1. GREEN CHEMISTRY AND ATOM EFFICIENCY
CHAPTER 6
GREEN CHEMISTRY AND
ATOM EFFICIENCY

2. Chapter Topics Definition of Green Chemistry.
Basic Principles of Green Chemistry.
Green Chemistry Methodologies.
- Alternative Feedstocks.
- Green Solvents.
- Synthesis Pathways.
- Inherently Safer Chemistry.
Case Studies.
References.
Narration : the following topics will be addressed in this chapter.

3. What is Green Chemistry ?
“The design of chemical processes, products and technologies that reduces or eliminates the use and generation of hazardous substances”
Narration : Green chemistry serves as a link between chemical processes, products and technologies design and the related environmental and health impacts. It also emphasizes a shift in focus from R&D (research and development) to more innovative and environmentally sound chemical products, processes and technologies. Reduction of energy and raw material use are both examples of pollution prevention methods green chemistry encourages.
Sources:

4. 12 principles 7 - Use of Renewable Feedstocks 2 - Atom Economy
1- Prevention
7 - Use of Renewable
Feedstocks
2 - Atom Economy
8 - Reduce Derivatives
3 - Less Hazardous
Chemical Syntheses
9 - Catalysis
12 principles
4 - Designing Safer
Chemicals
10 - Design
for Degradation
Narration : These 12 principles serve as a basis for green chemistry (according to the American Chemical Society Green Engineering Institute (website)). The topics found in bold will be covered in this chapter. The other topics are covered in various other chapters.
1. Prevention : it is more environmentally and economically sound to prevent waste rather then treat and/or clean it. 2. Atom Economy : Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product (defn on website). defn of atom economy atom economy : ratio of molecular weight of the starting materials and reagents and the molecular weight of the target molecule, provides a measure of the intrinsic efficiency of the transformation. (green engineering textbook) 3. Less Hazardous Chemical Syntheses Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.4. Designing Safer Chemicals Chemical products should be designed to effect their desired function while minimizing their toxicity (persistence and bioaccumulation?!?!).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. As well, their production and bioavailability should be minimized. 6. Design for Energy Efficiency Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. (If possible, synthetic methods should be conducted at ambient temperature and pressure to minimize impact...) 7. Use of Renewable Feedstocks A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable. If this is not possible, the next best option is obviously to attempt to find a recyclable and/or innocuous feedstock. 8. Reduce Derivatives Unnecessary derivatization (use of blocking groups, protection/ deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste. 9. Catalysis Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. 10. Design for Degradation Emphasis should be made on the production of chemicals so that they degrade as rapidly as possible after their use (to minimize environmental persistence) 11. Real-time analysis for Pollution Prevention Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.12. Inherently Safer Chemistry for Accident Prevention Substances and the form of a substance (functional group?) used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.
5 - Safer Solvents
and Auxiliaries
11 - Real-time Analysis for
Pollution Prevention
6 - Design for
Energy Efficiency
12 - Inherently Safer Chemistry
For Accident Prevention
Source :

5. Green Chemistry
The focus area of the EPA’s Green Chemistry Program considers :
- The use of alternative synthetic pathways
- The use of alternative reaction conditions
- The design of safer chemicals that are, for example, less toxic than current alternatives or inherently safer with regard to accident potential.
Narration : In the EPA’s green chemistry program, the focus area includes the following points.
Source :

6. An Ideal Chemical Reaction:
Is Simple.
Is Safe.
Has a High Yield and Selectivity.
Is Energy Efficient.
Uses Renewable and Recyclable Reagents and Raw Materials.
Narration : There are a wide range of things that need to be considered when designing a chemical reaction, including alternative raw materials, solvents, reaction pathways and reaction conditions (which will be discussed further in this chapter). The design of a chemical reaction can become quite tedious, specially when attempting to minimize or eliminate it’s environmental impact. The given criteria (on the slide) for an ideal chemical reaction should all be considered, but it can become extremely difficult to match all points, therefore the goal is to optimize the balance of the criteria. In this chapter, we will discuss how to best tackle this task.
Source : Green Engineering, Allen and Shonnard, p. 177

7. Brief Overview of Green Chemistry Methodologies
- Alternative Feedstocks.
- Green Solvents.
- Synthesis Pathways.
- Inherently Safer Chemistry.
Narration : In this section, we will briefly discuss the following topics. The information given will serve only as qualitative or semi-quantitative information as this topic is quite extensive and the design of chemical products, processes and technologies requires a lot of expertise.
Source : Green Chemistry, pp. 178

8. Feedstock Selection Always keep in mind the material’s :
- Persistence, Bioaccumulation and Toxicity.
- Availability and Renewability.
- Environmental Impact during Production (LCA – Life Cycle Management).
Narration : When selecting a material or alternative materials for feedstocks, it is important to keep in mind these aspects of the substance. These are not the main criteria upon which the selection is based, but it is important to consider these criterias because of their role in determining the substance’s effect on the environment. The last point is related to life cycle analysis.

