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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Green Chemistry Principle #8: Reduce Derivatives

By Trevor Janes, Member-at-Large for the GCI

8. Unnecessary derivatization (e.g. installation/removal of use protecting groups) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.

In Video #8, Cynthia and Devon look at one common example of derivatization, which is the use of protecting groups in chemical reactions. To help illustrate the concept of a protecting group, they use toy building blocks.

In this blog post, I will use cartoons such as the one shown below (a specific example of the use of protecting groups will be shown at the end of this post).

Principle 8 - unselective reaction

Figure 1 An unselective reaction.

In Figure 1, the starting material contains two reactive sites, represented by U-shaped slots. We only want the slot on the right to react with the reagent, shown as red circles. The starting material is reacted with the reagent in order to make the desired product, but an undesired product also forms, because both U-shaped slots react with the red circle. In other words, Figure 1 shows an unselective reaction because a mixture of products is made.

Formation of the undesired product can be avoided by carrying out a protection reaction before using the red reagent, and then carrying out a final deprotection reaction. This sequence of reactions is shown in Figure 2.

Principle 8 - selectivity through protecting groups

Figure 2 A selective reaction through the use of a protecting group, which temporarily blocks the reactive site on the left side. 

 

Figure 2 shows how a selective reaction is traditionally done – through the use of a temporary block, known as a protecting group. The starting material can be protected by blocking one of the reactive sites, represented by the blue rectangle covering the U-shaped slot on the left. This intermediate only has one reactive site left, so the second reaction with the red reagent can only happen at the empty U-shaped slot on the right. To get the same desired product as in Figure 1, the third and final deprotection step is carried out, which removes the protecting group.

Principle 8 - waste from protecting groups

Figure 3 The waste created by all three reactions in Figure 2.

Even though the product from Figure 2 is the desired product, we had to do three reactions to only make one change, which is inefficient. Also, each step generates waste products (shown underneath each reaction arrow in the above cartoon) , which are depicted in Figure 3.

Protecting groups are a useful tool that chemists use to make the molecules, because we often need to carry out selective reactions on a molecule that has multiple of the same reactive sites. However, as we have talked about here, they are also inefficient and wasteful.

An active area of research is the development of more selective reactions, which eliminate the need to use protecting groups altogether.[1] Selective reactions use slight differences in a molecule’s chemistry to make a reaction happen at only the desired reactive site. This is very similar to the installation of the protecting group in Figure 2.

As more and more highly selective reactions are discovered, our syntheses can be made more efficient by reducing the number of steps required and the amount of waste produced. Looking ahead, protecting groups will be less and less necessary – and that’s a good thing!

 

Appendix – Example from Real Chemistry

A simple, specific example of the use of protecting groups[2] is shown below. Both oxygen-containing sites are reactive, but we only want the one on the left side to react in this case. The first reaction is the installation of the protecting group, (CH3)3SiCl, on the OH oxygen only, protecting the right side. The second reaction shows the reagent, CH3CH2CH2MgBr (for those curious, this is called a Grignard Reagent), which now reacts with just the ketone C=O site on the left, adding the desired new CH3CH2CH2 segment. The last step shows a combination of removing the protecting group to return the OH group, and also removing the [MgBr] segment of the reagent with the help of acid (shown as H3O+), which leaves the desired product with a CH3CH2CH2 chain added only on one side of the molecule.

Principle 8 - real protecting group use in chemistry

This example of a selective reaction uses a protecting group, but this requires 3 steps to only make 1 change. Instead, we can eliminate the need for protecting groups by designing new and more selective reactions that are much more efficient.

References:

[1] I. S. Young and P. S. Baran, Nature Chem. 2009, 1, 193

[2] R. J. Ouellette and J. D. Rawn, in Organic Chemistry, 2014, Elsevier, Boston pp 491-534.

Green Chemistry Principle #7: Use of Renewable Feedstocks

By Trevor Janes, Member-at-Large for the GCI

7. A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.

In Video #7, Yuchan and Ian help us understand what a raw material or feedstock is, and why we need to choose feedstocks which are renewable.

