Green Chemistry Notes FE Sem 1 Applied Chemistry | MU

Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. This module applies its principles to three industrially important molecules, indigo dye, adipic acid and carbaryl, by comparing their conventional and green routes. It then studies two classes of green solvents, supercritical fluids and ionic liquids, and two green fuels, biodiesel and ethanol. These topics show how chemistry supports sustainable development and are asked as comparison and note questions in MU papers.

Summary

Green chemistry aims at pollution prevention at the source by following principles such as preventing waste, maximising atom economy, using less hazardous syntheses and safer solvents, using renewable feedstocks, using catalysts rather than stoichiometric reagents, designing energy-efficient processes and avoiding accidents. Atom economy, the percentage of the total mass of reactants that ends up in the desired product, is a simple measure of how wasteful a route is.

Indigo dye. The conventional (Heumann-Pfleger) route starts from aniline, which reacts with formaldehyde and hydrogen cyanide to give N-phenylglycinonitrile; hydrolysis gives N-phenylglycine, which is fused with sodamide in molten NaOH/KOH at high temperature to form indoxyl; air oxidation of indoxyl gives indigo. It uses toxic aniline, HCN and formaldehyde, strongly alkaline melts, high energy and gives salt-laden waste. The green route is biotechnological: genetically engineered E. coli converts glucose to tryptophan, the enzyme tryptophanase converts tryptophan to indole, and naphthalene dioxygenase oxidises indole to indole-2,3-dihydrodiol, which loses water to give indoxyl; indoxyl is oxidised by air to indigo. This route uses renewable feedstock, water as medium, mild temperatures and no toxic reagents.

Adipic acid, used to make nylon 6,6. In the conventional route benzene is hydrogenated to cyclohexane, which is air-oxidised to a mixture of cyclohexanol and cyclohexanone (KA oil), and this is oxidised with concentrated nitric acid to adipic acid. Benzene is carcinogenic and petroleum-derived, nitric acid is corrosive, and nitrous oxide N2O is released, a potent greenhouse gas that also depletes ozone. Green routes: (i) Draths and Frost: D-glucose is converted by genetically modified E. coli into cis,cis-muconic acid, which is hydrogenated to adipic acid, using a renewable, non-toxic feedstock; (ii) Noyori: cyclohexene is oxidised with 30% aqueous H2O2 in the presence of sodium tungstate and a phase-transfer catalyst without any organic solvent, giving adipic acid and water as the only by-product.

Carbaryl (Sevin), 1-naphthyl N-methylcarbamate, is an insecticide. In the conventional route methylamine reacts with phosgene to give methyl isocyanate (MIC), which is then reacted with 1-naphthol to give carbaryl. MIC is extremely toxic and volatile; its storage in large quantity led to the Bhopal gas disaster of 1984. In the greener route the order is changed: 1-naphthol is first reacted with phosgene to give 1-naphthyl chloroformate, which then reacts with methylamine to give carbaryl. The same overall product is obtained without making or storing MIC, so the process is inherently safer, though phosgene is still used.

Green solvents replace volatile, toxic and flammable organic solvents. A supercritical fluid is a substance above its critical temperature and critical pressure, where liquid and gas phases are no longer distinct. Supercritical CO2 (Tc = 31.1 C, Pc = 73.8 bar) has liquid-like density and dissolving power but gas-like low viscosity and high diffusivity, no surface tension, and its solvent power can be tuned by changing pressure and temperature. It is non-toxic, non-flammable, cheap and is removed simply by releasing pressure, leaving no residue. Applications: decaffeination of coffee and tea, extraction of flavours, essential oils and hops, dry cleaning, polymerisation and hydrogenation media, supercritical fluid chromatography, and supercritical water (Tc = 374 C, Pc = 221 bar) for oxidising hazardous wastes.

Ionic liquids are salts that are liquid below 100 C (many at room temperature), made of bulky, asymmetric organic cations such as imidazolium, pyridinium, quaternary ammonium or phosphonium, with anions such as Cl-, BF4-, PF6- or bis(trifluoromethylsulfonyl)imide. Characteristics: negligible vapour pressure (no evaporation losses or air pollution), non-flammability, high thermal stability, wide liquid range, ability to dissolve organic, inorganic and polymeric materials, wide electrochemical window and good ionic conductivity, recyclability, and tunable properties by choice of cation and anion (designer solvents). Applications: media for reactions such as Diels-Alder, Friedel-Crafts and hydrogenation, catalyst immobilisation, dissolution of cellulose, extraction and separation, and electrolytes for batteries, supercapacitors and electroplating.

