Gree N Chemist Ry

Gree n Chemist ry

Green Chemistry • • • • •

Introduction Percentage yield vs. atom economy The 12 principles of green chemistry Application of green chemistry in practice Feasibility of green chemistry for daily life

Introduction What is green chemistry? • Green chemistry is the use of chemistry for prevention of pollution problems. It involves the design of chemical products and processes that are environmentally benign. Green chemistry covers all aspects and types of chemical processes that reduce negative impacts to human health and the environment.

• Green chemistry can be used in the following areas: • • • • • • • • • • • • • • • • •

Aerospace Agricultural Automotive Biotechnology Ceramics and Materials Chemical Communications Computer Systems Consumer Products Dyes and Photography Educational Electronics and Electrical Equipment Environmental Food and Beverage Homeland Security Leather Medical

Percentage yield and atom economy

Percentage yield It does not indicate how efficiently the reactants have been used in generating the desired product. % Yield =

Actual yield Theoretical yield

x 100%

Calculating Percentage (%) Yield 2.3g of sodium reacts with an excess of chlorine to produce 4.0g of sodium chloride. What is the percentage yield? 2Na(s)

+

Cl2(g)

⇒

(Ar reactants: Na=23 Cl=35.5 2.3g Na = 2.3 mol Na 23

2NaCl(s) Mr product: NaCl= 58.5)

= 0.1 mol Na

Theoretically 0.1 mol Na should yield 0.1 mol NaCl Theoretical yield of NaCl = 58.5 x 0.1 = 5.85g % Yield =

Actual yield x 100% Theoretical yield

= 4.0g x 100% = 68% 5.85g

Calculating Percentage (%) Yield If 1.2g of magnesium reacts with an excess of oxygen to produce 0.8g of magnesium oxide… What is the percentage yield? 2Mg(s) +

O2(g)

⇒

2MgO(s)

(Ar reactants: Mg=24 O=16

Mr product: MgO= 40)

1.2g Mg = 1.2 mol Mg = 0.05 mol Mg 24 Theoretically 0.05 mol Mg should yield 0.05 mol MgO Theoretical yield of MgO = 40 x 0.05 = 2g % Yield =

Actual yield x 100% % Yield = 0.8g x 100% = 40% Theoretical yield 2g

Calculating Percentage (%) Yield If 2g of calcium carbonate reacts with an excess of hydrochloric acid to produce 1.11 g of calcium chloride…. What is the percentage yield? 2HCl + CaCO3 ⇒ H2O + CO2 + CaCl2 (Mr values are: CaCO3 = 100 CaCl2 = 111) 2g CaCO3 = 2 mol CaCO3 = 0.02 mol CaCO3 100 Theoretically 0.02 mol CaCO3 should yield 0.02 mol CaCl2 Theoretical Yield of CaCl2 = 111 x 0.02 = 2.22g % Yield =

Actual yield x 100% Theoretical yield

% Yield = 1.11 x 100 = 50% 2.22

2008-Al Chem Paper II 5(a) Upon irradiation of visible light, 0.450g of 2-4dimethylpentane undergoes monochloro-substitution to gives 0.200g of 1-chloro-2,4-dimethylpentane. 0.167g of 2-chloro-2,4dimethylpentane and 0,117g of 3-chloro-2,4-dimethylpentane. (ii)(I) calculate the overall percentage yield for the monchlorinated products formed. ANS Total no. of moles of monochloroinated products = (0.2+0.167+0.117)/134.5 = 3.60X10-3 No. of moles of 2-4-dimethylpentane = 0.45/100 = 4.5X10-3 Overall % yield = 3.60X10-3/4.5X10-3 = 80

1998-Al Chem Paper I 8(b) 20.0g of 4-nitrobenzoic acid reacted with PCl5 to give a product which reacted exothermically with ammonia to give T. After treatment with Br2 and NaOH(aq), T gave a soild. Crystallization of the soild from ethanol gave 9.3 g of U( C5H6N2O2) (i)Calculate the % yield of U from 4-nitrobenzoic acid ANS Molar mass of C7H5NO4 =167.12 Molar mass of C5H6N2O2 = 138.128 % yield = 9.3X 167.12 X100% 20.0X138.128 = 56.3%

Sustainable Development and Atom Economy Yield is not enough, because it •Ignores auxiliaries (reagents, catalysts, solvents, etc.) •Ignores work-up and purification •Ignores energy used, hazards involved, and any toxic chemicals used or produced.

