Green Chemistry What is it Why do we need it

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Green ChemistryWhat is it? Why do we need
it?
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Objectives

• You should be able to
• Describe a number of aspects of Green Chemistry.
• Explain the importance of Green Chemistry as part
  of a sustainable future.
• Explain what is meant by the term atom economy.
• Calculate the atom economy of a reaction from
  its balanced equation.

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Green Chemistry

• Means different things to different people.
• Its not just one thing there are many aspects
  to Green Chemistry.
• Lets consider some of the Principles of Green
  Chemistry.

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Principles of Green Chemistry
Reactions that use fewer reactants, particularly
ones that arent hazardous, are better.
Its better to develop reactions with fewer waste
products than to have to clean up the waste. i.e.
high atom economy
Reactants from renewable sources (e.g. plants)
are preferable.
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Principles of Green Chemistry
Reactions that use fewer reactants, particularly
ones that arent hazardous, are better.
Its better to develop reactions with fewer waste
products than to have to clean up the waste. i.e.
high atom economy
Reactants from renewable sources (e.g. plants are
preferable).
Processes should rely on renewable energy
resources, rather than fossil fuels.
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Photo credit Martin Bond / Science Photo Library
(Ref G350/762)
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Principles of Green Chemistry
Reactions that use fewer reactants, particularly
ones that arent hazardous, are better.
Its better to develop reactions with fewer waste
products than to have to clean up the waste. i.e.
high atom economy
Reactants from renewable sources (e.g. plants are
preferable).
Processes should rely on renewable energy
resources, rather than fossil fuels.
Solvent use should be minimised, solvents
should be benign in their impact on the
environment.
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Photo credit Keith Weller / US Dept of
Agriculture / Science Photo Library (Ref G350/762)
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Principles of Green Chemistry
Reactions that use fewer reactants, particularly
ones that arent hazardous, are better.
Its better to develop reactions with fewer waste
products than to have to clean up the waste. i.e.
high atom economy
Reactants from renewable sources (e.g. plants are
preferable).
Processes should rely on renewable energy
resources, rather than fossil fuels.
Solvent use should be minimised, solvents
should be benign in their impact on the
environment.
Materials produced by chemists should be
biodegradable so they dont persist in the
environment after theyve been used.
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Principles of Green Chemistry
Reactions that use fewer reactants, particularly
ones that arent hazardous, are better.
Its better to develop reactions with fewer waste
products than to have to clean up the waste. i.e.
high atom economy
Reactants from renewable sources (e.g. plants are
preferable).
Processes should rely on renewable energy
resources, rather than fossil fuels.
It is desirable to find ways to use waste
products in other chemical reactions.
Solvent use should be minimised, solvents
should be benign in their impact on the
environment.
Alternatively, waste products that are non-toxic
and biodegradable are favourable.
Materials produced by chemists should be
biodegradable so they dont persist in the
environment after theyve been used.
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Photo credit David Read / University of
Southampton
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Principles of Green Chemistry
Reactions that use fewer reactants, particularly
ones that arent hazardous, are better.
Its better to develop reactions with fewer waste
products than to have to clean up the waste. i.e.
high atom economy
Reactants from renewable sources (e.g. plants are
preferable).
Processes should rely on renewable energy
resources, rather than fossil fuels.
It is desirable to find ways to use waste
products in other chemical reactions.
Solvent use should be minimised, solvents
should be benign in their impact on the
environment.
Alternatively, waste products that are non-toxic
and biodegradable are favourable.
Materials produced by chemists should be
biodegradable so they dont persist in the
environment after theyve been used.
For a more detailed treatment, see Paul T.
Anstas and John C. Warner Green Chemistry Theory
and Practice, New York Oxford University Press,
1998
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Yield vs Atom economy

• Yield can be calculated as
• yield mass (g) of product obtained x 100
• theoretical yield (g)
• The yield tells us how efficient a reaction is
  in terms of the amount of product we obtained
  relative to the maximum we could get from the
  amounts of reactants we used.
• But it doesnt take account of waste products!

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Yield vs Atom economy

• Atom economy can be calculated as
• AE x 100
• A reaction may have a high yield but a low
  atom economy.

RFM desired product sum of RFMs of all
products
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Atom economy some examples

• Calculate the atom economy of CH2Cl2
• CH4 2Cl2 ? CH2Cl2 2HCl
• RFM CH2Cl2 85, HCl 36.5
• AE x 100
• AE x 100 53.8

RFM desired product sum of RFMs of all
products
85 85 (2 x 36.5)
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Atom economy some examples

• CH4 2Cl2 ? CH2Cl2 2HCl
• An atom economy of 53.8 may be considered to be
  quite low. How could a chemical company maximise
  their profits from this chemical process?
• The by-product is hydrogen chloride, which can
  be sold as a gas or made into hydrochloric acid.
  These can then be sold.

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Atom economy some examples

• Calculate the atom economy of ethylene oxide
• RFM C2H4O 44, CaCl2 111, H2O 18
• AE x 100 37.4

(2 x 44) (2 x 44)
111 (2 x 18)
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Atom economy some examples

• Ethylene oxide A case of Green Chemistry
• An atom economy of 37.4 is particularly poor,
  and this is a very wasteful process.
• Nonetheless, this was the preferred method for
  synthesising ethylene oxide for many years.

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Atom economy some examples

• Ethylene oxide A case of Green Chemistry
• Recently, a method of synthesising ethylene
  oxide from ethene and oxygen using a silver
  catalyst was developed.
• Whats the atom economy of this reaction?

100
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The role of catalysts

• Catalysts have a crucial role to play in the
  future of Green Chemistry.
• They allow the development of new reactions which
  require fewer starting materials and produce
  fewer waste products.
• They can be recovered and re-used time and time
  again.
• They allow reactions to run at lower
  temperatures, cutting the amount of energy
  required.

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Catalysts in Action
Animation credit Robert Raja / University of
Southampton
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The future of chemistry

• We need to reconsider the way we go about all
  aspects of our lives.
• The planet is feeling a burden.
• Science has the potential to solve our problems.
• Green Chemistry can play a significant role in a
  sustainable future.

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Photo credit NASA Earth Observatory / Science
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Photo credit John Cole / Science Photo Library
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