Tracking the Carbon Footprint in Drug Design-- Medical, Environmental, Social Implications
In this activity, students conduct a lab exercise over three lab sessions by taking a small sample of a pharmaceutical compound, slightly modifying its chemical structure, purifying the modified product sample and analyzing it for yield, purity and identity. "Big ideas" include the implementation of stoichiometric calculations in a real-world global context, and the formulation of thoughts regarding whether the benefits of such medical pursuits outweigh the global burden of carbon footprint and medical waste generation.
During the course of the experiment, students also keep track of the amount of each reagent used. Assessment of student learning is achieved through two assignments: (1) Submission of lab notebook pages, including calculations for percent yield, moles of carbon disposed of as waste for the entire process, and the liters of carbon dioxide potentially produced as a result of such waste; and (2) A reflection paper based upon lab data and literature sources, both peer-reviewed and from mass media, in which students discuss the pros and cons of generating such carbon footprints for the purposes of medical research and development
"Big Idea" 1
Implementation of stoichiometric calculations in a real-world global context. Here, students (after collecting experimental data and keeping track of quantities of chemicals used) will convert these mass and volume quantities to moles, then to total moles of carbon used, moles of carbon in the final product, and moles of carbon discarded immediately as waste. From these values, the volume of carbon dioxide generated from this waste (assuming an ideal gas and that this waste will be incinerated and therefore combusted) can be determined.
"Big Idea" 2Benefits of medical/pharmaceutical pursuits versus burden of carbon and medical waste generated by such pursuits. Following the lab exercise, each student writes a reflection paper (at least 3 pp. long) where thoughts and opinions on this big idea are addressed. As part of this paper, students conduct a literature search for at least one peer-reviewed article and one article from the general media, cite ideas from these articles in their paper, and discuss the persuasiveness of these articles.
Context for Use
For GOB students, the lab portion of the activity is conducted in pairs over three - three hour lab periods, to allow time to become acquainted with new lab concepts and techniques, as well as to provide enough drying time for the product prior to analysis. For science majors, the lab portion of the activity is conducted individually over two lab periods, as these students already will have had experience with these lab concepts and techniques by the time of this activity. In both cases, the activity is intended to be conducted late in the term.
Description and Teaching Materials
Experimental ProceduresProcedures for the methylation of chlorpromazine (product: mCPZ, chlorpromazine methiodide), as well as product purification and analysis, are modified from Brunauer et al. (2007) and associated online supplemental material. Briefly, a 250-mg sample of chlorpromazine hydrochloride (CPZ-HCl) dissolved in 2.5 mL deionized water is alkalized with dropwise additions of 1.0 M NaOH to a pH of about 10, producing chlorpromazine free base (CPZ). CPZ is extracted three times with 2-mL portions of diethyl ether, then dried with sodium sulfate (keeping the sample in the dark). The liquid then is transferred carefully to a clean vial and dried under a gentle air stream. Sample is stored cold, desiccated and in the dark until the next lab session. Alternatively, if time allows students may proceed forward in the procedure until the next point of storage.
The sample, dissolved in 1.5 mL ether, then is treated with two 12.5-microliter portions of methyl iodide (iodomethane), with a 15-minute benchtop incubation period (in the dark) following each addition. The sample then is washed three times via tabletop centrifugation, using 1.5-3.5 mL ether for each wash. The pellet is dried under a gentle stream of air to remove residual ether, then stored cold, desiccated and in the dark until the next lab session.
The sample then is analyzed for percent yield based upon methyl iodide quantity; Fourier-Transform infrared spectroscopy (FTIR) compared to CPZ-HCl; melting point compared to CPZ-HCl; and thin-layer chromatography in 3.4:1:0.2 (v/v) ethyl acetate:methanol:NH3 compared to CPZ-HCl. Students (after collecting experimental data and keeping track of quantities of chemicals used) will convert these mass and volume quantities to moles, then to total moles of carbon used, moles of carbon in the final product, and moles of carbon discarded immediately as waste. From these values, the volume of carbon dioxide generated from this waste (assuming an ideal gas under standard temperature and pressure conditions, and that this waste will be incinerated and therefore combusted) can be determined.
Student flow diagrams for lab procedures, as well as student instructions for the reflection paper, are shown below.
Teaching Notes and Tips
To facilitate the searching of peer-reviewed references, it may be beneficial to enlist the campus library to provide a brief lecture to the class about conducting such literature searches. Even if students are familiar with the process of literature searching, the peer-reviewed literature on sustainability issues is relatively small in comparison to that of many other subjects and as such may require further guidance.