Sustainable Development and Atom Economy Developing chemical reactions with a high atom economy is therefore crucial in moving towards sustainable development. High atom economy also makes good economic sense. However, some chemical reactions have a limited atom economy because:  the reaction may be reversible (the reactants might not be converted completely into the products)  the reaction may produce unexpected products  the products cannot be efficiently separated from the reactants

Sustainable Development and Atom Economy The atom economy is measure of the conversion of starting material (reactant) into desired product. It is different from percentage yield. In an ideal reaction, all the atoms of the reactants would end up as useful product. Such a reaction would produce no waste at all, but this is rarely possible.

Calculating Atom Economy The atom economy (also called atom utilisation) of a reaction, is a measure of the percentage of the starting materials that actually end up as useful products. The atom economy can be calculated in the following way: % atom economy =

mass desired product(s) x 100% total mass of reactants

Calculating Atom Economy In the production of ammonium nitrate... NH3(g) ammonia

+

HNO3(aq)

+ nitric acid

…17g of NH3 and 63g of HNO3

⇒ ⇒

NH4NO3(aq) ammonium nitrate

produce 80g of NH4NO3

Calculate the atom economy for this reaction: NH3= 17g

HNO3 = 63g

NH4NO3 = 80g

Atom economy = 80g x 100 = 100% 80g As there are no waste products in this reaction, it has an atom economy of 100%.

Calculating Atom Economy Example 2: In the smelting of iron: 2Fe2O3(s)

+ 3C(s)

⇒ 4Fe(s) +

3CO2(g)

iron oxide + carbon ⇒ iron + carbon dioxide …for every 320g of iron oxide 224g of iron is produced. Calculate the atom economy for this reaction: 2Fe2O3= 320g 3C= 36g 4Fe= 224g 3CO2 = 132g Atom economy = 224g x 100 = 63% 320 + 36g As the reaction produces carbon dioxide as a waste product, the reaction can not have an atom economy of 100%. The atom economy of this reaction could be improved, if a use could found for the waste carbon dioxide.

Atom Economy and Percentage Yield Nitrogen reacts with hydrogen to make ammonia. N2 + 3H2  2NH3 a) Calculate the maximum theoretical mass of ammonia that can be made by reacting 90g of hydrogen with an excess of nitrogen. b) In the reaction, only 153g of ammonia was produced. Calculate the percentage yield. c) Calculate the atom economy to make ammonia from the reaction of nitrogen and hydrogen.

a) Calculate the maximum theoretical mass of ammonia that can be made by reacting 90g of hydrogen with an excess of nitrogen. number of moles of H2 = 90/2 = 45moles number of moles of NH3 = 2/3 x 45 = 30 moles Maximum theoretical mass = 30 x (14+3) = 510 g b) In the reaction, only 153g of ammonia was produced. Calculate the percentage yield. Percentage yield = (153/510) x 100% = 30% c) Calculate the atom economy to make ammonia from the reaction of nitrogen and hydrogen. Atom economy = 100% (since all the reactants converts to products)

Conclusion: High atom economy is not equal to high percentage yield.

Reaction Type

Description

Atom Economy

Addition

Different molecules join together 100% as all reactant atoms end up to make a new substance in the product

Condensation

Two molecules join, with the Always a little less than 100% as production of a small molecule small molecules are produced, like water or ammonia which are usually waste.

Elimination

A group of atoms is removed Generally poor because an from a molecule, usually leaving additional product is always a double or triple bond formed

Rearrangement

Atoms are rearranged to create 100% as the same atoms are a different substance with the present in the product as in the same empirical formula reactant

Substitution

A group of atoms on a molecule The group replaced creates a is replaced by a different group product too, but as this may vary considerably, substitutions can be from fairly good to very poor

The 12 principles of green chemistry In 1998, the 12 principles of green chemistry were articulated by Anastas, P. T. & Warner, J.C. in the book ‘Green Chemistry: Theory and practice.’

