Green Chemistry Principle #10: Design for Degradation

By Shira Joudan, Chair of the Education Subcommittee for the GCI

10. Design for degradation: Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.

In video #10, Matt and I discuss designing chemicals that break down once their desired function is completed. Essentially, we want chemicals to degrade to molecules that are not harmful to humans, animals or the environment.

A lot of the chemicals we use in our day-to-day lives need to be stable to perform their function. For example, if your coffee mug dissolved when you poured your coffee into it, it wouldn’t be very helpful! Similarly, if lubricants degraded under high temperature and pressure, they may not work well in the engines of our cars or planes.

Once chemicals are done providing their main function, they might end up in a landfill or wastewater treatment plant where they can enter the waters, soil and air of our environment, or be taken up by animals or humans. The biggest challenge is making chemicals that are stable during usage, but don’t persist in the environment – or in other words, chemicals that can be degraded. Another important thing – we want the breakdown products to also be non-toxic and not persistent! It’s important to remember that there are different reasons a chemical can break down. It can be due to reactions with light (photodegradation), water (hydrolysis) or biological species, often with enzymes (biodegradation).

A common example that we hear about is biodegradation, especially with the well-known “biodegradable soaps.” We can use this as a good example about how we can design soaps, or detergents, to break down more easily in the environment.

Sodium dodecylbenzenesulfonate

Figure 1 Sodium dodecylbenzenesulfonate, an example of a linear alkylbenzene sulfonate (LAS) which is biodegradable.

Sodium dodecylbenzenesulfonate (Figure 1) is a common detergent, and is often referred to as LAS, for linear alkylbenzene sulfonates. Looking at its structure, you can see that it has a linear alkyl chain with a benzylsulfonate attached to it. It is useful as a detergent because it has a polar headgroup (sulfonate) and a non-polar alkyl group, making it a surfactant.

LAS is used in many things, especially laundry detergent. It degrades quickly in the environment under aerobic conditions, or when oxygen is present, because microbes are able to use to the linear alkyl chain as energy, via a process called β-oxidation, a process which breaks down the carbon chain. Once the long chain is degraded, the rest of the molecule can be degraded as well.

Branched alkylbenzene sulfonate.

Figure 2 A branched alkylbenezene sulfonate (does not biodegrade).

If you compare LAS to a branched version (Figure 2), you can immediately see that the alkyl chain looks very different. This molecule was also used as a detergent just like the linear version, but because of the location of the branches, microbes cannot perform β-oxidation since there are no good sites for that reaction to be initiated. Therefore, these branched detergents have been phased out in most developed countries because they are too persistent – they do not biodegrade.

The main way these molecules are degraded is through microbes, when oxygen is present. So if these soaps end up directly in water, like straight into a lake, they will not be broken down very quickly (even the linear version!). This is because there are fewer microbes in water as compared to in soil. Interestingly, the branched version is 4 times less toxic than the linear version, but can cause more damage because of its persistence. This is one of the reasons that it is very important to consider persistence, or a molecule’s resistance against degradation, and not only its toxicity.

You can see how designing chemicals to break down can be very challenging, but many researchers around the world are working on this right now. Some examples are biodegradable polymers that are used in plastics, like compostable cutlery.

Principle 10 is currently one of the largest challenges in green chemistry. If scientist designing new chemicals understand more about the mechanisms that can degrade them, we may be able to make chemicals that are reliable and stable during their intended use, but break down in the environment!

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ACS Summer School on Green Chemistry and Sustainable Energy 2017

ACS Summer School on Green Chemistry and Sustainable Energy 2017

By Samantha Smith, Yuchan Dong, and Shira Joudan

Yuchan Dong, who previously studied in China, had begun to miss life with roommates while in Canada. She reminisced about how you could talk about your lives late into the night, and spend meals chatting with friends in the cafeteria. “Luckily, at the ACS summer school, [she] got the chance to experience such life again and got to know a lot people who share same interests.” The summer school brought us back to the more carefree times of our undergraduate lives. Living in dormitories, sharing a floor with fifty-two other highly educated students, sharing every meal with our newly-formed friends, and even tackling homework assignments were just like the “good old days”. The level of diversity strengthened the value of peer-networking and real friendships were made throughout the week.