They use CO2 as an example of a feedstock which plants convert into sugar via photosynthesis. We humans use this sugar as our own feedstock for many different delicious things, including cookies! Yuchan and Ian explain that for a feedstock to be renewable, it must be able to be replenished on a human timescale, whereas depleting feedstocks take much longer to be replenished, and are being used up at a faster rate by human activity.

Many common feedstocks are depleting, such as petroleum and natural gas. The petrochemical industry uses petroleum and natural gas as feedstocks to make intermediates, which are later converted to final products that people use, such as plastics, paints, pharmaceuticals, and many others.

An example of a renewable feedstock is biomass, which refers to any material derived from living organisms, usually plants. In contrast to depleting feedstocks like petroleum, we can much more easily grow new plants once we use them up, and maintain a continuous supply. If we can use bio-based chemicals to do the same tasks that we currently accomplish using petrochemicals, we move closer to the goal of having a steady, reliable supply of resources for the future.

Existing chemical technology has developed based on using readily available petroleum as feedstock to make a majority of chemicals and end products. However, the chemical technology that enables conversion from biomass into bio-based chemicals into final products people use is not yet as well developed.1 Chemical scientists with various specializations are currently involved in improving our ability to use biomass.2, 3

So, how can we implement the principle of renewable feedstocks on a day-to-day basis? Yuchan and Ian illustrate principle 7 through their choice of solvent. As we explore in the video for principle #5, we choose a solvent for a particular purpose based on properties such as boiling point, polarity, and overall impact on health and the environment. One more aspect to consider is that we can choose to use a solvent based on is its renewability. Tetrahydrofuran (THF) is a useful ether solvent, but it is synthesized industrially from petrochemicals (see below for synthesis), so it isn’t renewable. A close relative of THF is 2-methyl THF. Its structure and properties are very similar to those of THF, but the difference is that 2-methyl THF can be synthesized from bio-based chemicals which are made from renewable feedstocks. So when we substitute 2-methyl THF in for THF, we are putting principle 7 into action.

Synthesis of THF4 vs. synthesis of 2-methyl THF5

screen-shot-2016-10-25-at-10-35-31-pm

The synthesis of THF.

An early step in the industrial production of THF involves reaction of formaldehyde with acetylene to make 2-butyne-1,4-diol. This intermediate is hydrogenated and cyclised in two more steps to yield THF. The acetylene input is derived from fossil fuels, which again are non-renewable.

screen-shot-2016-10-25-at-10-35-48-pm

The synthesis of 2-methyl THF.

An alternative to THF is 2-methyltetrahydrofuran, which has a very similar structure to THF.  It can be synthesized starting from biomass; after conversion to C5 and C6 sugars and subsequent acid-catalyzed steps, the intermediate levulinic acid can be hydrogenated to yield 2-methyl THF.

References:

  1. “Renewable Feedstocks for the Production of Chemicals” Bozell, J. J. ACS Fuels Preprints 1999, 44 (2), 204-209.
  2. “Conversion of Biomass into Chemicals over Metal Catalysts” Besson, M.; Gallezot, P.; Pinel, C. Chem. Rev. 2014, 114 (3), 1827-1870.
  3. “Transformation of Biomass into Commodity Chemicals Using Enzymes or Cells” Straathof, A. J. J. Chem. Rev., 2014, 114 (3), 1871-1908.
  4. “Tetrahydrofuran” Müller, H. in Ullmann’s Encyclopedia of Industrial Chemistry 2002, 36, 47-54.Wiley-VCH, Weinheim. doi:10.1002/14356007.a26_221
  5. “Synthesis of 2-Methyl Tetrahydrofuran from Various Lignocellulosic Feedstocks: Sustainability Assessment via LCA” Khoo, H. H.; Wong, L. L.; Tan, J.; Isoni, V.; Sharratt, P. Resour. Conserv. Recy. 2015, 95, 174.

