Year 12 ChemistryModule 8⏱ ~35 min6 MC · 3 Short AnswerLesson 16 of 16Core
Chemical Synthesis & Design
In 1993, Pfizer Inc. developed a green synthesis route for sertraline (Zoloft) that cut solvent use by 200 kg per kg of product, eliminated four hazardous reagent steps, and reduced process waste by 60%, winning the US EPA's Presidential Green Chemistry Challenge Award in 2002. The new route produced the same active molecule as the old one; only the sustainability metrics changed.
Today's hook: In 2002, Pfizer received the US EPA's Presidential Green Chemistry Challenge Award for redesigning the sertraline (Zoloft) synthesis. The new route still produced identical sertraline, same structure, same activity, but used 200 kg less solvent per kg of product, eliminated 4 hazardous reagent steps, and cut waste by 60%. The product was unchanged; only the process metrics improved. A student synthesises aspirin and gets an 85% yield, but their E-factor is 47 and they used three washings of dichloromethane. How can good yield still be environmentally poor, and what specific metrics would you use to show that?
0/5TASKS
1
Connect: yield is only one measure of a synthesis
Warm up first
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Worksheets
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
A student performs an aspirin synthesis and obtains crystals of product. They conclude: "The reaction was successful, so the process must also be sustainable."
Why is product formation not enough to judge whether a synthesis route is environmentally responsible?
What extra information would you need to evaluate the route properly?
Learning Intentions
Know
The reagents, conditions and products in aspirin synthesis
The stages of pharmaceutical drug development
The meanings of atom economy, E-factor and catalyst
Understand
Why green chemistry is about waste prevention, not only yield
How atom economy and E-factor assess sustainability in different ways
How catalysts can improve efficiency and reduce environmental impact
Can Do
Write the aspirin synthesis equation and identify the esterification step
Calculate atom economy and E-factor for a synthesis route
Evaluate a synthesis method using green-chemistry criteria
Key Terms
Green chemistryA design philosophy minimising hazardous substances, waste, and energy in chemical synthesis; guided by the 12 Principles of Green Chemistry.
Atom economyPercentage of reactant atoms incorporated into the desired product: atom economy = (MW of product / sum of MW of all products) × 100%.
Waste minimisationDesigning reactions to produce no or benign byproducts; reduces environmental impact and disposal costs.
Catalysis in green synthesisUsing catalysts (especially enzymatic or organocatalysts) reduces energy requirements and improves selectivity, minimising byproducts.
Solvent selectionReplacing hazardous organic solvents with water, supercritical CO₂, or ionic liquids reduces toxicity and flammability risks.
Step economyAchieving the desired product in the fewest steps possible; fewer steps reduce waste, cost, and opportunity for error.
Cross-lesson links: The acetylation example here (–OH → –OCOCH₃) revisits functional-group chemistry from Module 7 (Organic Chemistry). Green chemistry metrics apply to any chemical synthesis. Chromatographic purity analysis of the final product connects to L05 (HPLC).
Safety: Organic syntheses use flammable, volatile and often toxic solvents and reagents, so heat under reflux on a heating mantle rather than a naked flame, work in a fume cupboard, and wear gloves and eye protection. Collect organic and acidic waste in labelled containers for proper disposal, and purify and test the product before any further use.
Core Content
2
Explain: analyse a real synthesis pathway
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Synthesising Aspirin
+5 XP
An esterification route with a pharmaceutical purpose
Aspirin, or acetylsalicylic acid, is synthesised from salicylic acid and acetic anhydride. This is an esterification-style reaction in which the aspirin product is formed along with ethanoic acid.
The reaction is commonly carried out with an acid catalyst and gentle heating, then the product is crystallised and purified. In the lab this reaction is often taught as a manageable model of pharmaceutical synthesis.
Aspirin synthesis: salicylic acid + acetic anhydride → acetylsalicylic acid + ethanoic acid. Reaction type: esterification-style acetylation. Conditions: acid catalyst and gentle heating. Product isolated by crystallisation and filtration.
Pause, copy the highlighted aspirin synthesis summary into your book.
Aspirin Synthesis: salicylic acid + acetic anhydride → acetylsalicylic acid + ethanoic acid, salicylic acid is acetylated to form aspirin.
Aspirin anchor: The same core chemistry used in a school laboratory reflects industrial pharmaceutical thinking: choose reagents, manage conditions, isolate product, and then judge efficiency and sustainability.