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The week wasn’t just filled with relaxing chats in the Colorado sun; that was merely how we spent our free time. The days were jam-packed with riveting lectures during the day, assignments in the evening, and getting to know the local Golden beers at night (which was obviously a duty of ours as tourists). We also had the chance to take in the local scenery with hikes and whitewater rafting.

The ACS summer school on green chemistry is a competitive program offered to graduate students, post-doctoral fellows, and industry members every year in Golden, Colorado. Hosted by the Colorado School of Mines, the program consists of five days of lectures from green chemistry and sustainable energy experts, two poster sessions, a whitewater rafting trip, and lots of opportunity for networking. This program teaches global sustainability challenges with a focus on sustainable energy. The ACS Summer School is free of charge for successful attendees, including travel, accommodation on campus, and meals.

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Samantha, Yuchan, and Shira at the ACS Summer School

Jim Hutchison, a professor at the University of Oregon, spoke about how his department has completely reformatted their undergraduate chemistry curriculum to contain green and sustainable chemistry, something that particularly sparked Shira’s interest as lead of GCI’s Education Subcommittee. Bill Tolman, Chair of the University of Minnesota Chemistry Department, shared how students successfully cultivated the safety culture within his department. This had inspired Samantha to create new initiatives within our chemistry department. Queens University’s Professor Philip Jessop taught us about Life-Cycle Analysis (LCA) and assigned us multiple processes for which we calculated the gate-to-gate LCA. Mary Kirchhoff and David Constable from ACS gave talks on green chemistry and ACS resources, many of which would be useful to other departments. The format of the summer school allowed plenty of time to chat with the guest lecturers during coffee breaks, lunches, and poster sessions.

Many real-world issues were discussed. The worldwide energy usage and sources of energy were a main topic of discussion, as was the use of alternative sources. We were blown away by how multi-disciplinary green chemistry is, and we were enlightened on how we need experts in all fields to successfully create sustainable chemistry. We learned that to be able to effectively tackle environmental issues we need great synthetic chemists, whether they specialize in organic, materials or catalysis, as well as analytical chemists, engineers, environmental chemists, and toxicologists. We also need effective entrepreneurs and lobbyists.

Nearing the end of the summer school, a large group of us hiked up Tabletop mountain to get the most amazing view of the valley. A warm feeling of appreciation towards the summer school for bringing us out of the isolation of individual research in the busy city life was shared. We would like to thank ACS for giving us the chance to attend this amazing week. This experience has truly been beneficial to us, and we plan to use the knowledge gained during the week in our own studies as well as pass this knowledge on to our coworkers at the University of Toronto.

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Tabletop mountain in Golden, CO

We highly encourage anyone interested in green chemistry and sustainability to attend this beneficial program. Application deadlines are early in the year and submitted online. The application consists of the applicant’s CV, unofficial transcript, letter of nomination from faculty advisor or another faculty member, and a one-page essay describing your interest in green chemistry and sustainability as well as how it will benefit the applicant.

Challenges in Designing Non-Toxic Molecules: Using medicinal chemistry frameworks to help design non-toxic commercial chemicals

By Shira Joudan, Education Committee Coordinator for the GCI

Throughout the past 20 years, there have been numerous reports on the state of the science of designing non-toxic molecules, including three in this year alone.1–3 The idea of safe chemicals has been around for much longer than the green chemistry movement, however it is an important pillar in what it means for a chemical to be green. In fact, many scientists agree that the synthesis of safer chemicals is likely the least developed area of Green Chemistry, with lots of room for improvement.2 For more information, see our post and video on Green Chemistry Principle #4.

One expert on designing non-toxic molecules is Stephen C. DeVishira-blog-picto of the United States Environmental Protection Agency (US EPA). In a recent paper DeVito highlights some major challenges creating safer molecules, and discusses how we can approach this challenge.1 We require a societal change about how we think of toxicity, and this shift must begin with specific education.

How can we agree upon definition of a “safe” chemical?

