Química Ambiental · Green chemistry
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O que você vai aprender
- Match process changes to the twelve principles of green chemistry.
- Calculate atom economy, E-factor, process mass intensity, reaction mass efficiency and turnover number from stated masses and molar masses.
- Compare catalytic and stoichiometric routes, using the Boots and BHC syntheses of ibuprofen as the model case.
- Choose safer solvents and reagents, such as dimethyl carbonate in place of dimethyl sulfate or phosgene, and explain the mechanisms that make them work.
- Draw mechanisms that turn renewable feedstocks and CO₂ into products, and explain how molecular structure controls biodegradability.
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Twelve principles, one idea
Green chemistry designs hazards and waste out of a process instead of treating them afterwards. The twelve principles are: prevent waste; maximise atom economy; use less hazardous syntheses; design safer chemicals; use safer solvents and auxiliaries; design for energy efficiency; use renewable feedstocks; reduce derivatives such as protecting groups; prefer catalytic to stoichiometric reagents; design products to degrade; analyse in real time to prevent pollution; and choose inherently safer chemistry to prevent accidents. Most real improvements satisfy several principles at once.
Atom economy
Atom economy (AE) = molar mass of the desired product ÷ sum of the molar masses of all reactants in the balanced equation × 100%. It measures how much of the reactant mass can end up in the product even at 100% yield. Additions, cycloadditions and rearrangements reach 100%; substitutions and eliminations always make by-products. Making 1-bromobutane from butan-1-ol, NaBr and H₂SO₄ gives AE = 137.02 ÷ (74.12 + 102.89 + 98.07) = 49.8%, because NaHSO₄ and water are formed too. Catalysts and solvents are left out of atom economy because they do not appear in the balanced equation.
E-factor, PMI and reaction mass efficiency
The E-factor is the mass of waste per mass of product; it counts unrecovered solvent, excess reagents, by-products and yield losses, usually leaving water out. Process mass intensity (PMI) is the total mass put in per mass of product, so PMI = E + 1 when both are counted on the same basis. Typical E-factors rise from below 0.1 in oil refining to about 1–5 for bulk chemicals, 5–50 for fine chemicals and 25–100 or more for pharmaceuticals, mainly because of solvents and multistep syntheses. Reaction mass efficiency (RME) = mass of isolated product ÷ total mass of reactants; unlike atom economy, it falls when an excess of one reactant is used or the yield is low.
Catalysis: two routes to ibuprofen
The original Boots synthesis of ibuprofen took six steps with stoichiometric reagents, including AlCl₃ for a Friedel–Crafts acylation, and had an atom economy of about 40%. The BHC route takes three catalytic steps: acylation of isobutylbenzene with acetic anhydride in anhydrous HF, which is the catalyst and solvent and is recovered; hydrogenation of the ketone over Raney nickel; and palladium-catalysed carbonylation of the alcohol with CO. Its atom economy is about 77%, and close to 99% when the acetic acid by-product is recovered. AlCl₃ must be used in a full equivalent because the ketone product binds it; a true catalyst is regenerated, and its efficiency is measured by the turnover number, moles of product per mole of catalyst.
Safer solvents and reagents
Solvents are usually the largest part of the waste. Pharmaceutical solvent-selection guides such as the CHEM21 guide rank ethanol, water and ethyl acetate as recommended, toluene as problematic, dichloromethane as hazardous and benzene as highly hazardous. 2-Methyltetrahydrofuran, made from biomass, separates cleanly from water, unlike THF. Supercritical CO₂ (above 31 °C and 74 bar) is a non-toxic solvent that leaves no residue when the pressure is released. Dimethyl carbonate is a low-toxicity reagent that replaces dimethyl sulfate or methyl iodide for methylation, and phosgene for making carbamates: at about 90 °C amines attack its carbonyl carbon, while above about 120 °C phenoxides attack a methyl group and the methyl carbonate leaving group falls apart to CO₂ and methoxide.
