Presentation on theme: "CHAPTER 6 GREEN CHEMISTRY AND ATOM EFFICIENCY. Chapter Topics Definition of Green Chemistry. Basic Principles of Green Chemistry. Green Chemistry Methodologies."— Presentation transcript:
CHAPTER 6 GREEN CHEMISTRY AND ATOM EFFICIENCY
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.
What is Green Chemistry ? “ The design of chemical processes, products and technologies that reduces or eliminates the use and generation of hazardous substances” Sources:
1- Prevention 2 - Atom Economy 3 - Less Hazardous Chemical Syntheses 4 - Designing Safer Chemicals 5 - Safer Solvents and Auxiliaries 6 - Design for Energy Efficiency 7 - Use of Renewable Feedstocks 8 - Reduce Derivatives 9 - Catalysis 10 - Design for Degradation 11 - Real-time Analysis for Pollution Prevention 12 - Inherently Safer Chemistry For Accident Prevention Source :
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. Source :
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. Source : Green Engineering, Allen and Shonnard, p. 177
Brief Overview of Green Chemistry Methodologies - Alternative Feedstocks. - Green Solvents. - Synthesis Pathways. - Inherently Safer Chemistry. Source : Green Chemistry, pp. 178
Feedstock Selection Always keep in mind the material’s : - Persistence, Bioaccumulation and Toxicity. - Availability and Renewability. - Environmental Impact during Production (LCA – Life Cycle Management).
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.
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. Identifying Alternative Raw Materials in Order to Improve Environmental Performance (continued)
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.
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:
Pollutant Chemical Industries: Acid Catalysis and Partial Oxidation Acid catalysed reactions – liquid phase organic reactions. Problems – Reactions are catalysed by strong Brønstread (H 2 SO 4, HF) and soluble Lewis (AlCl 3, BF 3 ) 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.
Concerning Pollutant Chemical Industries A. Energy Production B. Petrochemical Manufacturing and Processing C. Pulp & Paper Mills D. Chemical Compounds Production E. Pesticides
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.
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. Alternative Reaction Pathway Selection (continued)
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 Source: Example : Addition Reactions
Electrophile SourceProduct Comment Hydrogen Halide HX Alkyl Halide RX H+ is electrophile H 2 SO 4 Alkyl hydrogen sulfateH+ is electrophile H2OH2OAlcohol Termed hydration In Mild Acid H2H2 Alkane Termed hydrogenation Requires palladium or platinum oxid Mercuric Acetate Alkyl Mercuric Acetate HgOCOOH Converted to alcohol in presence of sodium borohydrate (NaBH4) Halide (X 2 ) Alkyl dihalide Intermediate is halonium ion (RX+) Industrial Addition Processes
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. Source: Example : Substitution 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. Source: Example : Elimination Reactions
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”.
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. Functional Group Approach to Green Chemistry
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.
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.
References EPA’s Green Chemistry Program : Canada's Green Chemistry Network Green Chemistry Magazine pub.htm Other References