Teaching Green Chemistry with Epoxidized ... - ACS Publications

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Teaching Green Chemistry with Epoxidized Soybean Oil Homar Barcena,* Abraham Tuachi, and Yuanzhuo Zhang Kingsborough Community College, 2001 Oriental Boulevard, Brooklyn, New York 11235, United States S Supporting Information *

ABSTRACT: The synthesis of epoxidized soybean oil (ESO) provides students a vantage point on the application of green chemistry principles in a series of experiments. Qualitative tests review the reactions of alkenes, whereas spectroscopic analyses provide insight in monitoring functional group transformations.

KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Hands-On Learning/Manipulatives, Green Chemistry, Synthesis

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he integration of green principles1 in academic practices reflects the growing student interest in renewable resources and environmental stewardship.2 The motivation toward sustainable practices reflects the increasing awareness of global climate change and environmentalism, and application of these principles illustrates the scope of chemistry and how the science seeks opportunities to address the needs of our changing world. Thus, there have been many developments in curricula, hands-on activities, workshops, and other academic activities that capitalize on student interests in green chemistry.3−5 Inasmuch as the development of a green chemistry toolbox remains a challenge, educators need to expand the collection of green chemistry pedagogy to reflect the state of the art and the evolving viewpoints of industry and research. These experiments are meant not only to impart laboratory skills, but also for the application of chemistry toward global issues. Thus, students are equipped with a mindset to assess different sets of reagents for a chemical transformation and are better positioned to evaluate long-term considerations such as environmental fate, life cycle assessment, e-factor, and other such significances. In this experiment, students perform an epoxidation procedure and identify the green (and not-sogreen) elements of the exercise. Students further employ spectroscopy to determine the extent of functional group transformation as well as qualitative chemical tests for the presence of alkenes and epoxides.

triglycerides, which are esters of glycerol and three fatty acids. The high triglyceride content of soybean oil allows for its utility in many products such as soaps, coatings, paints, lubricants, and plastics.7,8 Fatty acids of vegetable oils vary in length from 14 to 22 carbons and contain one to three C−C double bonds, and for soybean oil, the main components are linoleic and oleic acids.9 High-olefin content vegetable oils make ideal precursors for highly functionalized materials, and a range of chemistries for their transformations have been explored.10 Industrially, epoxidation is primarily employed, often with peroxoacetic or peroxoformic acid generated in situ from hydrogen peroxide.11−13 The synthesis is economically viable, however suffers from long reaction times,14,15 and as such, epoxidation methods are an area of active research.16−21 Ring-opening of the epoxide is a further limiting factor in the optimization of the reaction, particularly in the presence of acids. Although many students have not heard of epoxidized soybean oil, it is a commercially successful product used in the plastics industry, particularly as a stabilizer for polyvinyl chloride polymers (PVC). Its utility is expected to grow with the increasing demand for phthalate-free plasticizers in the food and beverages, healthcare and pharmaceuticals, and adhesives and sealants industries. Epoxidation in Instruction

Epoxidation of alkenes using peroxyacids is among the fundamental reactions in organic chemistry, with an intricate mechanism that has been suggested to undergo a coarctate transition state.22 The reagent typically used is m-chloroperoxybenzoic acid (MCPBA), a crystalline solid that is safe to store and easy to weigh out.23 Because of the poor atom economy for

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EPOXIDIZED VEGETABLE OIL Vegetable oils are attractive alternatives to nonbiodegradable and nonrenewable petrochemical feedstocks, with applications ranging from biodiesel, biolubricants, and epoxy resins to polymer additives. They are readily available and cheap, with oils from palm, soybean, rapeseed, and sunflower among the most widely used worldwide.6 Vegetable oils are composed of © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: October 6, 2016 Revised: June 5, 2017

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DOI: 10.1021/acs.jchemed.6b00672 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

this reagent, and since it is typically used with chlorinated solvents, greener alternatives have been sought. For laboratory instruction, such a reagent is dimethyldioxirane, a secondary oxidant generated in situ by potassium monopersulfate (Oxone) and acetone.24,25 A greener procedure uses only hydrogen peroxide in water but is limited to α,β-unsaturated ketones.26 Catalytic systems have also been described such as Sharpless reagent27 and Jacobsen’s catalyst.28,29

