Chapter 17 - Green Chemistry
After completing this chapter, you should be able to:
 give reasons why a green approach to chemistry is
desirable
 give an outline of the stated principles of green
chemistry
 calculate the atom economy of a chemical reaction


describe what is meant by supercritical fluid
give some examples of the use of supercritical
carbon dioxide as a replacement for halogenated
solvents in industrial processes
Why is Green Chemistry important??
Many aspects of our lives have been enhanced by chemistry and the chemical industry. A significant factor in the dramatic
increase in life expectancy in most parts of the world in the 20th and 21st centuries is the development of pharmaceuticals
including antibiotics and antiseptics, the improved quality of our water supply and the availability of refrigeration to prevent food
spoilage.
Chemistry has also improved living conditions by the development of new products such as:
• fertilisers and pesticides that have improved the quantity and quality of our food production
• polymers such as nylon, polyesters, polyethene and polystyrene that are used to make synthetic fibres as well as a wide
range of consumer goods
• new ceramics, glasses and alloys have made modern materials more reliable and cheaper to produce
• semiconductors, including silicon chips, for our computers.
However, these new products have sometimes come at a cost to the environment and to human health. In the past 50 years we
have seen:
• major oil spills from crashed tankers that have devastated fish and bird communities
• rivers that have been seriously polluted from poor waste-disposal practices
• widespread use of tetraethyl lead as an anti-knock agent in petrol being subsequently linked to learning disabilities in
inner-city children
• the industrial accident in Bhopal, India, in 1984, that led to the release of toxic methyl isocyanate, killing almost 4000
people
• the industrial accident in Seveso, Italy, in 1976, that led to the release of other toxic chemicals into the environment. This
resulted in the death of many farm animals and long-term health problems for many people.
In recent years, an enormous amount of data has been collected on the hazards of some of commonly used chemicals. Scientists are
using this knowledge to devise processes to avoid the problems of the past. In a number of countries, laws have been enacted to
control the spread of pollutants in landfills and waterways.
Chlorofluorocarbons
Chlorofluorocarbons (CFCs) have been identified as a group of compounds that have contributed to the deterioration of the
ozone layer in the atmosphere. The ozone layer acts as a filter that prevents some ultraviolet radiation from reaching the Earth.
CFCs were first introduced in the 1930s. At that time, refrigerators were expensive and contained ammonia or Sulfur dioxide as
the coolant gases. Both of these gases are corrosive, smelly and toxic. Frequent leaks in the refrigerator pipes released these gases
into the home and the environment. So there was a need to replace ammonia and Sulfur dioxide as refrigerants. Chemists
identified a group of compounds of carbon, fluorine and chlorine as suitable alternatives. When first introduced, CFCs were
thought to be perfect for refrigeration, air conditioning and, a little later, propellants in aerosol cans and blowing agents to make
polystyrene foam.
The advantages of CFCs are that they:
• are non-toxic
• are very stable
• can be vapourised at just the right temperature to
make them ideal as refrigerants
•
•
made cheaper and safer refrigeration accessible to
more people who could keep food fresher for longer
periods of time.
are non-flammable
As the popularity of CFCs increased, and they became widely used over long periods of time, scientists started becoming
concerned that when they leaked, or were released into the atmosphere:
• their very stability meant that they survived for long periods of time in the environment
• in the presence of ultraviolet light in the atmosphere they took part in a series of complex reactions that resulted in the
breakdown of ozone in the atmosphere’s ozone layer.
On the basis of the available data, legislation was passed to limit and eventually phase out the use of CFCs and the hunt was on for
chemists to find a more environmentally friendly alternative. Research identified the chlorine atom in CFCs as the problem and
today, compounds known as HFCs, which contain hydrogen, fluorine and carbon, but not the offending chlorine, are commonly
used.
How does green chemistry
help?
The laws and treaties that were
enacted to reduce global pollution
were often aimed at dealing with
wastes after they had been
generated and did not address
methods to reduce the production
of waste.
Green chemistry, or
environmentally safe chemistry,
outlines a set of principles that
forms a framework that can be
used to evaluate the
environmental impact of a
chemical process. It focuses on
methods that reduce or eliminate
hazardous waste. The green
approach is that the best way to
minimise waste is not to produce
it in the first place. Its ultimate
goal is to implement energyefficient, hazard-free, waste-free,
efficient chemical processes
without sacrificing their
effectiveness.
Ideally:
• goods needed by society should be produced by methods that are not harmful to the environment
• fossil fuels, and other non-renewable resources, should be replaced by renewable ones
• goods produced by society, should either be recyclable or biodegradable
The processes used to manufacture the product should either produce no wastes or wastes that are recyclable or biodegradable.
Green chemistry practices have major long-term cost benefits to businesses and the reduction of long-term damage to the
environment. By switching to renewable energy sources, biomaterials and manufacturing chemicals that degrade into benign
substances, green chemistry is protecting the planet from long-term deterioration. Industry can benefit too from green chemistry
considerations, since greater efficiency leads
to reduced costs and improved profits.
The atom economy approach is a method of
accounting for the use of materials in a
manufacturing process. It tracks all the atoms
in a reaction and calculates the mass of the
atoms of reactants actually used to form
products as a percentage of the total mass of
reactants. From this, the mass of reactant
atoms that end up as waste can be calculated.
Supercritical carbon dioxide (scCO2)
Materials commonly exist in one of three
states of matter—solid, liquid or gas. When
liquids are heated, they tend to form a vapour.
When vapours are compressed, they tend to
condense into liquid. However, if a vapour is
heated above a certain critical temperature,
the vapour cannot be liquefied no matter what
pressure is applied. At these relatively high
temperatures and pressures, the distinction between the liquid and gaseous states blur. The material has properties similar to
those of gases in that it expands to fill any available space. At the same time, however, it has properties similar to those of liquids
and can be used as a solvent. At this stage, the material is a supercritical fluid. Carbon dioxide forms a supercritical fluid at a
pressure of about 73 atm and a temperature of about 31°C. This relatively low critical temperature makes supercritical carbon
dioxide easy to work with. Another useful feature is that its solvent properties can be altered by making slight adjustments to
temperature and pressure.
Replacement of halogenated solvents with scCO2 - A commonly used solvent in the dry-cleaning industry is tetrachloroethene
(C2Cl4) or ‘perc’. This solvent, like many other halogenated hydrocarbons, is volatile and a suspected carcinogen. Even at low
concentrations, perc has unpleasant side effects to humans and used perc must be disposed of as a hazardous waste.
Some dry-cleaners have now replaced this halogenated hydrocarbon with scCO2 mixed with small quantities of detergent. scCO2
is also being successfully used for various kinds of industrial cleaning. The use of scCO2 is a particularly environmentally friendly
solution as it can be obtained as a by-product from other industries. It is also relatively easy to recapture and reuse and does not
have the problems of toxicity associated with halogenated solvents.
Chapter 17 – Q1, 2, 3, 4, 5,
Pg 311 -312 Q 18, 19, 24