9. Identifying Alternative Raw Materials in Order to Improve Environmental Performance
Innocuous
Determining the hazards associated with the substance (using previously discussed methods) as well as alternative pathways if a hazardous material needs to be used.
Narration : These are basic guidelines to assist in the selection of feedstocks. (continued on following slide)

10. Minimizing Waste Generation
Identifying Alternative Raw Materials in Order to Improve Environmental Performance (continued)
Minimizing Waste Generation
Determining the quantity of waste produced by the given material and alternatives. Also important to consider the type of waste and its impact.
Selective
Does the selectivity of the substance minimize environmental impacts in separation, etc.?
Efficient
Offers many benefits... Not only based on yield and selectivity. Also consider the atom economy.
Narration : the following slide will discuss one approach and it’s steps to select appropriate feedstocks.

11. Selection of Feedstocks: Basic Guidelines
In surveying the field, it is useful to employ a taxonomy of methods that develop NGETs. To that end, we use the seven areas of green chemistry, a taxonomy that has been laid out to help describe green chemistry research:
A. Use of alternative feedstocks that are both renewable and less toxic to human health and to the environment.
B. Use of innocuous reagents that are inherently less hazardous and are catalytic.
C. Employment of natural processes—biosynthesis, biocatalysis, and biotech-based chemical transformations for both efficiency and selectivity.
Narration : These are basic guidelines to assist in the selection of feedstocks.
For instance, when considering what feedstocks to use in generating a particular compound, the green chemist will explore renewable feedstocks whenever practical. There is no guarantee that such renewable feedstocks are possible for a given reaction nor, if they are, that they will provide a net environmental benefit. Nonetheless, the principles of green chemistry provide a set of design criteria and goals that can help improve the environmental performance of new products and processes.
NGET : Next Generation Environmental Technologies.

12. G. Minimization of energy consumption.
D. Use of alternative solvents that reduce potential harm to the environment and serve as alternatives to currently used volatile organic solvents, chlorinated solvents, and other hazardous chemicals.
E. Safer chemical design—with principles of toxicology to minimize intrinsic hazards while maintaining needed functionality.
F. Development of alternative reaction conditions that increase selectivity and enable easier separations.
G. Minimization of energy consumption.
Source:

13. Pollutant Chemical Industries: Acid Catalysis and Partial Oxidation
Acid catalysed reactions – liquid phase organic reactions.
Problems – Reactions are catalysed by strong Brønstread (H2SO4, HF) and soluble Lewis (AlCl3, BF3) that are difficult to separate from the organic product and lead to large volumes of hazardous waste.
Alternative: using heterogeneous catalysis.
Partial Oxidation of organic molecules.
Problems – manufacturing methods include toxic and corrosive chemicals. Ex. processes based on cobalt- acetic acid- bromide, or using Cr(VI) and Mn(VII). They produce large volumes of an organic acid and toxic metal waste.
Alternative: less toxic catalytic agents.

14. Concerning Pollutant Chemical Industries
A. Energy Production
B. Petrochemical Manufacturing and Processing
C. Pulp & Paper Mills
D. Chemical Compounds Production
E. Pesticides

15. Narration : These are the main topics that need to be considered when selecting solvents. These criteria can also be applied to catalysts and other materials. Less Hazardous solvents are now relatively widely available, because of the high potential for accidents with the materials previously, new solvents have been developed to have a low risk, but it is still important to find the least hazardous solvent. Human Health is an important criteria because solvents are present in large quantities, therefore accidents with high exposure are likely. Also, solvents tend to have high vapor pressure and therefore this could result in significant exposure. Not only should the toxicity to humans be measured, but also the impact a solvent might have on the environment. Some solvents, specifically halogenated solvents, have been deemed potentially carcinogenic. Environment, both global and local, are also affected by solvents. On a global level, many solvents (including CFC’s) have proved to pose global warming potentials. On a local level, the use of VOC’s (volatile organic compounds) has raised concerns regarding rising air pollution levels. Alternative Solvents offer more environmentally sound options to the traditional solvents.

16. Alternative Reaction Pathway Selection
Addition ( A + B  AB)
No waste needs to be treated because the reaction is direct.
Substitution (AB + C  AC + B)
Necessarily generates stoichiometric quantities of substances as byproducts and waste that are not part of the target molecule.
Narration : This is a brief qualitative description of different equations that can be used to determine alternative reaction pathways. In order to assess the equations more, a semi-quantitative approach can be taken, using atom and math efficiency. These techniques and other tools will be discussed further along in the module. Also, an example of each possible alternative reaction pathway is given in the following slides.

17. Alternative Reaction Pathway Selection (continued)
Elimination (AB  A + B)
Does not require other substances, but does generate stoichiometric quantities of waste that are not part of the final target molecule.
Narration : the following slides contain an example of these reaction pathways.