Renewable feedstocks and design for degradation
Biomass supplies carbon that petroleum would otherwise provide. Fructose loses three molecules of water to give 5-(hydroxymethyl)furfural, a platform chemical; fermentation of sugars gives lactic acid, the monomer behind polylactic acid; vegetable oils are transesterified with methanol to biodiesel; and CO₂ adds to epoxides to give cyclic carbonates with 100% atom economy. Renewable and biodegradable are different properties: polyethylene made from sugarcane ethanol is renewable but persists. Molecules degrade when microbes can attack them, so ester links and linear chains help, while quaternary carbons and perfluorinated chains resist breakdown.
What to carry forward
Measure greenness with numbers: atom economy from the balanced equation, E-factor and PMI from real masses, turnover number for catalysts. Then improve the process by using catalysts instead of stoichiometric reagents, choosing safer solvents and reagents such as dimethyl carbonate, starting from renewable carbon and designing products that degrade.
Problemas de prática (40)
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Desenhe o mecanismo 6
Catalysis and the ibuprofen routes
- HF-catalysed Friedel–Crafts acylation of isobutylbenzene (BHC ibuprofen route)Draw the first step of the BHC ibuprofen synthesis: HF protonates acetic anhydride, the acetylium ion forms, it attacks isobutylbenzene para to the alkyl group, and fluoride restores the aromatic ring and regenerates HF.
Renewable feedstocks and degradation
- Base-catalysed methanolysis of a triglyceride (biodiesel)Draw the methoxide-catalysed transesterification that converts one ester of a triglyceride into a methyl ester, the reaction used to make biodiesel from vegetable oils.
- Diels–Alder reaction of bio-based furan with maleic anhydrideDraw the concerted Diels–Alder cycloaddition of furan and maleic anhydride, a 100% atom-economical reaction of a biomass-derived diene, and explain why the exo adduct is isolated.
- Bromide-catalysed addition of CO₂ to propylene oxideDraw the halide-catalysed cycloaddition of CO₂ to propylene oxide: ring opening by bromide, capture of CO₂ by the alkoxide, and ring closure that gives propylene carbonate and regenerates bromide.
Safer solvents and reagents
- Methyl carbamate from butylamine and dimethyl carbonateDraw the addition–elimination of butylamine at the carbonyl carbon of dimethyl carbonate that gives methyl N-butylcarbamate, a phosgene-free route to carbamates and isocyanates.
- Methylation of phenol with dimethyl carbonateDraw the base-catalysed O-methylation of phenol by dimethyl carbonate: SN2 attack of phenoxide on a methyl group, fragmentation of methyl carbonate to CO₂ and methoxide, and regeneration of phenoxide.
Preveja o produto 3
Catalysis and the ibuprofen routes
- BHC step three: carbonylationPredict the product of the palladium-catalysed carbonylation that completes the BHC ibuprofen route.
- BHC step two: hydrogenating the ketonePredict the product of the catalytic hydrogenation step in the BHC ibuprofen route.
Renewable feedstocks and degradation
- Dehydrating fructosePredict the platform chemical formed when fructose loses three molecules of water.
Classificar 7
Atom economy calculations
- Which reaction wastes no atoms?Recognise that addition reactions reach 100% atom economy while substitutions, eliminations and condensations do not.
Principles and green metrics
- Do not store the hazardConnect the storage of a large inventory of a toxic intermediate to the principle of inherently safer chemistry.
- Stop making the wasteIdentify the prevention principle from a change that removes a waste stream at its source.
- An in-line IR probeRecognise real-time analysis for pollution prevention in an in-process monitoring scheme.
- An enzyme removes a protecting group stepIdentify the reduce-derivatives principle when an enzymatic step removes the need for a protecting group.
Renewable feedstocks and degradation
- Renewable is not the same as biodegradableClassify polymers by whether they come from renewable feedstocks and whether they biodegrade, treating these as independent properties.