Biodiesel is a mixture of fatty acid methyl (or ethyl) esters made by transesterification of vegetable oils (jatropha, karanja, soybean) or animal fats and waste cooking oil. The triglyceride reacts with methanol (about 6:1 molar ratio) in presence of NaOH or KOH catalyst at about 60 to 65 C; the products separate into an upper biodiesel layer and a lower glycerol layer, and the biodiesel is washed and dried. Advantages: renewable and biodegradable, practically sulphur-free, lower emissions of CO, unburnt hydrocarbons and particulates, higher cetane number and flash point (safer to store), good lubricity, nearly carbon neutral, usable in existing diesel engines alone or blended (B20), and a useful by-product glycerol.

Ethanol is produced by fermentation of molasses, starch or cellulosic biomass. In the molasses process the diluted solution, acidified to pH about 4 to 5, is fermented by yeast at about 30 to 35 C: invertase converts sucrose to glucose and fructose, and zymase converts them into ethanol and CO2. The wash (about 8 to 10% ethanol) is distilled to rectified spirit (95.6%), and anhydrous ethanol for blending is obtained by azeotropic distillation or molecular sieves. Advantages: made from renewable agricultural material, high octane number, oxygenated fuel that burns cleaner with lower CO and hydrocarbon emissions, almost sulphur-free, nearly carbon neutral, blends with petrol (E10, E20) as power alcohol, reduces crude oil imports and supports farmers.

Mind map

Synthesis of biodiesel by transesterification

Important definitions

Formulas

Key points to remember

• Conventional indigo uses aniline, HCN and formaldehyde; green indigo uses engineered E. coli and glucose/tryptophan.
• Conventional adipic acid releases N2O, a greenhouse and ozone-depleting gas.
• Draths-Frost adipic acid: glucose to cis,cis-muconic acid to adipic acid.
• Noyori adipic acid: cyclohexene + 30% H2O2, Na2WO4, phase-transfer catalyst, no organic solvent, water by-product.
• Conventional carbaryl goes through MIC (Bhopal 1984); the greener route goes through 1-naphthyl chloroformate and avoids MIC.
• Supercritical CO2: Tc 31.1 C, Pc 73.8 bar; supercritical water: Tc 374 C, Pc 221 bar.
• Ionic liquids have negligible vapour pressure, so they do not cause VOC pollution.
• Biodiesel: triglyceride + methanol with base catalyst at about 60-65 C; by-product glycerol.
• Ethanol: invertase converts sucrose; zymase converts glucose to ethanol + CO2.
• Rectified spirit is 95.6% ethanol (azeotrope); anhydrous ethanol is needed for petrol blending.
• Atom economy alone does not make a route green; toxicity of reagents, by-products and hazards also matter.

Solved examples

Example 1. Calculate the atom economy of the Noyori synthesis of adipic acid: C6H10 + 4H2O2 gives C6H10O4 + 4H2O. (C6H10 = 82.14, H2O2 = 34.01, C6H10O4 = 146.14)

Step 1: Sum of reactant masses = 82.14 + 4 x 34.01 = 82.14 + 136.04 = 218.18. Step 2: % AE = 146.14/218.18 x 100 = 66.98%. The only by-product is water, which is harmless.

Example 2. Calculate the atom economy of the conventional oxidation C6H12O + 2HNO3 gives C6H10O4 + N2O + 2H2O and compare with the Noyori route. (C6H12O = 100.16, HNO3 = 63.01)

Step 1: Reactant mass = 100.16 + 2 x 63.01 = 226.18. Step 2: % AE = 146.14/226.18 x 100 = 64.61%. Step 3: The atom economies are similar (64.6% vs 67.0%), but the conventional route uses corrosive HNO3, starts from benzene-derived cyclohexanol and releases N2O, while the Noyori route gives only water. Hence the Noyori route is greener; atom economy is not the only criterion.

Example 3. Calculate the atom economy for the final step of carbaryl synthesis in the conventional route: 1-naphthol + CH3NCO gives carbaryl. (1-naphthol = 144.17, MIC = 57.05, carbaryl = 201.22)

Step 1: Reactant mass = 144.17 + 57.05 = 201.22. Step 2: % AE = 201.22/201.22 x 100 = 100%. This is an addition step with a single product. However, the route needs storage of extremely toxic MIC; the greener chloroformate route avoids this hazard.

Example 4. Show that the overall atom economy for carbaryl from 1-naphthol, phosgene and methylamine is the same for both routes. (COCl2 = 98.92, CH3NH2 = 31.06)

Step 1: Both routes have the same overall equation: C10H7OH + COCl2 + CH3NH2 gives carbaryl + 2HCl. Step 2: Reactant mass = 144.17 + 98.92 + 31.06 = 274.15. Step 3: % AE = 201.22/274.15 x 100 = 73.40% for both. Step 4: The green advantage of the chloroformate route is therefore not atom economy but inherent safety: no MIC is formed or stored.