We need to decide and agree upon parameters that deem a molecule safe, or non-toxic. Generally, most chemists agree that an ideal chemical will have no (or minimal) toxicity to humans or other species in the environment. It should also not bioaccumulate or biomagnify in food chains, meaning it should not build up in biota, or increase in concentration with increased trophic levels in a food chain. After its desired usage, an ideal chemical should break down to innocuous substances in the environment. Potency and efficacy are also important, as well as the “greenness” of its synthesis. Setting quantitative thresholds to these parameters and enforcing them is the largest challenge.

How do we tackle the over 90,000 current use chemicals?

Although not all of these chemicals are actually in use, they are all registered under the US EPA’s Toxic Substances Control Act (TCSA), which contains both toxic and non-toxic chemicals. Many chemicals that are being used should be replaced with safer alternatives, but there are so many that it seems terrifying to know where to begin. Another replacement option is designing new technologies that don’t require the function that these chemicals provide. About two-thirds of the chemicals registered in TCSA or Environment and Climate Change Canada’s Chemicals Management Plan were in use before registration was required. Unlike pharmaceuticals and pesticides which are heavily regulated by Health Canada, commercial chemicals do not require stringent toxicity tests. But things are changing in the US and in Canada. For example, Canada has just listed 1550 priority chemicals that will be addressed by 2020. When considering replacement for chemicals of concern, the most common barrier to reducing the use is currently “no known substitutes or alternative technologies”.

How do we ensure sufficient training on the concepts of safer chemical design?

Many people making new chemicals are unfamiliar with green chemistry and basic toxicology principles. Without the proper toolbox of knowledge designing safer chemicals is challenging. [The Green Chemistry Commitment is a great place to start!] DeVito discusses the need for “toxicological chemists” which would be analogous to medicinal chemists, but instead produce non-toxic commercial chemicals. Medicinal chemists have the training to design appropriate pharmaceuticals, however commercial chemicals do not receive the same attention in terms of designing safe and efficacious products. Since humans are exposed to the commercial chemicals as well, often in intimate settings, the same attention to detail should be used during the synthetic process in order to produce safe chemicals.

Synthetic organic chemists are the ones designing the new chemicals, and we can no longer keep traditional chemists and toxicologists an arm’s length apart. Instead, there is a need for a new type of scientist that considers the function of the chemical for its desired usage and its toxicity potential to humans and the environment. Similar to the training of medicinal chemists, these chemists should receive training in biochemistry, pharmacology and toxicology, and also in environmental fate processes. DeVito suggests adding topics into an undergraduate curriculum, some of which are highlighted here:

  • Limit bioavailability: A common way to prevent toxicity has been to reduce the bioavailability of molecules. Essentially, the idea is that if the chemical cannot be absorbed into the bloodstream of humans or other species, it will not be able to cause significant toxic effects. A common predictor for bioavailability is the “Rule of 5”, where a molecule will have poor absorption if it contains more than five hydrogen bond donors or 10 hydrogen bond acceptors, a molecular weight of more than 500 amu, and a logP (or log Kow) of greater than 5.4 More sophisticated prediction methods also exist based on linear free energy relationships. A good example of low bioavailability is the artificial sweetener sucralose, where only 15% of the chemical is absorbed through the gastrointestinal tract into the bloodstream.5
  • Isosteric substitutions of molecular substituents: By removing parts of the molecule and replacing it with another functional group with similar physical and chemical properties (isosteric) toxicity can be reduced. This is common in medicinal chemistry, where it is referred to as bioisosterism, and is used to reduce toxicity, alter bioavailability and metabolism. A simple substitution can be replacing a hydrogen atom for a fluorine atom, but there can also be much larger isosteric substitutions.
  • Designing for degradation: A toxic molecule that persists in the environment can lead to global long term exposure. Understanding common environmental breakdown mechanisms can allow us to design molecules that will break down to innocuous products after their desired usage. A good starting point is understanding aerobic microbial degradation, since most of our waste ends up at a wastewater treatment plant. An important thing to keep in mind is that if a non-toxic molecule degrades to a toxic molecule, the starting material will still be of concern.

