Chemistry • Year 12 • Module 8 • Lesson 15

Drug Synthesis & Green Chemistry

Build HSC Band 5–6 extended-response technique: evaluate synthesis routes using both quantitative metrics and green chemistry principles, and critique chemical claims using evidence.

Master · Extended Response (Band 5–6)

1. Data + scenario — evaluate aspirin synthesis routes against green chemistry criteria (Band 5–6)

8 marks   Band 5–6

Scenario. A research group at CSIRO Clayton is optimising the synthesis of acetylsalicylic acid (aspirin) for incorporation into a controlled-release formulation. They have identified three candidate routes (X, Y, Z). Route X uses the traditional acid-anhydride esterification with an acid catalyst and organic solvent. Route Y uses a biocatalyst (immobilised lipase enzyme) in aqueous solution at room temperature. Route Z uses a step-shortened route via a different acetylating agent but in a halogenated solvent.

Route Atom economy (%) E-factor Solvent Temperature Catalyst type
X 75 2.8 Ethyl acetate (organic) 80°C Acid (H3PO4)
Y 75 1.2 Water 25°C Biocatalyst (lipase)
Z 68 1.9 Dichloromethane (DCM) 0–5°C None (stoichiometric reagent)

Q1. Evaluate the three routes using green chemistry principles. In your response you must:

  • Define atom economy and E-factor and explain what each measures.
  • Compare all three routes on at least three green chemistry criteria (e.g. atom economy, E-factor, solvent selection, energy efficiency, catalysis).
  • Identify the single most and least sustainable route, providing quantitative evidence from the data.
  • Explain why Route Y's biocatalyst is consistent with green chemistry even though atom economy is the same as Route X.
  • Reach a justified overall recommendation that acknowledges trade-offs, not just one metric.
Stuck? Plan first: define both metrics → compare each route on 3 criteria → use data as evidence → address the biocatalyst question specifically → make a recommendation that weighs atom economy, E-factor, solvent and energy together. Revisit lesson Cards 3, 4 and 5.

2. Source critique — evaluate a pharmaceutical industry claim (Band 5–6)

7 marks   Band 5–6

“The pharmaceutical manufacturer's newly optimised aspirin synthesis achieves an atom economy of 92%, so the production process is now highly sustainable and waste concerns have been effectively resolved. The high atom economy also means the E-factor will be correspondingly low, and no further green chemistry improvements are necessary.”

Source: Adapted from a fictional industry sustainability brochure (illustrative example for HSC practice).

Q2. Critique this claim. In your response:

  • Identify the one element of the claim that is scientifically defensible.
  • Identify and explain at least two scientific errors or unjustified assumptions in the claim.
  • Explain how the errors would be detected by measuring or reporting additional data.
  • Reformulate the claim into a scientifically accurate statement about sustainability in pharmaceutical synthesis.
Stuck? Think about what atom economy does and does not measure. Does a high atom economy guarantee a low E-factor? Does it capture solvent waste? Does it reflect energy use? Revisit lesson Cards 3, 4 and the Misconceptions box.
Answers — Do not peek before attempting

Q1 — Sample Band 6 response (8 marks), annotated

Atom economy is a percentage metric defined as (molar mass of desired product / total molar mass of all products) × 100%. It measures how efficiently reactant atoms are incorporated into the wanted compound at the reaction level. E-factor is the mass of waste produced divided by the mass of product obtained (kg/kg or g/g). It measures practical waste burden across the entire process, including solvents, auxiliaries and byproducts. Both metrics are needed because atom economy reflects the reaction equation but E-factor reflects the real process. [1 — both metrics defined and contrasted]

Comparing on atom economy: Routes X and Y share the highest atom economy at 75%, meaning 75% of product-side atom mass is the desired aspirin. Route Z is lower at 68%, indicating more product-side atom mass is diverted to unwanted species. [1 — atom economy comparison with data]

Comparing on E-factor: Route Y has by far the lowest E-factor (1.2), meaning only 1.2 g of waste is generated per gram of aspirin. Route Z is next (1.9) and Route X is highest (2.8). Route X therefore generates 2.3 times as much waste per gram of product as Route Y. [1 — E-factor comparison with quantitative evidence]

Comparing on solvent selection: Route Y uses water (non-toxic, non-flammable, abundant, no VOC concerns), which aligns directly with the green chemistry principle of safer solvents. Route X uses ethyl acetate (flammable, VOC risk, but lower hazard than halogenated solvents). Route Z uses dichloromethane, a probable carcinogen and persistent environmental contaminant — a major sustainability concern. [1 — solvent selection criterion compared]

