Drug Synthesis & Green Chemistry
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.
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?
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
Core Content
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.
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.
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.
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 drug 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.
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 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.
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
Worked Example 2 — Calculating E-Factor
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.
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.
Atom economy = MW(aspirin) / [MW(salicylic acid) + MW(acetic anhydride)] x 100 = 180 / (138 + 102) x 100 = 75%. This does not meet the ideal of 100% - 25% of atoms end up as acetic acid by-product. However, acetic acid has commercial value and can be recovered, which partially offsets this. Green chemistry also considers solvent choice, energy use, and catalyst efficiency - not just atom economy alone.
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Activities
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.
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
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?
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.
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
1. B — aspirin is synthesised from salicylic acid and acetic anhydride.
2. D — this is the correct sequence from discovery to approval.
3. A — higher atom economy means more atoms end in the desired product.
4. C — E-factor is mass of waste divided by mass of product.
5. B — catalysts can improve efficiency and selectivity without being consumed.
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 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
What are the reagents, products and catalyst used in aspirin synthesis?
How do you calculate atom economy? What does a higher value mean?
How do you calculate E-factor? What does a lower value mean?
List the stages of pharmaceutical drug development in order.
Why does a catalyst improve sustainability without changing the atom economy formula?