Table 1. Overview of the Main Experimental Tasks Task Description Introduction to epoxidation and experimental overview Experiment setup and temperature control Addition of peroxide Workup Distillation Bromine test and periodic acid test, including overview FT-IR spectroscopy overview and experiment Overview 1H NMR use and sample preparation 1 H NMR Spectroscopy

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EXPERIMENTAL OVERVIEW Herein, we describe an adaptation of an industrial process as a laboratory exercise to illustrate epoxide formation while applying green principles. The procedure employs soybean oil, a renewable resource. Formation of the oxirane ring is catalyzed by acid, with hydrogen peroxide as the oxidant. Thus, the reaction conditions are mild, the byproducts benign, and the product nontoxic and biodegradable. The experiment has been performed in five sections of a second-semester, secondyear organic chemistry laboratory course, with 16−20 students in each session, lasting 4 h each. In this activity, peroxyformic acid is generated in situ using hydrogen peroxide, with para-toluenesulfonic acid (PTSA) as a catalyst, to oxidize the olefins in soybean oil (Scheme 1). Table

a

Laboratory Session

Time, mina

1

15

1 1 1 2 2

30 120 45 60 60

2 3 3

90 30 120

The laboratory experiment sessions are 4 h.

on the student’s bench. Glassware used to handle concentrated peroxide (addition funnel, graduated cylinder, or syringe) must be rinsed with water right away. Skin contact with hydrogen peroxide causes bleaching and a stinging sensation. Rinse under cold water. Eye contact with concentrated peroxide can cause corneal damage; rinse with copious amount of water and consult a physician. After the reaction, the outside of the flask was rinsed with petroleum ether to remove traces of oil, which could make glassware handling difficult. Neutralization of the remaining formic acid with sodium bicarbonate was performed in a separation funnel. Make sure students do not stopper the flask, and wait until the bubbling has subsided before shaking and inverting to minimize gas buildup. Formic acid and PTSA are corrosive and hazardous in case of skin contact, eye contact, and inhalation. Ethyl acetate, petroleum ether, and isopropanol are flammable solvents. Bromine in chloroform is corrosive and very hazardous in case of skin contact. Concentrated nitric acid and silver nitrate are very hazardous in case of skin contact, eye contact, ingestion, or inhalation. Chloroform-d is hazardous in case of skin contact, eye contact, ingestion, or inhalation. Soybean oil and epoxidized soybean oil are skin and eye irritants and cause glassware to be slippery.

Scheme 1. Synthesis of Epoxidized Soybean Oil

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RESULTS AND DISCUSSION

Synthesis

Starting with soybean oil, the synthesis of ESO was adapted for practical application in an organic chemistry teaching laboratory. Using standard glassware, students first heated soybean oil, formic acid, and a catalytic amount of paratoluenesulfonic acid (PTSA). Students were advised to keep the reaction temperature between 60 and 80 °C, and they had to practice temperature control with an oil bath (60 °C) to maintain the temperature range. Students then carefully added hydrogen peroxide dropwise using a syringe or an addition funnel. The reaction is exothermic, and in the event of boiling, the students were forewarned to stop the addition of peroxide until the temperature was within the recommended range. This should not be an issue provided patience is exercised when the oxidant is added. The reaction was allowed to stir for 10 min within the temperature range, and usually the students did not need to adjust the heat during the reaction. The use of PTSA in lieu of other acid catalysts promoted the reaction much faster than reported procedures, which required several hours.30,31 Our modifications allowed students to finish

1 outlines the experimental tasks divided over three laboratory sessions, with each session having a separate but related instructional goal. First, students learn the synthesis of epoxides, followed by epoxide ring-opening and a review of trans-dihalogenation of alkenes for the chemical tests. Lastly, students perform spectroscopy on the crude product mixture to analyze the extent of the reaction.