This workflow emphasises that synthesis is not just one reaction equation. Product quality depends on reaction conditions, isolation, crystallisation, and drying as well as the chemistry itself.
Which pair of reagents is used to synthesise aspirin in this course?
3
Represent: move from discovery to approval
2
From Discovery to Approval
+5 XP
A medicine is not "finished" when the molecule is made
We just saw that aspirin synthesis is a manageable model of pharmaceutical chemistry. That raises a question: is making a molecule in the lab the end of the story for a new medicine? This card answers it → a candidate must pass a long sequence of testing and regulatory stages before it can be supplied.
Making a candidate molecule is only the start of pharmaceutical development. A successful drug must also pass a long sequence of testing and regulatory stages.
Pharmaceutical development stages in order: Discovery → Preclinical (lab and model testing) → Phase I (initial human safety) → Phase II (early efficacy) → Phase III (broader confirmation) → Regulatory approval for supply.
Pause, copy the highlighted development stages into your book.
Main question at each stage
Can we identify a promising candidate?
Does it show useful activity and acceptable safety before human trials?
Is it safe at relevant doses?
Does it show effectiveness in target patients?
Does it work safely at larger scale?
Does the evidence support use?
Stage purpose
Discovery and initial design
Preclinical (lab and model testing)
Phase I (initial human safety)
Phase II (early efficacy testing)
Phase III (broader confirmation)
Regulatory approval for supply
Big picture: Pharmaceutical chemistry connects synthesis with safety, efficacy and regulation. A good molecule alone is not enough.
Which sequence correctly describes pharmaceutical development?
4
Respond: apply a whole-process sustainability lens
3
Green Chemistry Principles in Chemical Synthesis
+5 XP
Designing routes that waste less and risk less
We just saw that a molecule must pass regulatory approval before reaching patients. That raises a question: beyond safety and efficacy, what obligations does chemistry have to the environment? This card answers it → green chemistry asks designers to prevent waste and hazard at the source, rather than treating problems afterward.
Green chemistry asks chemists to design processes that minimise waste and hazard from the start, rather than cleaning up problems afterward.
Key principles relevant to chemical synthesis include: waste prevention (produce less waste rather than treating it), atom economy (design reactions so atoms end up in the product), less hazardous synthesis (use and generate substances with little toxicity), catalysis (prefer catalytic over stoichiometric reagents), and safer solvents (avoid hazardous solvents wherever possible).
Green chemistry: design to prevent waste and hazard from the start. Key principles: waste prevention, atom economy, less hazardous chemistry, catalysis, safer solvents. High yield alone does not mean the route is green.
Pause, copy the highlighted green chemistry principles into your book.
Common error: "High yield means green synthesis." Not necessarily. A route can give a good yield yet still produce too much waste or rely on poor solvent or reagent choices.
What does a higher atom economy generally indicate?
5
Apply: distinguish atom economy from actual waste
Beyond the syllabus. Reagents, conditions, yield, purity and the environmental, social and economic evaluation of a synthesis are core. Atom economy and E-factor calculations and pharmaceutical approval stages are extension — one well-understood synthesis case is what the exam needs.
4
Atom Economy and E-Factor
+5 XP
Two ways to judge how clean a route really is
We just saw that green chemistry requires more than good yield. That raises a question: how do chemists put actual numbers on how green a synthesis route is? This card answers it → atom economy and E-factor are two quantitative measures of different aspects of waste and efficiency.
Green-chemistry evaluation needs numbers, not just impressions. Two useful measures are atom economy and E-factor.
Atom economy = (MW of desired product / total MW of all products) × 100%, higher atom economy means more atoms end up in the wanted product.
E-factor = mass of waste / mass of product, lower E-factor means less waste is generated per unit product.
Atom economy = (MW desired product / total MW all products) × 100%; higher = better. E-factor = mass of waste / mass of product; lower = better. Both metrics are needed, a route can have good atom economy but still produce a lot of practical waste.
Misconception: high atom economy guarantees an environmentally sustainable process. It does not measure solvent waste, toxicity, energy demand, selectivity or work-up materials, so it must be evaluated alongside E-factor and the wider green-chemistry principles.
Pause, copy the highlighted formulas and interpretations into your book.
These measures are related, but they are not identical. Atom economy focuses on how reaction atoms are distributed among products, while E-factor reflects the practical mass of waste generated in the process.
Worked Example 1, Calculating Atom Economy
1
Given: Aspirin is the desired product (MW = 180 g mol−1). Total molar mass of all products = 240 g mol−1.