1. Prevent waste 2. Design safer chemicals and products 3. Design less hazardous chemical syntheses 4. Use renewable feedstocks 5. Use catalysts, not stoichiometric reagents

6. Avoid chemical derivatives 7. Maximize atom economy 8. Use safer solvents and reaction conditions 9. Increase energy efficiency 10. Design chemicals and products to degrade after use 11. Analyze in real time to prevent pollution 12. Minimize the potential for accidents

Applications of green chemistry

Microscale Experiment • a teaching method widely used at school and at university levels, working with small quantities of chemical substances • use low-cost and even no-cost material

Advantages of Microscale Experiment • less starting materials are required for testing the possibility of some unknown reactions • very useful when the starting materials are very expensive • less reactant, less reagent and less solvent are required • lower cost • save time and money • use small amount of reactants can make the experiment more safe • less chemical waste is deposited

Disadvantages of Microscale Experiment • equipment and textbooks about microscale experiment are • but the costs can be recovered in a relatively short period of time due to savings realized on purchase and disposal costs of reduced quantities of chemicals

Use of hydrogen peroxide as a bleaching agent • Traditional chlorine bleach (active ingredient – hypochlorite OCl )

• Disadvantages:

1. Toxic chlorine gas may evolve 2NaOCl + 2NH3  2NaONH3 +Cl2 2. Poisonous chlorinated organic compounds may be found 3NaOCl +NH3  3NaOH +NCl3 3.The reaction of byproducts hydrazine and monochloramine is highly exothermic NH3 + NaOCl  NaOH + NH2Cl NH3 + NH2Cl + NaOH  N2H4 + NaCl +H2O

• Bleached by oxidation : H2O2(aq) + dye  H2O(l) + (dye+O)

Applications:

• bleaching agent for hair • the bleaching of pulp for paper manufacturing • household disinfectant • Hydrogen peroxide should be stored in a cool, dry, wellventilated area and away from any flammable or combustible substances, it should be stored in a container composed of non-reactive materials such as stainless steel or glass

Use of fuel cell • It is a device that converts fuel i.e. hydrogen into electrical energy • The cell, the reactants flow in and products flow out electrolyte in the presence of electrolytes such as H2SO4 • The anode and cathode are coated with Pt, which acts as a catalyst. + At anode : 2H2  4H +4e -

+

At cathode : O2 + 4e + 4H  2H2O

• The elctrons produred at the anode flow through the wire connecting the external circuit, electrical energy is generated • At the end, pure water and heat are produced.

Advantages of using H-cell:

• they are environmentally friendly • no air pollutants are emitted • do not contribute to global warming as combustion of fossil fuel is not involved • high efficiency • hydrogen is the most plentiful element in the universe

Applications of H-cell: • fuel cell vehicles (still in development stage) • electrical system of rockets and shuttles

Manufacture of nylon-6,6 by Beckmann rearrangement

oxime

lactam

Monomer for nylon-6

• a typical Beckmann rearrangement • The oxime from cyclohexanone has identical carbonyl substituents and exists as a single isomer • The product of the rearrangement is a lactam (a cyclic amide), which can be hydrolyzed to an omega-amino acid • This lactam serves as an important industrial precursor to nylon-6

Advantages • no wastage • water is a renewable source • rearrangement – 100% atom economy

Obtain saturated fats by catalytic hydrogenation

• adds hydrogen atoms at the double bonds of a fatty acid chain to produce an artificial fatty acid • operate at pressures of 3 – 6 atm and temperatures of 100 – 180oC • using nickel / palladium / nickel / rhodium as catalysts

Advantages of using catalytic hydrogenation • • • • • • •

cheaper than animal source fats are available in a wide range of consistencies increased oxidative stability longer shelf life a harder consistency a higher melting point better oxidation stability

Ionic liquid

Fig. 1 Examples of simple room temperature ionic liquid

• Ionic liquid is used to refer to a molten salt that is a liquid at ambient temperatures. They generally are compounds that have both a large anion and cation, and possess a low degree of symmetry.

Ionic liquids can be: • simple salts, or • binary ionic liquid Example • 1-alkyl-3-methylimidazolium tetrafluoroborate salts are miscible with water at 25 °C where the alkyl chain is less than 6, but at or above 6 carbon atoms, they are form a separate phase when mixed with water. • This makes solvent extraction easier.

Advantages • • • •

Prevent waste Use renewable feedstocks Use catalysts, not stoichiometric reagents Avoid chemical derivative

Biofuels • Fuels derived from organic biomass from recently living animals or plants or their byproducts, has transformed from a niche alternative to fossil fuels. • Vegetable oils, animal fats, ethanol and biodiesel are biofuels.

• Landfill sites generate gases by anaerobic digestion. Landfill gas contains approximately 50% methane, the gas found in natural gas. • Ethanol is produced by enzyme digestion fermentation of the sugars, distillation and drying.