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HAZARDS Hydrogen peroxide (30%) is a strong oxidizer and must be handled with care. Gloves and goggles must be worn at all times, and gloves must be rinsed with water before they are taken off to wash off any residual hydrogen peroxide. In our lab, students were first instructed to set up the reaction and allow the temperature to equilibrate. Once ready, the instructor inspected the apparatus and dispensed the hydrogen peroxide B




Students were tasked to perform 1H NMR analyses using their own products, and representative spectra are included in the Supporting Information. Samples were prepared in CDCl3 and referenced accordingly. The instructor provided a demo for the soybean oil spectrum, where the glycerol peaks were identified and used as an internal standard for the integration (1:2:2).13,35 The alkene peaks (5.5−6.0 ppm) were also integrated to provide a measure of olefin protons. When students obtained the spectrum for their product, they were tasked to identify and integrate the oxirane protons. The percent conversion may then be calculated from the olefinic protons since the glycerol backbone of the fatty ester was used as the internal standard.36 The hydroxyl protons are difficult to assign due to the number of overlapping signals or trace amounts of water, and, in cases where the epoxide has opened, C−H peaks for the diols (3.2−4.1 ppm) are discernible from the upfield oxirane protons (2.8−3.2 ppm).37 We note that the NMR portion of the activity may be omitted without detracting too much from the instructional goals; however, our students appreciated the access to the instrument to analyze their own products.

the synthesis in a single laboratory period, and the transformation to the oxirane is quantitative according to IR and NMR spectroscopy, as discussed below. It is advisable to maintain rapid stirring, as the reaction tends to become biphasic as more hydrogen peroxide is added. In cases where the reaction became too hot and boiled, no significant product loss was observed, although this increases the chance of epoxide ring opening as a side reaction. Workup consisted of washing the oil with cold water to remove the excess peroxide and the acids. Ethyl acetate was added to facilitate product recovery during extraction due to the viscosity of the oil and the small scale of the reaction. This solvent was easily removed by simple distillation in our case and could be expedited using a rotary evaporator. On the basis of this procedure, students obtained 80−90% yield of crude product. No further purification was performed. Characterization

Product characterization using chemical tests and spectroscopy was performed by students individually. The bromine test is commonly used in qualitative tests for olefins, and recalls the formation of the halonium bridge, followed by nucleophilic attack on the more substituted carbon. This further highlights the formation and reactivity of three-membered rings such as epoxides. Performance of the bromine test yields a positive result for the soybean oil starting material, whereas the ESO product does not decolorize the bromine. Dropwise addition of the bromine reagent gives the most evident result. A substitute for this reagent that contains no chlorinated solvents and generates bromine in situ has been described, although we have not tried it in our laboratory.32 A qualitative test for epoxides using periodic acid is also performed,33 which reviews the oxidation of alkenes via cleavage of diols to carbonyl compounds. Parallels may be drawn for the periodic acid mechanism with KMnO4 and OsO4 oxidation of alkenes. This simple test may be completed within 15 min, and with only two drops of material, with the soybean oil exhibiting little or no precipitate for control. Trace yellow precipitates indicate the presence of AgBr, whereas trace white precipitates may be due to trace epoxides in the starting material. The ESO product gives a palpable white AgIO3 precipitate under this test. The periodite test indicates the presence of epoxides and 1,2-diols, warranting further characterization to elucidate if the epoxide is intact after the reaction, or if it opened to give the glycol. Fourier transform infrared (FT-IR) spectroscopy was performed to examine the functional groups present in the starting material and products, and representative student data are included in the Supporting Information. Students were able to identify the characteristic carbonyl peak at 1700 cm−1, as well as the Csp2−H and Csp3−H stretches, with the peaks above 3000 cm−1 disappearing in the product. The weak CC (1650 cm−1) absorption disappears after the reaction. The broad OH signal at 3300 cm−1 should be absent for both the starting material and product, indicating that the absence of the alcohol functional group. However, for most students, the OH peak is present, indicating some ring-opening of the epoxide or the presence of trace water. The C−H protons of the diol sideproduct may be identified in 1H NMR spectrum, as discussed below. Lastly, ESO has a characteristic peak in the fingerprint region from the C−O−C stretching of the oxirane ring (823 cm−1).34

