Green Chemistry

Overview

"Such prosperity as we have known it up to the present is the consequence of rapidly spending the planet's irreplaceable capital"

Aldous Huxley (1894 -1963)

By promoting life-changing concepts for these students to practice, it becomes important to first understand the past, and what chemical synthesis has created and what it has developed into today. The most important objective of all synthesis is to gain a new product, one that is more desirable to your lifestyle and the culture that you live in. To understand the history of synthesis, students will be engaged in a learning of the ancient world and how some early metallurgy evolved into the first aspects of chemical synthesis. This early history will show how ancient people first developed the use of smelting naturally occurring ores for use in making better tools, and military instruments, as well as in creating dyes for cloth and glazes for pottery that help make pottery stronger. All of these ancient devices were the first aspects of using chemical synthesis although in a haphazard way as there was no periodic table or knowledge of what objects could blend well together.

It is due to this first accidental aspect of chemical synthesis that we have arrived in a world filled with environmental problems that we have today. These cultures only knew what materials would mix with each other to make new substances; they were not aware of what waste products they were also creating in the process, or if they were, it was not a potential problem as they still acquired the substance that was initially sought after. This created generations only looking at what they were able to create without regard to what was left behind. This internal philosophy of creating new substances with disregard has continued through the modern day. The remainder of ancient history will look at the Greek and Roman cultures, and the rise of alchemy as the predecessor to modern chemistry. ⁵

The late eighteenth and early nineteenth centuries mark a point on the chemistry timeline where students can fundamentally see what chemical synthesis is. With the creation of the periodic table and an understanding of chemical properties came about the science of stoichiometry which shows two or more reactants being combined through chemical means to create one or more products. Once one knows what they want to create, they can work backwards to understand what objects in nature they can obtain for the creation of the new substance. Through a thorough analysis of stoichiometry, students will understand the different chemical processes and how to balance an equation out. Through the use of the understanding of the measurement of a mole, students will be able to predict how much of different elements are needed to generate whatever new substance they desire. This will be beneficial in the ultimate understanding of Green Chemistry.

The 20 ^{t h} Century marks a point in history essential to understanding the environmental effects that have occurred due to past chemical processes. The advent of chemical warfare through the first and second world wars as well as the explosive use of chemical pesticides demonstrates the need for a solution. The original solution to the problem was to get rid of the extremely toxic chemicals. This was brought on by the environmental movement that Rachel Carson and Silent Spring had a lot of help in creating and increasing the public's awareness of the problem of chemicals in the environment. Although written in an apocalyptic style, it had the result of helping to formulate new legislation for the chemical industry, thus helping the environment overall. The impact of the environmental movement did not, however, change the practice of all of the chemical industry. Many companies continued their practices of creating as much product as possible without care or conviction to the amount of by-product or waste that was also resultant from their chemical practices. This ultimately ends in the eventual learning and use of the principles of green chemistry as demonstrated in Paul Anastas' book, Green Chemistry Theory and Practice. Once one understands the principles and practices of this new approach to chemistry it becomes evident how the chemical industry should embrace this form of chemical practice as a way of not only helping the environment, but also at increasing their profitability in the market place.

Green Chemistry: A Basic Understanding

"Pollution is nothing but the resources we are not harvesting. We allow them to disperse because we've been ignorant to their value"

R. Buckminster Fuller (1895 - 1983)

Green chemistry is a new approach to an old science. It is taking what we already know about chemistry and how to form new substances and redesigning that method to be both more economical and also better for the environment by preventing pollution and containment problems. It is essentially finding a better approach for this method. Paul Anastas writes that in 1994 there were 2.26 billion pounds of the approximately 300+ hazardous substances released into the environment. Of that most of these hazardous substances come directly from the chemical industry and not the end user of a final product. ⁶ Most end-user or consumer products have already been reduced of their potentially high levels of toxicity to the environment through the legislative acts of the environmental movement. Synthetic chemists play a central role in developing green chemical methods for pollution prevention and as a result the end-user benefits from products being made in ways to better help the environment. ⁷ Through public awareness of this problem and solution to the problem, the environment and chemical practice around the world will adapt to this new approach