18. Example : Addition Reactions
The addition of HX to an alkene is an organic reaction in chemistry where HX, or a halogen sigma bonded to a hydrogen atom, adds to the carbon-carbon double bond of an alkene following Markovnikov's rule
(Markovnikov's rule is observed).
The general chemical formula of the reaction is as follows:
C=C + HX H-C-C-X
Narration: Addition reactions incorporate the starting materials into the final product and, therefore, do not produce waste that needs to be treated, disposed of, or otherwise dealt with.
Source:

19. Alkyl hydrogen sulfate Alkyl Mercuric Acetate HgOCOOH
Industrial Addition Processes
Electrophile Source
Product
Comment
Hydrogen Halide HX
Alkyl Halide RX
H+ is electrophile
H2SO4
Alkyl hydrogen sulfate
H2O
Alcohol
Termed hydration In Mild Acid H2
Alkane
Termed hydrogenation Requires palladium or platinum oxid
Mercuric Acetate
Alkyl Mercuric Acetate HgOCOOH
Converted to alcohol in presence of sodium borohydrate (NaBH4)
Halide (X2)
Alkyl dihalide
Intermediate is halonium ion (RX+)
Narration: Electrophilic addition reactions are used in many industrial synthetic processes. Alkenes are widely used "raw materials" for industrial synthesis.
Source:

20. Example : Substitution Reactions
In chemistry, Nucleophilic Substitution is a type of chemical reaction in which one nucleophile (electron donor) replaces another as a covalent substituent of some atom. In the examples given here, the nucleophilic atom is carbon.
An example of nucleophilic substitution is the hydrolysis of an alkyl bromide, R-Br, under alkaline conditions, where the "attacking" nucleophile is hydroxide ion, OH-:
R-Br + OH R-OH + Br-
The bromide ion, Br-, is said to be the leaving group.
Narration: Substitution reactions, necessarily generate stoichiometric quantities of substances as byproducts and waste.
Source:

21. Example : Elimination Reactions
Halogenoalkanes also undergo Elimination Reactions in the presence of sodium or potassium hydroxide.
                                                        
The 2-bromopropane has reacted to give an alkene - propene.
Notice that a hydrogen atom has been removed from one of the end carbon atoms together with the bromine from the centre one. In all simple elimination reactions the things being removed are on adjacent carbon atoms, and a double bond is set up between those carbons.
Narration: Elimination reactions do not require input of materials during the course of the reaction other than initial input of a starting materials, but they do generate stoichiometric quantities of substances that are not part of the final target molecule.
Source:

22. Functional Group Approach to Green Chemistry
Structure Activity Relationship
Used to determine a potential structural modification that may improve the substance’s safety.
Elimination of Toxic Functional Groups
Substances in the same functional group tend to have the same toxicity. If it is possible, eliminate any substances from a given group, or mask the toxic substance’s property rendering it “safe”.
Narration : These are some of the functional group approaches to green chemistry (qualitatively described). Structure Activity Relationships : This is a powerful tool (as seen in chapters 4 and 5) and in this case can help improve a chemical’s safety even if the mechanism of action is not known. Elimination of Toxic Functional Groups : If there is no data proving the toxicity of a given substance, it is possible to assume that it has the same toxic properties as other compounds in the same functional group. This is even more correct if there is data proving that other substances in the functional group are toxic. Often it is impossible to eliminate a functional group because it is the group that possesses properties required in the process, however, it may be possible to mask the toxic properties of the necessary substance in order to eliminate it’s toxicity.

23. Reduction of Bioavailability
Functional Group Approach to Green Chemistry
Reduction of Bioavailability
Modifying or eliminating certain properties that cause toxic substances to be bioavailable.
Design for Innocuous Fate
Designing substances to ensure they degrade after their useful life.
Narration : Reducing Bioavailability : If it is impossible to change the properties that cause a substance to be toxic it may be possible to change the properties that render the substance bioavailable. Designing Substances for Innocuous Fate : In the past, substances were made to last as long as possible, but now it is important to design or use substances that can degrade rapidly after their use in order to minimise persistence as well as potential harmful effects on the environment.

24. Quantitative/Optimization-Based Frameworks for the Design of Green Chemical Synthesis Pathways
Step 1 : select a set of molecular or functional group building blocks from which a target molecule can be constructed.
Step 2 : identify a series of stoichiometric, thermodynamic, economic and other constraints that might occur.
Step 3 : a set of criteria can be used to identify reaction pathways that deserve further examination.
Narration : Many of the previously discussed methods need very specific knowledge and research. This slide presents the outline of a more simple combinatory method. The first step is detailed on the next slide. Ranking the different materials can be very difficult, but is usually based on economical and environmental performances.

25. Step 1 : Construction of Alternative Chemical Pathways
Selection of functional group building blocks include the groups :
Present in the product.
Present in any existing industrial raw materials, co -products or by-products.
Which provide the basic building blocks for the functionalities of the product or of similar functionalities.
- Select sets of groups associated with the general chemical pathway employed (cyclic, acyclic or aromatic).
- Reject groups that violate property restrictions.
Narration : As outlined in the previous slide, the first step to Quantifying and/or Optimizing Frameworks for the Design of Green Chemical Synthesis Pathways is to select a set of functional group building blocks. The present list demonstrates criteria that encourages the widest possible selection for potential materials.

26. References EPA’s Green Chemistry Program :
Canada's Green Chemistry Network
Green Chemistry Magazine
Other References