixed stack organic semiconductors: Neutral and ionic phases of tetrathiafulvalene-chloranil (TTF-CA) charge transfer complex. J. Chem. Phys. 1983, 79, 1075−1085. (11) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: Hoboken, NJ, 1991. (12) Smith, B. C. Infrared Spectral Interpretation: A Systematic Approach; CRC Press: Boca Raton, FL, 1999.
D
dx.doi.org/10.1021/ed4002267 | J. Chem. Educ. XXXX, XXX, XXX−XXX