Advantages • Prevent waste • Use renewable feedstocks • Design chemicals and products to degrade after use

Haber process in the manufacture of ammonia • Area: Chemical + Ceramics and Materials • N2(g) + 3 H2(g)  2 NH3(g) (ΔH = −92.4 kJ·mol−1) • catalyst: a form of magnetite, iron oxide

• Preparation: • First, methane is cleaned to remove sulphur impurities that will poison the catalysts. • Steam reforming: over a catalyst of nickel oxide (CH4 + H2O → CO + 3 H2 ) • Secondary reforming: addition of air to convert the methane that did not react during steam reforming. CH4 + O2 → 2 CO + 4 H2 CH4 + 2 O2 → CO2 + 2 H2O • Then the water gas shift reaction yields more hydrogen from CO and steam. CO + H2O → CO2 + H2 • Last step: the gas mixture passing into a methanator, which converts most of the remaining CO into methane for recycling as carbon monoxide poisons the catalyst CO + 3 H2 → CH2 + H2O

Use of supercritical carbon dioxide (e.g. decaffeinating coffee)

Critical temp. (31.1 °C) and critical pressure (72.9 atm/7.39 Mpa) are low

• In a fluid state while also being at or above both its critical temperature and pressure • Yields rather uncommon properties, e.g. dissolving organic substances • Low toxicity and environmental impact • An important commercial and industrial solvent • It replaces a chlorinated organic solvent tetrachloroethene (Cl2C=CCl2) which may cause cancer in dry cleaning, metal cleaning and other polluting industrial processes.

H2O2 and O2 as environmentally benign oxidising agents • Many industrial processes involves oxidising agents, e.g. KMnO4, K2Cr2O7, conc. NHO3, etc. • These oxidising agents are not green. • Conc. NHO3 on reduction produces toxic N2O, which induces greenhouse effect and leads to ozone depletion • H2O2 is one of the green replacement for these harmful oxidisng agents

• Aqueous hydrogen peroxide is an ideal oxidant, because the atom economy is excellent and water is the only theoretical side product H2O2  [O] + H2O or 2H2O2  O2 + 2H2O • Example • Cyclohexane is converted directly to pure, crystalline hexanedioic acid in a very high yield

• Molecular oxygen is another environmental benign oxidising agent which has been used industrially, e.g. RCH2OH

O2, catalyst, sunlight

RCOOH

• The product from the reduction of O2 is environmentally benign water

Bioplastic • Bioplastics (also called organic plastics) are a form of plastics derived from renewable biomass sources, such as vegetable oil, corn starch, or microbiota, rather than fossilfuel plastics which are derived from petroleum. • On the other hand, bioplastic can be made from agricultural byproducts and also from used plastic bottles and other containers using microorganisms.

Plastics made from corn are already on the market.

Advantages of bioplastic: • Because of their biological degradability, the use of bioplastics is especially popular for disposable items, such as packaging and catering items. • In these areas, the goal is not biodegradability, but to create items from sustainable resources. • The production and use of bioplastics relies less on fossil fuel as a carbon source and also introduces fewer greenhouse emissions if it biodegrades. They significantly reduce hazardous waste caused by oilderived plastics, which remain solid for hundreds of years.

Disadvantages of bioplastic: • There are fears that bioplastics will damage existing recycling projects. • Shelf life is limited because the plastic is permeable to water - the bottles lose their contents and slowly deform.

Catalytic converter

Catalytic converter • used to reduce the toxicity of emissions from an internal combustion engine • used on generator sets, forklifts, mining equipment, trucks, buses, trains, and other engine-equipped machines • provides an environment for a chemical reaction wherein toxic combustion by-products are converted to less-toxic substances

Catalytic converter Types of catalytic converter: • Two-way • Three-way

Two-way catalytic converter • There are two simultaneous tasks : • Oxidation of carbon monoxide to carbon dioxide : 2CO + O2 → 2CO2 • Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water : CxH2x+2 + 2xO2 → xCO2 + 2xH2O • (a combustion reaction) • Widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions • However, cannot control amount of nitrogen oxides NOx

Three-way catalytic converter • There are three simultaneous tasks : • Reduction of nitrogen oxides to nitrogen and oxygen : 2NOx → xO2 + N2 • Oxidation of carbon monoxide to carbon dioxide : 2CO + O2 → 2CO2 • Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water : CxH2x+2 + 2xO2 → xCO2 + 2xH2O

Three-way catalytic converter • Three-way catalytic converters can store oxygen from the exhaust gas stream, when the air fuel ratio goes lean • When oxygen is insufficient, the stored oxygen is released and consumed • Unwanted reactions : formation of hydrogen sulphide and ammonia • But can be limited by precious metals used • Nickel or manganese is added to block the adsorption of sulphur by the washcoat