Green Chemistry Principle #5: Safer Solvents and Auxiliaries

By Laura Reyes, Co-Chair for the GCI

5. Safer Solvents and Auxiliaries: The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.

The 5th principle of green chemistry promotes the use of Safer Solvents and Auxiliaries. This includes any substances that do not directly contribute to the structure of the reaction product but are still necessary for the chemical reaction or process to occur. In the video for Principle #5, we talk about the impact of solvent waste and illustrate it by substituting dichloromethane, a commonly-used solvent, with a safer alternative.

Solvents are the most common example of auxiliary substances. Usually, solvents themselves do not react with the reagents but are still necessary in reactions in order to dissolve reagents, mix all reaction components, and control the temperature of the reaction. After the reaction, more solvents are then often used to separate and purify the product from other reaction components and any side-products.

This reliance on solvents means that a massive amount of solvent waste is generated during a typical chemical reaction. Reducing solvent use is therefore usually a high priority for chemists working on making their reactions greener, especially when working on an industrial scale.

For example, Pfizer was able to reduce the amount of solvent waste generated in its synthesis of Viagra from 1300 kilograms to just 22 kilograms for every kilogram of Viagra made.[1] This huge reduction, and others like it throughout the chemical industry, ends up making a big difference in the resulting environmental impact and demand on resources.

Although reducing solvent amounts altogether is certainly important, it’s also good to remember that every solvent has its own properties. A toxic and environmentally persistent solvent like dichloromethane should be avoided whenever possible. Many guides have been created to help chemists replace solvents of concern, such as these guides by Pfizer, GlaxoSmithKline, and Sanofi. A recent paper also compiled these guides into a more comprehensive overview.

In our video, we used column chromatography to show an example of solvent substitution in action. Column chromatography is a separation method commonly used by chemists. It works very well for separations but, like many other solvent-based separation methods, the downside is that a large amount of solvent is required.

Columns

We separated compounds found in spinach extract using column chromatography, to show how solvents can be substituted for safer choices.

We used a convenient guide for substituting dichloromethane in our column chromatography demo.[2] Using this guide, we replaced the dichloromethane/ethyl acetate (95:5) mixture in Column 1 with its alternative of heptane/isopropanol (85:15) mixture in Column 2. We then compared the separation of compounds in spinach extract between the two columns.

Column chromatography is a complicated process with a lot of factors to consider, so we had to simplify it for the purpose of the video. This YouTube video explains the basics very well for those who want to learn more. Essentially, the compounds that flow through the column are passed between the solid phase (the silica gel in the column) and the liquid phase (the solvent) at different rates, which causes them to separate as they travel downwards. The solvent choice greatly influences how well compounds separate. Although no two solvents work exactly the same way, substitution guides like the one we used in our video have already done the tedious work to help chemists choose the greenest option that will work for their reactions.

Considering the integral use of solvents throughout chemistry, the implementation of Principle #5 in even seemingly small ways can end up drastically reducing the amount of solvents used altogether and move towards safer options whenever solvents are required.

References:

[1] Pfizer’s reduction of waste in Viagra production: P. J. Dunn, et al., Green Chem. 2004, 6, 43-48.

[2] Dichloromethane use, concerns, and substitution in column chromatography: J. P. Taygerly, et al., Green Chem. 2012, 14, 3020-3025.

Solvent selection guides:

Pfizer: K. Alfonsi, et al., Green Chem. 2008, 10, 31-36.

GlaxoSmithKline: R. K. Henderson, et al., Green Chem. 2011, 13, 854-862.

Sanofi: D. Prat, et al., Org. Process Res. Dev. 2013, 17 (12), 1517-1525.

Compilation of guides: D. Prat, et al.Green Chem. 201416, 4546-4551.