What remains still to this day are a large number of waste products generated by the chemical industry as a result of low yield synthesis in their production of obtainable marketable products. ⁸ A lot of this comes from the use of petroleum, and petroleum constituents, as the base feed stock (the main additive component of any chemical reaction) for many of these procedures. ⁹ This low yield results in a large number of waste products not intended for resale and, thus, are dumped or burned resulting in potential ecological disasters. Due to modern awareness of the damaging effects humans have on the environment, strategies have been implemented to streamline this new approach to chemistry so that it might be embraced by an industry slow to change. This is a modern day scientific revolution unfolding that will forever change human practice of a generation of new synthetic compounds. ^{1 0} A list of the twelve principles of green chemistry for the industry is found in the appendix.

The proper execution of these principles will result in a newly established approach to chemical synthesis. To understand the twelve principles in full, please see teacher appendix for the list of principles. So what does this list mean in layman's terms and how can these principles be applied into education. The latter of this statement will be derived in the section under strategies. Since this is such a new science, this section shall be devoted to a basic understanding of the knowledge so as to apply it properly in a classroom situation.

If we look at a common chemical synthesis, we can break down the differences between traditional chemistry and that of green chemistry. The equation for photosynthesis is 6 CO ₂ + 6 H ₂O → C ₆H _{1 2}O ₆ + 6 O ₂ with the understanding that light energy is required to start the reaction causing the oxidation of the water molecules and reduction of carbon dioxide. When one looks at this equation, it can be understood that there are two reactants being combined (carbon dioxide and water) and reformulated to become two products (glucose and molecular oxygen). The beauty of the equation though is that there is essentially no waste. This is not because the plant needs the oxygen that is released; instead the oxygen is being utilized by animals and, thus, is reclaimed as opposed to becoming waste. The glucose that is created (C ₆H _{1 2}O ₆ ) and maintained in the plant is essential for animal survival as it is used in cellular respiration. The oxygen that is generated becomes atmospheric oxygen which is also used by the animal kingdom. The result of this is a true green reaction in that there are no waste by-products being generated, as the waste oxygen has been reclaimed elsewhere. The resulting products of cellular respiration (carbon dioxide and water) are the reactants needed for the photosynthesis side of the equation to begin again, making the process a continuous cycle. This is the major goal and aim of green chemistry in creating a sustainable system with minimal waste and ideally no waste.

Unfortunately chemistry for the most part does not create new products so easily. Instead what normally happens is the creation of the product with a lot of matter that is not needed and, therefore, wastes. To illustrate this idea, the pharmaceutical company represents a great opportunity. This industry is driven by the profitability of the end result product, not the amount of waste generated during the process. Often the product is in ratio to its waste anywhere in order of magnitude of 1 part product to 25 (and up to 100) parts of waste by-product. This is to say that for every 1 gram of useful pharmaceutical product being utilized by a consumer there is anywhere from 25 to 100 grams of waste being discarded by the pharmaceutical company. Ibuprofen, which has been around since 1969, is a great example to illustrate how the chemical industry had synthesized a product one way unchanged for many years, producing massive amounts of waste. Yet when they focused on another route, they were able to synthesize the same product with much less waste. To illustrate the idea, we will disseminate through the history of ibuprofen through its original synthesis and the modern synthesis being used today. ^{1 1}

The Boots Company Synthesis of Ibuprofen

Ibuprofen is the active ingredient in a number of brand name products including Advil, Motrin and Nuprin. Ibuprofen acts as an analgesic (pain reliever) and is also

effective as a Non-Steroidal Anti-Inflammatory Drug. The world production of ibuprofen exceeds 30 million pounds per year. The Boots Company PLC of Nottingham,

England first patented the synthesis of ibuprofen in the 1960's (U.S. Patent 3,385,886) which served as the main method of synthesis for many years. The Boot's synthesis of ibuprofen is a six-step synthesis with much waste being generated. When this process was first created and patented it was the usefulness of the end product that made it worthwhile without worry to how much waste was generated (this is similar to many chemical process and routines, making it an essential understanding of Green Chemistry). If you look at table 1, you can see the characteristics in each step of the process. For further clarification of this process, please go to teacher appendix that includes a link to see the visual model for use as an aid during class instruction. ^{1 2}