For diesel engines • Most commonly used catalytic converter is the diesel oxidation catalyst • They uses excess O2 in the exhaust gas stream to oxidize CO to CO2 and HC to H2O and CO2 • Reach 90% efficiency, eliminate diesel odor and help to reduce visible particulates (soot) • However they are incapable of reducing NOx

Catalyst Poisoning • Catalyst poisoning occurs when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces • catalyst cannot contact and treat the exhaust • The most notable contaminant is lead, so vehicles equipped with catalytic converters can only be run on unleaded gasoline. • Other common catalyst poisons : manganese, silicon, phosphorus, zinc

Treatment for Catalyst Poisoning • Catalyst poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time • The increased exhaust temperature can sometimes liquefy or sublimate the contaminant, removing it from the catalytic surface • However, removal of lead deposits in this manner is usually not possible due to lead's high boiling point

Potential applications (1) Corn waste converted to chemicals

• conversion of glutamic acid to -aminobutyric acid (GABA) (nitrogen-containing)using a decarboxylase enzyme • producing nitrogen-containing industrial chemicals more cheaply than the energy intensive from the waste from bioethanol production • improving the green credentials and the economics of biofuel production

(2) Expanding waste to reduce waste

• expanding the structure of waste polyvinyl-alcohol (PVA) from liquid crystal display (LCD) screens to form a mesoporous material with a high surface area • use of enzyme immobilisation, tissue scaffolds or drug delivery due to the biocompatibility of PVA • providing an exciting new method for recovering materials that would otherwise go to waste • iodine, which is present in waste PVA, is essential for the expansion process

Feasibility of green chemistry for daily life applications • • • • •

A number of difficulties involves: Chemical (what technologies are available?) Economic (who pays, who benefits?) Social (who are affected?) Political (who is responsible?)

• Possible contribution of green chemistry in achieving sustainability in the 21st century: • (1) Renewable energy technologies E.g. Array of solar cells to supply electric power to space shuttle • (2) Reducing our dependence on the dwindling fossil carbon • (3) The replacement of existing polluting technologies by environmentally benign alternatives

Past Paper Analysis (05, 07, 08)

Past Paper 05 II 5.(a)(iii) Besides cutting down petroleum consumption, suggest one additional advantage of using the alternative fuel over using gasoline. (1M)

Past Paper 05 II 5.(a)(iii)  Additional Information: • Gasoline (petrol) is commonly used as motor car fuel. • Lead petrol: Straight chain alkanes cause knocking ( 死火 ) in the engine when they are burnt as oxygen is used up very quickly for complete combustion. • The problem is solved by adding tetrathyllead (TEL) >> leaded petrol (Toxic to human, was ban by the HK government since 1994.) • Unlead petrol: Straight chain alkanes >> branched chain

Past Paper 05 II 5.(a)(iii) Ans:

Alternative fuel produces less air pollutants than petroleum when burn.

Past Paper 07 I 8.(a)(i)

(2M)

Past Paper 07 I 8.(a)(i) Ans:

Past Paper 07 I 8.(a)(ii)

(1M)

Past Paper 07 I 8.(a)(ii) Ans:

Past Paper 07 I 8.(a)(iii)

(1M)

Past Paper 07 I 8.(a)(iii) Ans:

Past Paper 08 I 10.

10. Write an essay on the application of the principles of green chemistry in industry. (20M)

Ans:

Past Paper 08 I 10.

Ans:

Past Paper 08 I 10.

Past Paper 08 I 10.  Additional Information: • Supercritical carbon dioxide: • carbon dioxide that is in a fluid state while also being at or above both its critical temperature and pressure, yielding rather uncommon properties • usually as a gas in air at standard conditions for temperature and pressure / as a solid called dry ice when frozen

Past Paper 08 I 10.  Additional Information: Phase diagram

Past Paper 08 I 10.  Additional Information: - if temperature & pressure are both increased from standard conditions for temperature and pressure to be at or above the critical point for carbon dioxide >> adopt properties midway between a gas and a liquid - behaves as a supercritical fluid above its critical temperature (31.1 °C) and critical pressure (72.9 atm), expanding to fill its container like a gas but with a density like that of a liquid

Past Paper 08 I 10.  Additional Information: - low toxicity and environmental impact - is becoming an important commercial and industrial solvent - relatively low temperature of the process and the stability of CO2 also allows most compounds to be extracted with little damage or denaturing

Past Paper 08 I 10. Ans:

Past Paper 08 I 10. Ans:

End of presentation