Example 5. Calculate the atom economy of biodiesel production from triolein: C57H104O6 + 3CH3OH gives 3C19H36O2 + C3H8O3. (Triolein = 885.45, CH3OH = 32.04, methyl oleate = 296.50)

Step 1: Reactant mass = 885.45 + 3 x 32.04 = 885.45 + 96.12 = 981.57. Step 2: Product mass = 3 x 296.50 = 889.50. Step 3: % AE = 889.50/981.57 x 100 = 90.62%. The by-product glycerol is itself a useful chemical.

Example 6. Calculate the atom economy for fermentation of glucose to ethanol. (C6H12O6 = 180.16, C2H5OH = 46.07)

Step 1: C6H12O6 gives 2C2H5OH + 2CO2. Step 2: % AE = (2 x 46.07)/180.16 x 100 = 92.14/180.16 x 100 = 51.14%. Although nearly half the mass leaves as CO2, the feedstock is renewable and the CO2 was absorbed by the crop during growth, so the fuel is nearly carbon neutral.

Important questions and answers

Q1. Compare the conventional and green routes for synthesis of indigo. (5 marks)

Conventional (Heumann-Pfleger): aniline + formaldehyde + HCN gives N-phenylglycinonitrile; hydrolysis gives N-phenylglycine; fusion with NaNH2 in NaOH/KOH at high temperature gives indoxyl; air oxidation gives indigo. Drawbacks: toxic aniline, HCN and formaldehyde; corrosive alkali melt; high energy; large salt waste; traces of aniline in product. Green (biocatalytic): engineered E. coli converts glucose to tryptophan; tryptophanase converts tryptophan to indole; naphthalene dioxygenase converts indole to indole-2,3-dihydrodiol, which dehydrates to indoxyl; air oxidation gives indigo. Advantages: renewable feedstock, aqueous medium, ambient temperature and pressure, no toxic reagents, biodegradable waste.

Q2. Compare the conventional and green synthesis of adipic acid. (6 marks)

Conventional: benzene + H2 (Ni) gives cyclohexane; air oxidation (Co catalyst) gives cyclohexanol + cyclohexanone; oxidation by conc. HNO3 gives adipic acid + N2O. Drawbacks: carcinogenic, non-renewable benzene; corrosive HNO3; N2O emission (greenhouse gas about 300 times CO2 and ozone depleting); high temperature and pressure. Green route 1 (Draths-Frost): D-glucose with genetically modified E. coli gives cis,cis-muconic acid, which is hydrogenated (H2, Pt) to adipic acid; renewable, non-toxic, no N2O. Green route 2 (Noyori): cyclohexene + 30% H2O2 with Na2WO4 and a phase-transfer catalyst [methyltrioctylammonium hydrogen sulphate], solvent-free, about 75-90 C, gives adipic acid and water only.

Q3. Discuss the conventional and green routes for the synthesis of carbaryl. (5 marks)

Conventional: CH3NH2 + COCl2 gives CH3NCO (MIC) + 2HCl; MIC + 1-naphthol gives carbaryl. MIC is highly toxic and volatile; large stored quantities leaked at Bhopal in 1984, killing thousands. Green(er) route: 1-naphthol + COCl2 gives 1-naphthyl chloroformate + HCl; the chloroformate + CH3NH2 gives carbaryl + HCl. The same raw materials and product are used, but MIC is never formed or stored, so the process is inherently safer. Phosgene is still used, so further greener options aim at phosgene-free carbonylation.

Q4. What are supercritical fluids? Give characteristics and applications of supercritical CO2. (5 marks)

A supercritical fluid is a substance above its critical temperature and pressure, where gas and liquid are indistinguishable. scCO2: Tc = 31.1 C, Pc = 73.8 bar. Characteristics: liquid-like density and solvating power; gas-like low viscosity and high diffusivity; zero surface tension so it penetrates pores; solvent power tunable by P and T; non-toxic, non-flammable, cheap, recyclable; removed by depressurisation leaving no residue. Applications: decaffeination of coffee, extraction of hops, flavours and essential oils, dry cleaning, polymerisation and hydrogenation medium, supercritical fluid chromatography, cleaning of electronic parts; supercritical water for destruction of hazardous wastes.