Atoms of each reagent that are incorporated into the final desired product (ibuprofen) are shown in center column and those that end up in unwanted products are shown in the last column to right. Table 1 illustrates the atom economy of the Boots Company synthesis and allows one to calculate an atom economy of 40% (atom economy will be further defined under the heading strategies). As was indicated above, about 30 million pounds of ibuprofen are manufactured on a yearly basis. If the entire world's supply of ibuprofen were manufactured by the Boots process, then this would generate about 35 million pounds of waste! ^{1 3}

TABLE 1 Atom Economy of the Boots Company Synthesis of Ibuprofen

% Atom Economy = (FW of atoms utilized/FW of all reactants) X 100
= (206/514.5) X 100 = 40%

The BHC Company Synthesis of Ibuprofen

In the eighties, ibuprofen was approved for over-the-counter use and the Boots Company patent expired. Recognizing the financial opportunities that the manufacture and sales of this drug could offer, several companies embarked upon setting up facilities and developing new methods for the preparation of ibuprofen. Due to competition in the marketplace to make a cheaper product, it was pertinent to develop a more stream-lined approach to synthesizing ibuprofen. The Hoechst Celanese Corporation discovered a new three-step synthesis of ibuprofen. Together with the Boots Company, they formed the BHC Company to prepare (by this new synthesis) and market ibuprofen. The BHC Company synthesis is show below in Table 2 with the utilized atoms in center column and the unutilized atoms in the final column. The atom economy is further illustrated in Table 2 and calculation of the % atom economy gives 77%, a significant improvement over the 40% of the original process. ^{1 4} This new process not only stream-lined the synthesis by reducing the number of steps, it also greatly reduced the amount of waste being generated. This is the key to Green Chemistry in that the identical final substance was created, yet it took less energy, and produced less waste making the entire process both more profitable to the company but also more beneficial to the environment.

Table 2 Atom Economy of the BHC Company Synthesis of Ibuprofen

% Atom Economy = (FW of atoms utilized/FW of all reactants) X 100
= (206/266) X 100 = 77%

The atom economy of the BHC Company process jumps to ~99% if one considers that the acetic acid generated in Step 1 is recovered and reused.

Not only does the BHC Company process offer a dramatic improvement in the atom economy, it offers other environmental advantages. These include a three-step catalytic process vs. the six-step Boots Company process that requires auxiliary reagents in stoichiometric amounts. For example, the first step in each process yields the same product from the same reactants. However, the Boots Company process utilizes aluminum trichloride in stoichiometric amounts while the BHC Company process uses HF in catalytic amounts that is recovered and reused repeatedly. The aluminum trichloride produces large amounts of aluminum trichloride hydrate as a waste product which is generally land-filled. The nickel and palladium catalysts used in Steps 2 and 3 of the BHC Company process are also recovered and reused. ^{1 5}

Because the BHC Company process is only three steps (vs. six steps for the Boots Company process) and it has a much improved atom economy, it not only results in a dramatic decrease in the waste produced, it also allows for a greater profit margin (more ibuprofen in less time and with less equipment). These factors translate into economic benefits for the company as a result of the fact that less money is required to deal with the waste that is generated and less capital expenditure is required to produce the same amount of ibuprofen. Thus, not only does the environment benefit, but the company's bottom line is strengthened and good public relations can be reaped as a result of a greener process. ^{1 6}

One should recognize that you do not have to be an expert in chemistry, or understand each phase of the production to see the importance and relevance of the two separate processes. Instead, it is important to recognize the number of step and be able to see the waste products produced in each step vs. the amount of actual desired product by the end of the set of reactions. The ideas encompassed in the illustration of ibuprofen show the main principle behind the green chemistry movement. This is based on the principle of the atom economy. This concept states that reactions for environmental and economic reasons should be designed for efficiency. The atom efficiency looks at all of the products that are generated and bases their value on what products are actually desired vs. the total of all chemicals used in the process. A simpler example for illustration is that of the production of propene which will be further discussed in the section on strategies (a detailed understanding of propene manufacture can be found using a URL found in the teacher appendix). Once this is understood the process behind the synthesis of ibuprofen becomes clearer. The products that are not desired are considered waste; this is what the chemical industry for generations has created a lot of just to get a small yield of the desired product. By looking at alternative feedstock, catalysts, and solvents for use in chemical reactions, chemists are finding new ways to generate the same products with less waste associated with it.

