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:http://center.acs.org/applications/greenchem/

http://www.ec.gc.ca/p2progress/2000-2001/en/sec2_3_2.cfm

1- Prevention

2 - Atom Economy

3 - Less Hazardous Chemical Syntheses

4 - Designing SaferChemicals

5 - Safer Solvents and Auxiliaries

6 - Design for Energy Efficiency

7 - Use of RenewableFeedstocks

8 - Reduce Derivatives

9 - Catalysis

10 - Designfor Degradation

11 - Real-time Analysis forPollution Prevention

12 - Inherently Safer ChemistryFor Accident Prevention

Source : http://www.chemistry.org/portal/a/c/s/1/general.html?DOC=greenchemistryinstitute\gc_principles.html

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 : http://www.epa.gov/greenchemistry/docs/general_fact_sheet.pdf

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 GenerationDetermining 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.?

• EfficientOffers 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: http://www.rand.org/publications/MR/MR1682/MR1682.ch2.pdf

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.

Concerning Pollutant Chemical Industries

A. Energy ProductionB. Petrochemical Manufacturing and Processing C. Pulp & Paper MillsD. Chemical Compounds ProductionE. Pesticides

L ess Hazardous :Scrutinize diff erent options to minimize all hazards.

Human Health :T he potential impact it might have, because of it’s large quantity.A lso important to consider the environmental impact.

E nvironment (G lobal and L ocal) :Determine the potential eff ects.

A lternative Solvents :Include supercritical fluids, aqueous applications, polymerized/ immobilized solvents,ionic liquids, solvent- less systems and reduced hazard organic solvents

C riteria to Select Solvents

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: http://www.encyclopedia4u.com/a/addition-of-hx-to-an-alkene.html

Example : Addition Reactions

Electrophile Source Product Comment

Hydrogen HalideHX

Alkyl HalideRX

H+ is electrophile

H2SO4 Alkyl hydrogen sulfate H+ is electrophile

H2O AlcoholTermed hydrationIn Mild Acid

H2 AlkaneTermed hydrogenationRequires palladium or platinum oxid

Mercuric AcetateAlkyl Mercuric Acetate

HgOCOOH

Converted to alcohol in presence of sodium borohydrate (NaBH4)

Halide(X2)

Alkyl dihalideIntermediate is halonium ion (RX+)

Industrial Addition Processes

http://xnet.rrc.mb.ca/martins/Organic%203/addition.htm

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: http://www.encyclopedia4u.com/n/nucleophilic-substitution-reaction.html

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: http://www.chemguide.co.uk/mechanisms/elim/elimvsubst.html#top

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 BioavailabilityModifying or eliminating certain properties that cause

toxic substances to be bioavailable.

• Design for Innocuous FateDesigning 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 PathwaysStep 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 :

http://www.epa.gov/greenchemistry/index.html

Canada's Green Chemistry Networkhttp://www.greenchemistry.ca/

Green Chemistry Magazinehttp://www.rsc.org/is/journals/current/green/

greenpub.htm

Other Referenceshttp://www.chemistry.org/portal/a/c/s/1/

acsdisplay.htmlDOC=greenchemistryinstitute\index.html