Assessment

In the beginning of the course, students were introduced to the 12 principles of green chemistry,1 which provided them the necessary background to evaluate the synthesis. Thus, the epoxidation may be viewed from an atom economy standpoint, which students can calculate from the ratio of the molecular weight of the product and the molecular weight of all reactants.38,39 Ignoring the catalyst, the ESO synthesis described herein has a 90% atom economy, higher in comparison to MCPBA (51%). Students may also be quizzed on evaluating how green other epoxidation methods are, such as the halohydrin route, which is an additional alkene derivative, and less ideal. From the standpoint of chemical hazards, formic acid is not an ideal solvent based on life cycle assessment and environmental, health, and safety assessment methods.40 However, the amounts used of this chemical are minimized, accounting for 15% of the mass of the reaction, compared to standard batch chemistry that necessitates 50−80% of the mass. The use of concentrated hydrogen peroxide also poses a hazard, particularly since it is a strong oxidizer; however, this reagent has been used in instructional experiments published in this Journal.41−43 On the other hand, the byproduct of this reagent is water, which is benign. The high molecular weight of ESO, its nonvolatility, and its nonflammability make it a benign product for epoxidation experiments in the teaching lab. Soybean oil is obtained from renewable feedstocks. Although we did not obtain formic acid from a renewable source, it may be derived from biomass.44 Hydrogen peroxide has been recognized as a green oxidation reagent, minimizing the need for organic solvents and halides.45 Students also recognized that the ESO product may be biodegradable, although we disposed of it as chemical waste for incineration. The reaction was catalyzed by PTSA, which was removed after the reaction by washing with water. This catalyst further provided the advantage of minimizing energy consumption both during the reaction and by reducing the time for the reaction to be completed. Although the synthesis employs many green principles, students were tasked to look at the activity in its entirety to evaluate its adherence to green chemistry. For example, the use C




Table 2. Discussion Points To Engage Students in Green Chemistry Principles Green Chemistry Principle 1 2 3 4 5 6 7 8 9 10 11 12

Prevention Atom Economy Less Hazardous Chemical Syntheses Designing Safer Chemicals Safer Solvents and Auxiliaries Energy Efficiency Use of Renewable Feedstocks Reduce Derivatives Catalysis Degradation Real-time analysis for pollution prevention Inherently Safer Chemistry for Accident Prevention

Discussion Points Use of a more environmentally friendly oxidizing agent compared to MCPBA. The procedure has a higher atom economy than using MCPBA. Concentrated H2O2 is hazardous, yet its byproduct is water, which is benign. No chlorinated solvents are used in the synthesis, although halogenated solvents and reagents are used in the chemical tests. The epoxidized soybean oil product is nontoxic. Although formic acid is hazardous, a minimum amount of it is used both as a solvent and oxidant. Reaction was performed at moderate temperatures, 60−80 °C. Soybean oil is a plant-derived starting material. One-step formation of epoxide, as opposed to the halohydrin synthetic sequence. p-TSA and formic acid were used as a catalyst for the formation of a peroxyformic acid. Epoxidized soybean oil is biodegradable; degradation of the oxidant produces only water. Not used in this experiment, although hypothetically 1H NMR or IR can be used to follow the functional group transformation. The use of concentrated H2O2 is hazardous.

ORCID

of H2O2 as an environmentally friendly oxidizing agent may adhere to the principle of Prevention but is disadvantageous due to its inherent hazard. In critiquing the activity under a green chemistry framework, students gained an appreciation of the considerations of experimental design (Table 2). The chemical tests review the reactions of alkenes and epoxides, and many students recognized the use of halogenated reagents as a drawback. For their FT-IR results, students were able to recognize the functional groups as well as the characteristic fingerprint of the epoxide. The 1H NMR spectra presented more of a challenge, especially in calculating the percent conversion. Students were at first daunted by the number of signals but gained an appreciation of the technique once they were able to identify the protons from the different functional groups of the molecule.

Homar Barcena: 0000-0001-9572-0833 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for Jose Lenis, Aleksandr Gorbenko, and our organic chemistry students for their feedback in the development of this experiment.

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CONCLUSIONS The integration of green principles in academic practices reflects the growing interest of students and educators in green chemistry.2,46 In response to this demand, we report an adaptation of the transformation of soybean oil to ESO. The prevalence of epoxides in commercial products and in chemical group transformations offers an opportunity to engage students in green chemistry. Students successfully performed the epoxidation and characterized their products using qualitative tests and spectroscopy. More importantly, students learned to assess how procedures may adhere to some of the principles of green chemistry, while violating others.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00672. Student handout; detailed procedure; discussions on mechanism, qualitative analyses, spectroscopic analyses (PDF, DOCX) Instructor notes; laboratory preparation; expected student results (PDF, DOCX) 1 H NMR spectra; FT-IR spectra (PDF, DOCX)

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