2
Find: Atom economy.
3
Method: atom economy = (180 / 240) × 100% = 75%
4
Answer: The atom economy is 75%, meaning 75% of the product-side atom mass appears in the desired aspirin.
Worked Example 2, Calculating E-Factor
1
Given: A synthesis produces 4.0 g of product and 10.0 g of waste.
2
Find: E-factor.
3
Method: E-factor = 10.0 / 4.0 = 2.5
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Answer: The E-factor is 2.5, so 2.5 g of waste are produced for every 1 g of product.
Which statement best describes E-factor?
6
Feedback: optimise the system, not one number
5
Catalysts and Sustainable Synthesis
+5 XP
Improving efficiency without being consumed
We just saw that atom economy and E-factor measure different aspects of sustainability. That raises a question: what practical tool can improve both without appearing in either formula? This card answers it → catalysts improve rate and selectivity without being consumed, and they can reduce energy use and by-product formation in practice.
In Pfizer's redesigned sertraline synthesis, replacing a stoichiometric titanium reagent with a catalytic palladium-based catalyst in one key step eliminated an entire waste stream and reduced metal waste by 85%. The catalyst was not in the product, it was recovered and reused. In aspirin synthesis, the phosphoric acid catalyst operates the same way: it accelerates the reaction and lowers the energy barrier without appearing in the product or being consumed overall. That is why catalysts are the single most powerful lever for improving sustainability metrics without changing what the final molecule does.
Catalysts improve rate and selectivity without being consumed. They can lower energy use and reduce by-product formation. A catalyst does not directly change the atom economy formula, but it improves practical sustainability by increasing efficiency and reducing wasted reagents and energy.
Pause, copy the highlighted catalyst role into your book.
Catalyst role: A catalyst does not change the formula for atom economy directly, but it can still improve sustainability by increasing efficiency and reducing wasted reagents, energy and by-products.
Data Interpretation, Comparing Route Sustainability
Route
Atom economy
Waste / g
Product / g
E-factor
Route A
74%
6.0
3.0
2.0
Route B
62%
12.0
3.0
4.0
Route C
74%
3.0
3.0
1.0
Route C is strongest overall because it combines high atom economy with the lowest E-factor. Route A is better than Route B, but still produces more waste than Route C. This shows why one metric alone is not enough.
Interpret: A good HSC response does not just pick the highest atom economy. It checks whether the practical waste burden also supports that conclusion.
Why can catalysts improve sustainability in pharmaceutical synthesis?
The aspirin workflow above shows the sequence. These are the factors that decide which route you run in the first place.
6
Beyond Pharmaceuticals, Cleaning Products and Biofuels
+5 XP
The same atom-economy and E-factor lens applies to any industrial synthesis
We just saw that a catalyst improved sustainability in the aspirin route without changing the product. That raises a question: does this same evaluation apply outside pharmaceuticals? This card answers it → the syllabus also names cosmetics, cleaning products and fuels as synthesis contexts, and two worked case studies show the identical atom-economy and E-factor thinking applied there.
Green-chemistry evaluation is not unique to medicines. Any industrial synthesis, a detergent, a fuel, a cosmetic ingredient, can be judged with the same atom economy and E-factor tools already used for aspirin.
Case Study 1, Surfactant Synthesis (Saponification)
Household and industrial cleaning products rely on surfactants, soap is the simplest example. Soap is made by saponification: a triglyceride (fat or oil) is heated with concentrated NaOH(aq), producing soap (the sodium salt of a fatty acid) and glycerol as a by-product. This is the same base-catalysed hydrolysis of an ester link met with fats and oils in Module 6/7.
Saponification: triglyceride + 3NaOH(aq) → 3 soap (sodium fatty-acid salt) + glycerol, an ester-hydrolysis reaction run at industrial scale.
1
Given: A tristearin-based saponification produces soap (desired product, total mass 918 g mol−1 across the 3 units formed) and glycerol (by-product, 92 g mol−1).
Answer: Soap-making has a high atom economy because glycerol, the only by-product, is itself a valuable, saleable co-product (used in cosmetics and pharmaceuticals), so very little of the reaction mass is true waste.
Case Study 2, Biodiesel Synthesis (Transesterification)
Biodiesel is produced by transesterification: a triglyceride reacts with methanol under acid or base catalysis, producing biodiesel (fatty-acid methyl esters) and glycerol as the by-product, the same by-product seen in soap-making.