Toxicity is complicated. The best way to arm the next generation of chemists with the skills needed to design smart, safe chemicals is to tailor the undergraduate education to our new goals.

Numerous institutions, including the University of Toronto, are working towards this by signing onto the Green Chemistry Commitment!

(1)         DeVito, S. C. On the design of safer chemicals: a path forward. Green Chem. 2016, 18 (16), 4332–4347.

(2)         Coish, P.; Brooks, B. W.; Gallagher, E. P.; Kavanagh, T. J.; Voutchkova-Kostal, A.; Zimmerman, J. B.; Anastas, P. T. Current Status and Future Challenges in Molecular Design for Reduced Hazard. ACS Sustain. Chem. Eng. 2016, 4, 5900–5906.

(3)         Jackson, W. R.; Campi, E. M.; Hearn, M. T. W.; Collins, T. J.; Voutchkova-Kostal, A. M.; Kostal, J.; Connors, K. A.; Brooks, B. W.; Anastas, P. T.; Zimmerman, J. B.; et al. Closing Pandora’s box: chemical products should be designed to preserve efficacy of function while reducing toxicity. Green Chem. 2016, 18 (15), 4140–4144.

(4)         Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Experimental and computational approaches to estimate solubility and permeability in drug discovery and development setting. Adv. Drug Deliv. Rev. 2001, 46, 3–26.

(5)         Roberts, A.; Renwick, A. G.; Sims, J.; Snodin, D. J. Sucralose metabolism and pharmacokinetics in man. Food Chem. Toxicol. 2000, 38, 31–41.

UofT Demonstrates its Commitment to Sustainable Chemistry

“We’re very pleased and proud to announce that the Chemistry Department has recently joined the Green Chemistry Commitment (GCC)!” – Dr. Andy Dicks, University of Toronto, Associate Professor

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GCI Members Fall 2016

The University of Toronto has recently signed the GCC making us the first school outside of the United States to sign onto this impactful commitment, which now contains 33 colleges and universities. The GCC is overseen by Beyond Benign, a United States not-for-profit organization created by Dr. Amy Cannon and Dr. John Warner, a founder of the principles of green chemistry. Within the GCC, academic institutions collaborate to share resources and know-how in order to positively impact how the next generation of scientists are educated about sustainability issues. Participating departments commit to green chemistry instruction as a core teaching mandate. The aim is to provide undergraduates and graduates with the required understanding to make green chemistry become standard practice in laboratories around the world. This, in turn, ensures that when graduates of the university enter the workforce, they are armed with the knowledge of how to make molecules and processes more sustainable and less toxic by adhering to the Twelve Principles of Green Chemistry.

The GCC unites the green chemistry community around shared goals and a common vision to grow departmental resources to allow a facile integration of green chemistry into the undergraduate laboratories as well as to improve connections with industry which creates job opportunities for sustainability-minded graduates. Their website offers many resources for those interested in reading actual case studies and laboratory exercises, so please click here to visit their website and be informed!

Our chemistry department has already improved the green chemistry content in our undergraduate laboratories by updating the first year courses and upper year synthetic chemistry courses to include various graded questions about the Twelve Principles as well as ensuring the undergraduates are thinking about how they could make their current lab protocols more sustainable. Additionally, students can choose to study the fate of chemicals in our environmental chemistry courses offered. Of course there’s always room to improve, so the Green Chemistry Initiative (GCI), in collaboration with Dr. Andy Dicks, is working on evaluating the undergraduate chemistry curriculum’s current focus on sustainable chemistry and toxicology, in hopes to further improve our undergraduate’s learning experience. The GCI also provides many educational opportunities to department members such as our Seminar Series as well as many outreach opportunities, making our group a driving force in the integration of green chemistry principles to the department. Lastly, the University of Toronto chemistry courses reach thousands of students a year, and by being the first Canadian university to sign this commitment, we are working towards a greener future in Canada!

Thank you for celebrating this very momentous achievement with us!
Karl Demmans, Ian Mallov, Shira Joudan, and Laura Reyes