Chemical Waste FAQs

By Peter Mirtchev, Member-at-Large for the GCI and Laura Reyes, Co-Chair for the GCI

All chemists create chemical waste, it’s simply part of our job. Recently, we started a Waste Awareness Campaign to track the amount of waste being generated by our chemistry department. Aside from this, the chemical waste disposal process was a bit of a mystery. Learning how to properly sort, label, and dispose of chemical waste should be part of every chemists’ early training, but typically gets overlooked. In academic research labs, waste disposal habits tend to get passed down from one person to the next, and often stem from tradition rather than regulation. With this post, we hope to clarify some of the confusion surrounding proper disposal of different types of chemical waste.

We recently co-hosted a seminar about waste disposal with the Chemistry Students’ Union. Our speakers were Ken Greaves (Chemistry Department Supplies & Services Supervisor) and Rob Provost (Environmental Protection Manager at the UofT EH&S Office). We found that many members of the department had important questions regarding proper disposal practices and what happens to chemical waste after it is picked up. We have summarized the crucial points of the talk in Q&A format below. There’s also very useful information at this EH&S website on chemical waste. If you have other unanswered questions regarding chemical waste, leave them in the comments and we’ll get them answered for you!

Disclaimer: the information below is specific to the Department of Chemistry at the University of Toronto, and may change according to institution. Check with your own institution regarding the rules of chemical waste disposal.

Q: What are the most common types of chemical waste produced in a research laboratory?

A: Solvents. At the University of Toronto, the three most common chemicals are 1) acetone, 2) hexanes, and 3) dichloromethane. The Department of Chemistry purchases over 10,000L of acetone per year alone.

Q: What is considered ‘flammable’ waste?

A: A flammable liquid is one that has a flashpoint of 23.8°C or lower.

Q: Is a mixture of water and organic solvents considered aqueous or flammable waste?

A: If the mixture is more than 50% water, it is considered aqueous waste. If it is less than 50% water, it is considered flammable waste. If the amounts are uncertain, treat as aqueous (see below for more about the treatment of waste types).

Q: What mixtures are treated as chlorinated waste? Continue reading

Waste Awareness Campaign

By Karl Demmans, Workshop Coordinator for the GCI

Welcome back to the GCI’s monthly updates about our recent endeavors and findings in green research! Today I’d like to discuss the efforts put forth by various faculty and graduate students over the past eight months to collect and distribute data about the amount of waste produced in the Lash Miller Chemical Laboratories, the main building for the Department of Chemistry at UofT. Our hope is that once students are presented with this information, they will be more conscious about their chemical procedures and consider alternate green methods to help reduce waste. For an example of the types of data we have collected, take a look at the poster found below.

Waste Poster GCI

Waste data for Lash Miller Chemical Laboratories (Department of Chemistry, University of Toronto).

In Lash Miller, waste collection occurs every Friday. Each research group gathers their labelled waste containers and brings them to  our waste department for sorting and temporary storage, before eventual disposal.  The rather colourful chart in the poster displays the amount of solid or solvent waste, broken down for each chemistry discipline, concluding with the percent of total waste each discipline produces, as well as the type of waste that is made.

The colour gradient of the five waste categories denotes the combined environmental and economic concerns ranging from solid decontaminated waste (‘best’) to acidic waste (worst). Overall, the waste picture for Lash Miller looks pretty good, with only 9% of the total waste produced in the building coming from the two red categories (acidic and chlorinated). By specifically targeting these types of waste for reduction, we can continue to improve and make the waste profile of our department even better.

For Lash Miller graduate students, if you’d like to know specifically how much waste your group is producing, send an inquiry e-mail to green [at] chem.utoronto.ca!

Lastly, the final part of the poster describes what each type of waste is, and explains the disposal process. The topic of how chemical waste is disposed of was recently discussed during our last GCI seminar. Click here to read our Chemical Waste FAQs!

In the upcoming months there will be another poster displaying the percent reductions in waste produced per discipline, to see how graduate students react to the current information. Thanks for stopping by to learn about our Waste Awareness Campaign and how we are helping to reduce our environmental footprint.