Transesterification: triglyceride + 3CH3OH(l) → 3 biodiesel (methyl ester) + glycerol, catalysed by NaOH(aq) or KOH(aq).
1
Given: A transesterification produces biodiesel (desired product, total mass 895 g mol−1 across the 3 units formed) and glycerol (92 g mol−1).
Answer: Biodiesel production reaches a comparably high atom economy to soap-making. Large-scale biodiesel plants also typically report a much lower E-factor than pharmaceutical synthesis, because the process runs at bulk scale with a single, reusable by-product rather than multiple purification and testing steps.
Green-chemistry evaluation extends beyond pharmaceuticals. Soap-making (saponification: triglyceride + NaOH → soap + glycerol) and biodiesel production (transesterification: triglyceride + methanol → biodiesel + glycerol) both reach roughly 90% atom economy, and both treat glycerol as a valuable co-product rather than pure waste, which is why their practical E-factors tend to be lower than most pharmaceutical routes.
Pause, copy the highlighted case-study summary into your book.
Common thread: Aspirin, soap and biodiesel are different products from different industries, but every route is judged with the same tools: what fraction of atoms end up in the wanted product, and how much waste is generated per gram of product.
Soap-making (saponification) and biodiesel production (transesterification) both produce the same by-product. What is it?
🔬Predict, Then Reveal+8 XP
Aspirin synthesis from salicylic acid and acetic anhydride produces aspirin and acetic acid as the only by-product. Calculate the atom economy and predict whether this meets the green chemistry ideal of 100% atom economy.
Your predictionExpert answerCompare
Atom economy = 180 / (138 + 102) × 100 = 75%. It is below the 100% ideal because some atoms enter the acetic-acid co-product. Sustainability still also depends on recovery, solvents, energy, hazards and catalysis.
Your calculation and interpretation matched.
Separate the numerical metric from the wider evaluation.
Account for both products before evaluating sustainability.
P
Practice: evaluate a synthesis with multiple criteria
Complete the Learn phase to unlock Practice.
Activities
ACTIVITY 1, Calculate the Green-Chemistry Metrics
Do the calculation, then explain what the number means for sustainability.
1. A route has desired product molar mass 150 g mol−1 and total molar mass of all products 250 g mol−1. Calculate atom economy.
2. A reaction produces 5.0 g of product and 7.5 g of waste. Calculate E-factor.
3. Which is more sustainable: a route with atom economy 80% and E-factor 1.0, or a route with atom economy 65% and E-factor 3.0? Explain briefly.
4. A biodiesel synthesis (transesterification) produces 895 g mol−1 of methyl-ester biodiesel (the desired product) alongside 92 g mol−1 of glycerol by-product. Calculate the atom economy and explain why the glycerol is not simply treated as waste.
A2
Activity 2
Use the aspirin example to connect reaction chemistry with development and sustainability decisions.
1. Identify the reagents and products in aspirin synthesis and state the type of reaction.
2. Explain why a catalyst can improve sustainability even though it does not become part of the final balanced equation products.
3. Why does a successful synthesis still need preclinical testing, clinical trials and regulatory approval before a drug can be supplied widely?
Check Your Understanding
DO
Adaptive practice
Calculate accurately, then explain what each metric includes and leaves out.
MC
Multiple Choice
1. Which pair of reagents is used to synthesise aspirin in this course?
2. Which sequence correctly describes pharmaceutical development?
3. What does a higher atom economy generally indicate?
4. Which statement best describes E-factor?
5. Why can catalysts improve sustainability in pharmaceutical synthesis?
6. Soap-making (saponification) and biodiesel production (transesterification) both produce the same by-product. What is it, and why does this improve their practical sustainability?
SA
Short Answer
1. Describe the synthesis of aspirin from salicylic acid and acetic anhydride, including reagents, conditions, products and reaction type. (4 marks)
2. Explain the difference between atom economy and E-factor, and why both are useful when evaluating a synthesis route. (5 marks)
3. Evaluate the sustainability of an aspirin synthesis route that has moderate yield, atom economy of 75%, E-factor of 3.0 and requires a catalyst. In your answer, refer to waste, atom use and the role of the catalyst. (5 marks)
Show All Answers
Activity 1
1. Atom economy = (150 / 250) × 100% = 60%.
2. E-factor = 7.5 / 5.0 = 1.5.
3. The route with atom economy 80% and E-factor 1.0 is more sustainable because it uses atoms more efficiently and generates less waste per gram of product.