Safety Issues. See Brunauer et al. (2007) and associated online supplemental material. Briefly, safety goggles must be worn throughout the experiment, and appropriate gloves must be worn and changed frequently during the experiment, particularly when handling methyl iodide. Steps involving volatile reagents should be performed in a fume hood. Avoid inhalation, ingestion and skin contact with mCPZ, CPZ-HCl and CPZ since they are bioactive. Chlorpromazine is a potent antipsychotic agent (Brunauer, 2007); however, so long as students observe standard precautions when handling toxic substances, there should be no need for protective respiratory equipment (though instructors may consider having a supply of surgical masks on hand for concerned students). Avoid direct contact with methyl iodide since it is a potent methylating agent even though used in very small quantities here. It is recommended that instructors pre-measure quantities of CPZ-HCl and methyl iodide for student use (at least roughly) so as to minimize student exposure. Since ether is extremely flammable, the experiment should NOT be performed in the presence of open flame or other laboratory ignition sources. Solid waste (including gloves) should be collected in waste containers in the hood. And prior to starting the experiment each student should construct a chemical table containing names, formulas, relevant physical constants and important hazard information for all chemicals to be used, all of which information can be acquired online.
Other Lab-Related Notes. Chlorpromazine is light-sensitive, thus instructors should make sure that drug samples are kept sequestered from light as much as possible (such as with amber vials, aluminum foil or inverted cardboard boxes). Based on the stated quantities of reagents used, and with methyl iodide as the limiting reactant, the theoretical yield would be 0.185 g of mCPZ produced. Students with a reasonable percent yield should have more than enough product to complete all of the analysis tasks on the final day of the lab procedure. However, students with poor yield may find themselves too low on product to complete the full analysis as described. To enhance yield, the instructor may have the students increase the total added quantity of methyl iodide from 25 to about 35 microliters. However, to reduce the possibility of multiple methylation, methyl iodide always should be the limiting reactant.
Regarding FTIR analysis, the major difference between spectra for CPZ-HCl and mCPZ occurs at 2500 cm-1. There, a small peak is observed for CPZ-HCl but not for mCPZ, indicating the presence of a tertiary amine (Brunauer et al., 2007; Silverstein et al., 1981).
Regarding melting point, mCPZ melts around 148-149°C (Brunauer et al., 2007). Meanwhile CPZ-HCl melts (or decomposes) at a temperature at least 30 degrees higher (Budaveri, 1996).
Regarding thin-layer chromatography, mCPZ has a much lower Rf value than CPZ-HCl, since the presence of ammonia will convert CPZ-HCl to the neutral free-base form while leaving mCPZ intact and cationic (Brunauer et al., 2007).
Reflections. Based on the quantities of reagents used in this exercise, the calculated predictions of carbon dioxide generated by waste incineration per setup can range from roughly 21 L (taking into account that the ether used for extraction was evaporated rather than collected, and that the synthesized product in theory would not be disposed of as chemical waste) to roughly 27 L (assuming that ALL sources of carbon in the lab procedure would be collected as waste and incinerated). Thus far this activity has been attempted by roughly 30 GOB students, and in assessing these initial outcomes, students have offered a variety of thoughts on this CO2 value. Some have viewed this as a relatively small amount of CO2 expended when compared to the amount of CO2 being exhaled by each person on a daily basis. Others compared this expenditure to the much larger amount of CO2 that is being generated through everyday fuel consumption, leading to more general thoughts on consumerism, daily habits and carbon footprint. Still others utilized the 21-27 L calculated value to discuss the impact of having this quantity multiplied by the number of students, the number of academic chemistry labs and the number of industrial chemistry labs, leading to a very significant carbon footprint being generated by the field as a whole. Many of these students looked to the ether washes and the TLC solvent of this experiment as possible places where solvent usage could be reduced, leading to discussion about the impact of reduced solvent use on product purity, as well as some discussion that perhaps the usage of a little more organic material (and disposable gloves) during the learning of lab technique may pay off in the long run with the greater dexterity and confidence that would allow for a much greater carbon savings in the future, while at the same time providing a SAFER lab experience now, an issue certainly that cannot be discounted.
A few students did some research into alternative methods of waste disposal that claim to generate reusable by-products while reducing CO2 emissions. And a few others delved more thoroughly into the environmental justice perspective as it pertains to medical research and health care, some offering rather interesting assertive stances as to whether the pursuit of increased life expectancy and quality of life by individuals is worth the legacy of chemical and medical waste being left for others, present and future, to handle.
References and Resources
Budaveri S, Ed. (1996). The Merck Index (12th ed.). Merck Research Laboratories, Whitehouse Station, NJ.
Sheetz MP, Singer SJ (1974). Proc. Natl. Acad. Sci. U.S.A. 71, 4457-4461.
Silverstein RM, Bassler GC, Morrill TC (1981). Spectrometric Identification of Organic Compounds (4th ed.). John Wiley & Sons, New York.