Chapter 17 – Green Chemistry

Week 6, Lesson 3

Development of CFCs

• Chloroflurocarbons (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 1930’s.• CFCs were thought to be perfect for refrigeration,

air conditioning and propellants in aerosol cans.

Advantages of CFCs

• Are non-toxic• Are very stable• Are non-flammable• Can be vapourised at just the right

temperature to make them ideal for refrigerants

• Made cheaper and safer refrigeration accessible

CFCs and the Atmosphere

• Scientists became concerned when they leaked or were released into the atmosphere:– Their stability meant that they survived for long

periods of time in the environment.– In the presence of UV light in the atmosphere they

took part in a series of complex reactions that resulted in the breakdown of ozone in the ozone layer.

The Solution

• Research identified that the chlorine atom ins CFCs as the problem.

• Today compounds known as HFCs, which contain hydrogen, fluoride and carbon are commonly used.

What is Green Chemistry?• Green 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 to not produce it in the first place.

• Its ultimate goal is to implement energy-efficient, hazard-free, waste-free, efficient chemical processes without sacrificing their effectiveness.

Green Chemistry

• 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.

Principles of Green Chemistry (1-3)1. PREVENT WASTE – it is better to design chemical

processes to prevent waste than to treat waste or clean it up after it is formed.

2. DESIGN SAFER CHEMICALS AND PRODUCTS – design chemical products to be fully effective, yet have little or no toxicity.

3. DESIGN LESS HAZARDOUS CHEMICAL SYNTHESES – Methods should be designed that use and generate substances with little or no toxicity to humans and the environment.

Principles of Green Chemistry (4-6)

4. USE RENEWABLE RAW MATERIALS – Use starting materials that are derived from renewable resources such as plant material rather than those such as from fossil fuels that will eventually run out.

5. USE CATALYSTS, NOT STOICHIOMETRIC REAGENTS – Minimise waste by using catalysts in small amounts that can carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and work only once.

6. AVOID CHEMICAL DERIVATIVES – Avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.

Principles of Green Chemistry (7-9)

7. MAXIMISE ATOM ECONOMY – Design syntheses so that the final product contains the maximum proportion of the starting materials. There should be few, if any, wasted atoms.

8. USE SAFER SOLVENTS AND REACTION CONDITIONS – Avoid using toxic solvents to dissolve reactants or extract products.

9. INCREASE ENERGY EFFICIENCY – Energy requirements should be minimised. Run chemical reactions at room temperature and pressure whenever possible.

Principles of Green Chemistry (10-12)

10. DESIGN FOR DEGRADATION – Chemical products should be designed to break down harmless substances after use so that they do not accumulate in the environment.

11. ANALYSE IN REAL TIME TO PREVENT POLLUTION – Include continuous monitoring and control during process to minimise or eliminate the formation of by-products.

12. MINIMISE THE POTENTIAL FOR ACCIDENTS – Design chemicals and their forms (solid, liquid or gas) to minimise the potential for chemical accidents including explosions, fires and releases to the environment.

Atom Economy

• The atom economy approach is a method of accounting for the use of materials in a manufacturing process.

• It tracks all 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.

Atom Economy Example…• Calculate the percentage atom economy in the formation of

1-iodiopropane from 1-propanol according to the following reaction.

CH3CH2CH3OH + NaI + H2SO4 CH2CH2CH2I + NaHSO4 + H2O

Formula of Reactants

Molar Mass of Reactants

Atoms used in Product

Sum of Molar Mass of Used Atoms

Unused Atoms

Sum of Molar Mass of Unused Atoms

CH3CH2CH2OH

60.1 3C, 7H 43.1 HO 17.0

NaI 149.9 I 126.9 Na 23.0

H2SO4 98.0 - 0 2H, S, 4O 98.0

Total Atoms in Reactants, 3C, 10H, 5O, Na, S, I

308.0 3C, 7H, I 170.0 HO, Na, 2H, S, 4O

138.0

Atom Economy Example Continued….

Percentage Atom Economy = (molar mass used atoms / molar mass of all reactants) x 100

= (170.0/308.0) x 100 = 55.2%

Formula of Reactants

Molar Mass of Reactants

Atoms used in Product

Sum of Molar Mass of Used Atoms

Unused Atoms

Sum of Molar Mass of Unused Atoms

CH3CH2CH2OH 60.1 3C, 7H 43.1 HO 17.0

NaI 149.9 I 126.9 Na 23.0

H2SO4 98.0 - 0 2H, S, 4O 98.0

Total Atoms in Reactants, 3C, 10H, 5O, Na, S, I

308.0 3C, 7H, I 170.0 HO, Na, 2H, S, 4O

138.0

Green Chemistry in Action

• Most commonly used solvents are flammable and volatile organic compounds which are toxic.

• Some have significant environmental impact and are associated with the deterioration of the ozone.

• There has been a lot of research in finding alternative solvents.

• One alternative is carbon dioxide.

Supercritical Carbon Dioxide (scCO2)

• Usually when liquids are heated they turn into a vapour and when vapour is compressed it condenses into a 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 temperatures the distinction between liquid and gas is blurred.

• The material has similar properties to gas in that it expands to fill any space, however, it also has similar properties to a liquid and can be used as a solvent.

• At this stage, the material is said to be a supercritical liquid.

Supercritical Carbon Dioxide cont…

• Carbon dioxide forms a supercritical fluid at a pressure of 73atm and a temperature of 31°C.

• This relatively low temperature makes superficial 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.

• scCO2 is an environmentally friendly option also because it can be obtained as a by-product from other industries. It is also easy to recapture and rescue.

Other examples of Green Chemistry

• Petroleum is the raw material for the manufacture of polystylerene. Polystyrene is a very good heat insulator and shocker absorber so is commonly used in food containers nad packaging.

• In the past, this packaging was expanded with the use of CFCs. Now these have been replaced with CO2 or restaurants are using cardboard containers.

• Adipic acid is a compound used in large quantities to make nylon and other useful products. Usually this is made from benzene, a known carcinogen.

• Scientists have found a way, by using genetically altered bacteria as catalysts to make adipic acid from glucose.