4. Atom economy = (895 / (895 + 92)) × 100% ≈ 90.7%. The glycerol is not treated as pure waste because it is a valuable co-product, sold for use in cosmetics, food and pharmaceutical industries, which improves the real-world sustainability of the process beyond what the atom economy figure alone suggests.
Activity 2
1. The reagents are salicylic acid and acetic anhydride. The products are aspirin and ethanoic acid. The reaction is an esterification-style acetylation process.
2. A catalyst can improve sustainability by increasing rate and selectivity, lowering energy demand and helping reduce wasted reagents or by-products even though it is not consumed overall.
3. Synthesis success is not enough because a drug must still be shown to be safe and effective through preclinical testing, clinical trials and regulatory approval.
Multiple Choice
Aspirin route: salicylic acid reacts with acetic anhydride.
Development: discovery proceeds through preclinical and phased clinical evidence before approval.
Atom economy: a higher value means more reactant atoms enter the desired product.
E-factor: mass of waste divided by mass of product.
Catalysis: catalysts can improve rate and selectivity without being consumed overall.
6. A both saponification and transesterification release glycerol, which is captured and reused as a co-product rather than discarded, improving practical sustainability.
Short Answer Model Answers
Q1 (4 marks): Aspirin is synthesised by reacting salicylic acid with acetic anhydride, usually under acid-catalysed conditions with gentle heating. The desired product is acetylsalicylic acid, and ethanoic acid is also formed. The reaction is an esterification-style acetylation process. After reaction, aspirin can be crystallised, filtered and dried.
Q2 (5 marks): Atom economy measures the fraction of product-side atom mass that appears in the desired product. E-factor measures the mass of waste produced per mass of product obtained. Atom economy is useful because it shows how well atoms are directed into the wanted molecule at the reaction level. E-factor is useful because it reflects the practical waste burden of the process. Both are needed because a route may look good by one metric but still generate too much waste overall.
Q3 (5 marks): This route has some strengths but is not ideal. An atom economy of 75% suggests that a reasonable proportion of atoms end up in the desired aspirin, so the route is moderately efficient in atom use. However, an E-factor of 3.0 means the process still generates 3 g of waste for every 1 g of product, which is a significant waste burden. The catalyst is a positive feature because it can improve rate and selectivity and may reduce energy use or unwanted side products. Overall, the route is workable and moderately sustainable, but there is still room to improve waste reduction and overall process efficiency.
Return to Think First
Return to the 2002 Pfizer sertraline green synthesis challenge. Now that you can calculate atom economy and E-factor, evaluate the original versus redesigned routes.
Why would a route that achieves 85% yield but uses 200 kg of solvent waste per kg of product score poorly on E-factor, even if it scores well on yield and atom economy?
How did Pfizer's catalytic palladium step improve the E-factor of the sertraline synthesis without changing the molecular structure of the final product at all?
Write one sentence explaining why a student who achieves an 85% yield in aspirin synthesis but uses three dichloromethane washes should still be concerned about the sustainability of their procedure.
✓
Review: check what actually stuck
QUIZ
Quick Quiz
Review
What are the reagents, products and catalyst used in aspirin synthesis?
How do you calculate atom economy? What does a higher value mean?
Atom economy = (MW of desired product / total MW of all products) × 100%. A higher value means a greater fraction of atoms from the reactants end up in the desired product, more efficient use of atoms.
How do you calculate E-factor? What does a lower value mean?
E-factor = mass of waste / mass of product. A lower E-factor means less waste is generated per gram of product, a more environmentally efficient process.
List the stages of pharmaceutical drug development in order.
Discovery → Preclinical testing → Phase I clinical trials (safety) → Phase II (early efficacy) → Phase III (broader comparison) → Regulatory approval.
Why does a catalyst improve sustainability without changing the atom economy formula?
A catalyst is not consumed in the overall reaction and does not appear in the balanced equation as a product or reactant, so it does not change the atom economy calculation. However, it improves sustainability in practice by increasing rate and selectivity, reducing energy demand and minimising by-products and wasted reagents.
Name two other industries (besides pharmaceuticals) that use the same atom-economy/E-factor evaluation, and state their shared by-product.
Soap-making (saponification of a triglyceride with NaOH) and biodiesel production (transesterification of a triglyceride with methanol) both produce glycerol as a by-product, which is reused as a valuable co-product rather than discarded.