Q5. What are ionic liquids? Discuss their characteristics and applications. (5 marks)

Ionic liquids are salts with melting points below 100 C, composed of bulky asymmetric organic cations (1-alkyl-3-methylimidazolium, pyridinium, ammonium, phosphonium) and anions (Cl-, BF4-, PF6-, NTf2-); the bulky ions cannot pack well, so the lattice energy is low. Characteristics: negligible vapour pressure; non-flammable; thermally stable; wide liquid range; dissolve many organic, inorganic and polymeric compounds; good ionic conductivity and wide electrochemical window; recyclable; properties tunable (designer solvents). Applications: reaction media for Diels-Alder, Friedel-Crafts, hydrogenation; immobilising catalysts; dissolving cellulose; extraction; electrolytes in batteries, fuel cells, supercapacitors and electroplating.

Q6. Explain the synthesis of biodiesel and state its advantages. (5 marks)

Synthesis: vegetable oil (triglyceride) is treated with methanol (about 6:1 molar ratio) in presence of about 1% NaOH or KOH at about 60-65 C with stirring. Triglyceride + 3CH3OH gives 3RCOOCH3 + glycerol. On standing, glycerol settles as the lower layer and is removed; the methyl ester layer is washed and dried. Advantages: renewable, biodegradable, non-toxic; practically sulphur-free; lower CO, hydrocarbon and particulate emissions; higher flash point, safer to handle; higher cetane number; good lubricity; can be used in diesel engines with little or no modification and blended (B20); nearly carbon neutral; glycerol by-product.

Q7. Explain the production of ethanol by fermentation and its advantages as a fuel. (5 marks)

Molasses is diluted to a suitable sugar concentration, acidified to pH 4-5 and supplied with nutrients (ammonium salts); yeast is added and fermentation is carried out at about 30-35 C for 2-3 days. C12H22O11 + H2O (invertase) gives glucose + fructose; C6H12O6 (zymase) gives 2C2H5OH + 2CO2. The wash (8-10% ethanol) is distilled to rectified spirit (95.6%), and anhydrous ethanol is obtained by azeotropic distillation or molecular sieves. Advantages: renewable, high octane number, oxygenated so burns more completely with less CO, almost sulphur-free, nearly carbon neutral, can be blended with petrol (E10, E20), reduces crude oil import.

Q8. Define atom economy and calculate it for the Noyori synthesis of adipic acid. (3 marks)

% atom economy = molar mass of desired product/sum of molar masses of reactants x 100. C6H10 + 4H2O2 gives C6H10O4 + 4H2O: % AE = 146.14/(82.14 + 136.04) x 100 = 66.98%.

Q9. Why are green solvents needed? Compare supercritical CO2 and ionic liquids. (4 marks)

Conventional organic solvents are volatile, toxic and flammable and form a large part of chemical waste and VOC emissions. scCO2: a supercritical fluid; non-toxic, cheap, removed by depressurisation; needs high pressure equipment; good for non-polar solutes. Ionic liquids: salts liquid below 100 C; negligible vapour pressure; tunable; dissolve polar and non-polar compounds; act as electrolytes; but are costly and some are toxic or poorly biodegradable. Both reduce VOC pollution and can be recycled.

Q10. What is sustainable development? How does green chemistry contribute to it? (3 marks)

Sustainable development is development that meets present needs without compromising the ability of future generations to meet their own needs. Green chemistry contributes by preventing waste, using renewable feedstocks (glucose-based adipic acid, biofuels), using safer solvents (scCO2, ionic liquids), avoiding hazardous intermediates (MIC-free carbaryl), improving atom and energy efficiency, and producing cleaner fuels (biodiesel, ethanol).

Exam tips

• For each synthesis, write the reaction steps in a sequence with conditions, then list drawbacks of the conventional route and advantages of the green route in a table.
• Mention the Bhopal disaster (MIC, 1984) in the carbaryl answer.
• Mention N2O emission in the adipic acid answer; it is the key reason for the green route.
• Learn critical constants of CO2 (31.1 C, 73.8 bar).
• In biodiesel and ethanol answers, write the balanced reactions and at least five advantages.
• Atom economy numericals: always use the balanced equation with coefficients.

FAQs

Why is CO2 the most popular supercritical solvent?

Its critical temperature (31.1 C) and pressure (73.8 bar) are easily reached, and it is non-toxic, non-flammable, cheap and leaves no residue.

Does the green carbaryl route avoid phosgene?

No, it still uses phosgene, but it avoids forming and storing methyl isocyanate, the extremely toxic intermediate responsible for the Bhopal disaster.

Why are ionic liquids called designer solvents?

Their properties such as melting point, viscosity, polarity and miscibility can be tuned by choosing different cation-anion combinations.

Is biodiesel the same as vegetable oil?

No. Vegetable oil is a viscous triglyceride; biodiesel is its methyl ester obtained by transesterification, with much lower viscosity suited to diesel engines.

These notes follow the University of Mumbai first-year engineering syllabus (NEP 2020, Applied Chemistry) and were prepared with AI help and checked.