Safer solvents and reagents
- An oxidant that leaves only waterIdentify the oxidant whose only by-product after oxygen transfer is water.
Comparar 6
Atom economy calculations
- Rank four reactions by atom economyCompute and rank the atom economies of an elimination, a substitution, a condensation and a cycloaddition.
Catalysis and the ibuprofen routes
- Why the BHC ibuprofen route makes less wasteCompare the Boots and BHC syntheses of ibuprofen by step count, reagent type and atom economy.
Principles and green metrics
- How E-factor and PMI relateRelate the E-factor to process mass intensity using the mass balance of a process.
- Waste per kilogram across the chemical industryRank industry sectors by their typical E-factor and relate the trend to volume and the number of steps.
Safer solvents and reagents
- Rank solvents with a selection guideOrder common solvents by their standing in the CHEM21 pharmaceutical solvent-selection guide.
Renewable feedstocks and degradation
- Which surfactant do microbes break down?Compare surfactant structures to decide which is readily biodegradable, applying the principle of design for degradation.
Calcular 8
Catalysis and the ibuprofen routes
- Atom economy of the BHC ibuprofen routeCalculate the overall atom economy of the three-step BHC synthesis of ibuprofen from its net equation.
- Turnover number of a palladium catalystCalculate a catalyst's turnover number from its loading and the reaction yield.
Atom economy calculations
- Atom economy of making 1-bromobutaneCalculate the atom economy of an alcohol-to-alkyl bromide substitution that uses NaBr and sulfuric acid.
- Atom economy of making a cyclic carbonate from CO₂Calculate the atom economy of the catalytic addition of CO₂ to an epoxide.
- Reaction mass efficiency of an esterificationCalculate reaction mass efficiency when one reactant is used in excess and the yield is below 100%.
- Atom economy of a Wittig methylenationCalculate the atom economy of a Wittig reaction and see the cost of the triphenylphosphine oxide by-product.
Safer solvents and reagents
- Atom economy of methylation with dimethyl carbonateCalculate the atom economy of the O-methylation of phenol by dimethyl carbonate.
Principles and green metrics
- E-factor with solvent recoveryCalculate an E-factor from a batch mass balance in which most of the solvent is recovered.
Encontre o erro 2
Principles and green metrics
- A perfectly green Diels–Alder?Diagnose why a 100% atom economy does not make a process green when solvent and purification waste dominate.
Renewable feedstocks and degradation
- Biodiesel from waste cooking oil goes wrongDiagnose why free fatty acids in a waste oil spoil a base-catalysed biodiesel reaction and choose the fix.
Interprete as evidências 1
- High yield, low atom economyInterpret yield and atom economy data together to judge how much reactant mass becomes product.
Trabalhe de trás para frente 1
- Working back from polylactic acidIdentify the fermentation-derived monomer of polylactic acid from the polymer's repeat unit.
Planeje uma síntese 2
Catalysis and the ibuprofen routes
- Choose the greenest alcohol oxidationPlan an alcohol oxidation by comparing catalytic and stoichiometric oxidants by their by-products and hazards.
Safer solvents and reagents
- Replace THF in a Grignard workupChoose a biomass-derived ether solvent that separates from water to replace THF.
Explique por quê 4
Catalysis and the ibuprofen routes
- Why AlCl₃ is not a catalyst in acylationExplain why Friedel–Crafts acylation consumes a full equivalent of AlCl₃ and how the BHC route avoids that waste.
Safer solvents and reagents
- Similar atom economy, different hazardExplain why dimethyl carbonate is preferred to dimethyl sulfate even though their atom economies are similar.
- Decaffeinating coffee with supercritical CO₂Explain why supercritical CO₂ is regarded as a green solvent for extraction.
Atom economy calculations
- Using excess ethanolExplain how a stoichiometric excess of one reactant affects atom economy and the E-factor differently.
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Depois, Scan Standard: /mês