Transcript Slide 1

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
7 - Use of Renewable
Feedstocks
2 - Atom Economy
8 - Reduce Derivatives
3 - Less Hazardous
Chemical Syntheses
9 - Catalysis
4 - Designing Safer
Chemicals
10 - Design
for Degradation
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 : 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:
•
•
•
•
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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.
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.
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 cobaltacetic 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
Criteria to Select Solvents
Less Hazardous :
Scrutinize different options to minimize all hazards.
Human Health :
The potential impact it might have, because of it’s large quantity.
Also important to consider the environmental impact.
Environment (Global and Local) :
Determine the potential effects.
Alternative Solvents :
Include supercritical fluids, aqueous applications, polymerized/immobilized solvents,
ionic liquids, solvent-less systems and reduced hazard organic 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.
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.
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
Source: http://www.encyclopedia4u.com/a/addition-of-hx-to-an-alkene.html
Industrial Addition Processes
Electrophile Source
Product
Hydrogen Halide
HX
Alkyl Halide
RX
H+ is electrophile
H2SO4
Alkyl hydrogen sulfate
H+ is electrophile
H2 O
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
http://xnet.rrc.mb.ca/martins/Organic%203/addition.htm
Comment
Intermediate is
halonium ion (RX+)
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.
Source: http://www.encyclopedia4u.com/n/nucleophilic-substitution-reaction.html
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.
Source: http://www.chemguide.co.uk/mechanisms/elim/elimvsubst.html#top
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”.
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.
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 :
http://www.epa.gov/greenchemistry/index.html
Canada's Green Chemistry Network
http://www.greenchemistry.ca/
Green Chemistry Magazine
http://www.rsc.org/is/journals/current/green/green
pub.htm
Other References
http://www.chemistry.org/portal/a/c/s/1/acsdisplay
.htmlDOC=greenchemistryinstitute\index.html