For a better understanding of green chemistry, it is suggested that you read the book Green Chemistry: Theory and Practice by Paul Anastas and John Warner. This book is written in easy to understand terms for people who are not specialized in chemistry yet want to understand the process and principles behind green chemistry.

Prerequisite Knowledge

"The world can only be grasped by action not by contemplation. The hand is the cutting edge of the mind"

Jacob Bronowski (1908-1974)

In order for this unit to be a success and beneficial for students, it must be understood that a certain amount of prerequisite knowledge must be obtained either before the start of this unit or simultaneously with this unit. With that understood, this section outlines the basic understandings students should be aware of and be able to practice concepts of before beginning. Otherwise the information on green chemistry is only information, and not practical knowledge that can then be put into practice by your students.

Students should have a firm understanding of the basic units of an atom and how they operate. They should know that the electron (e-) has minimal mass, but most of the volume of an atom and has a negative electrical charge. They should also know that the proton (p+) is in the nucleus with a positive charge and coupled with the neutron (n) of neutral charge they make up the majority of the mass of the atom but very little of the volume. Students should be able to use and utilize the periodic table, understanding the difference between the vertical groups or families and how they have similar chemical properties to the horizontal periods that have increasing number of electrons in their valence shell or energy level. Students should be most familiar with elements 1 through 20 and the basic properties of said atoms. They should understand the difference between atomic number and the atomic mass of an element. With said knowledge, they should be able to determine with speed and efficiency the number of e-, p+, and n of any given atom. Through this information, students should also be presented with the identification and understanding of isotopes and how certain atoms can have more or less n found in the nucleus, thus changing the properties of the atom. From this basic understanding of isotopes, students should be presented with radioactive decay in order to understand how certain elements behave in nature by decaying to other chemical forms.

With this basic information, students should be given time to understand the principles behind the valence shell of an atom and the maximum number of 8 e- to be found in that area. With that information, students can harvest information about both covalent bonding (the sharing of e-) and ionic bonding (the giving up or taking of extra e- with the understanding of the names of the cation and anion). With students gaining the ability to understand the bonding of atoms, they are able to solidify that information through chemical equations and the ideas based on stoichiometry. Stoichiometry is the branch of chemistry that deals with the application of the laws of definite proportions and of the conservation of mass and energy to chemical activity. This process should also have a component of the understanding of what a mole is and how it can be used in obtaining information about chemical formulations and the amount of materials used in the equation as compared to the amount of materials that result from the synthesis. Students should understand the types of chemical reactions and formations that can take place such as:

Synthesis is the reaction of 2 or more elements to form a compound.

General formula A + B → C

Example

4 Fe + 3 O ₂ → 2 Fe ₂O ₃

Decomposition is the breaking down of a large molecule into its elements or into smaller substances.

General formula C → A + B

Example

2 KClO ₃ → 2 KCl + 3 O ₂

Single replacement is when 1 element reacts with a compound to form a new compound.

General formula x + yA → xA + y

Example

Cu + 2 AgNO ₃ → Cu(NO ₃) ₂ + 2 Ag

Double replacement is when 2 compounds react to produce 2 new compounds. The cation of one compound replaces the cation in another compound.

General formula xA + yB → yA + xB

Example

Ca(OH) ₂ + H ₂SO ₄ → 2 HOH + CaSO ₄ (Ca ^{2 +} replaces 2H ⁺) note: HOH = H ₂O

Combustion is usually the burning of hydrocarbons in the presence of oxygen. One product is carbon dioxide when there is an excess amount of oxygen and the other is water.

Example

CH ₄ + 2 O ₂ → CO ₂ + 2 H ₂O

This constitutes the basic principles of middle school and high school chemistry that need to be understood for students to coherently understand the principles of green chemistry and, thus, make a conviction to them to practice the principles they learn. Without this prior knowledge being obtained, students will not be able to fully understand the severity of the environmental effects current chemical practices have on the environment. If in need of any further understanding of these basic principles listed here, please look at appendix A for teacher resources.