Comparing on energy efficiency: Route Y operates at 25°C (ambient), requiring minimal energy input. Route X requires heating to 80°C. Route Z requires cooling to 0–5°C. Both X and Z consume energy for temperature control; Y aligns with the green chemistry principle of energy efficiency. [1 — energy criterion]

Route Y's biocatalyst (lipase) is consistent with green chemistry even though atom economy equals Route X, because atom economy is a property of the reaction stoichiometry, not the conditions or auxiliaries. The biocatalyst operates at ambient temperature in water, eliminating high-temperature heating (reducing energy use) and replacing a mineral acid with a biodegradable, selective catalyst that generates no corrosive or inorganic waste. This reduces the practical E-factor and hazard profile without altering the atom economy of the reaction. [1 — biocatalyst explanation]

Most sustainable route: Route Y, because it combines the joint-highest atom economy (75%), the lowest E-factor (1.2), the safest solvent (water), and the most energy-efficient conditions (25°C). Least sustainable: Route X for E-factor, but Route Z is arguably least sustainable overall because DCM solvent and no catalyst (stoichiometric reagent) introduce serious hazard concerns even though its E-factor (1.9) is intermediate. [1 — most/least sustainable with quantitative evidence]

Recommendation: Route Y should be pursued as the primary route. Its advantages on E-factor, solvent and energy efficiency outweigh any scale-up challenges with biocatalyst production. Route X is a viable fallback if biocatalyst stability proves difficult to optimise at industrial scale; Route Z should be avoided due to DCM hazard. [1 — justified recommendation acknowledging trade-offs]

Marking criteria:

  • 1 mark — Defines both atom economy and E-factor correctly and distinguishes what each measures.
  • 1 mark — Compares all three routes on atom economy using data.
  • 1 mark — Compares all three routes on E-factor using quantitative data.
  • 1 mark — Compares routes on solvent selection, linking to the green chemistry principle.
  • 1 mark — Compares routes on energy efficiency (temperature).
  • 1 mark — Explains why the biocatalyst in Route Y improves sustainability even with equivalent atom economy.
  • 1 mark — Identifies most and least sustainable route(s) with specific quantitative evidence.
  • 1 mark — Provides a justified recommendation that acknowledges trade-offs (not a one-metric conclusion).

Q2 — Sample Band 6 critique (7 marks), annotated

Defensible element: An atom economy of 92% is genuinely high and indicates that almost all product-side atom mass ends up in the desired aspirin, with minimal atom loss to byproducts. A high atom economy is a positive green chemistry indicator. [1 — correctly identifies the one valid element]

Error 1 — Atom economy does not determine E-factor: The claim assumes that high atom economy implies a correspondingly low E-factor. This is scientifically incorrect. Atom economy is a stoichiometric property of the reaction equation; E-factor is a process metric that includes all masses of solvent, auxiliaries, catalyst and other waste beyond what appears in the balanced equation. A synthesis can achieve 92% atom economy and still have a high E-factor if large quantities of organic solvent, washing agents or purification reagents are used. These are invisible in the atom economy calculation. [1 — identifies the error; 1 — explains the mechanism correctly]

Error 2 — Sustainability requires more than atom economy: The claim says “waste concerns have been effectively resolved” on the basis of atom economy alone. This ignores the other 11 Green Chemistry Principles: solvent selection, energy efficiency, design for degradation, avoidance of auxiliaries, safety, renewable feedstocks, and others. For example, even with 92% atom economy, if the process uses a hazardous solvent at high temperature, the process is not sustainable in those dimensions. [1 — identifies second error with named green chemistry principles]

How to detect the errors experimentally: Measure the E-factor directly (weigh all waste streams including solvent, measure product mass) and report it alongside atom economy. Conduct a full process mass intensity (PMI) audit or life cycle assessment (LCA) to capture solvents, water and energy use. These measurements would reveal whether the process E-factor is actually low, and whether waste concerns have genuinely been resolved beyond the reaction stoichiometry. [1 — identifies correct experimental / measurement approach]

Reformulated accurate statement: “An atom economy of 92% indicates that the reaction step efficiently incorporates most reactant atoms into the desired aspirin product, which is a positive green chemistry outcome. However, overall process sustainability requires evaluating E-factor (actual waste burden), solvent selection, energy efficiency and other green chemistry principles before concluding that waste concerns have been resolved.” [1 — defensible reformulation using precise terminology]

Marking criteria:

  • 1 mark — Identifies the one defensible element (high atom economy is a positive indicator).
  • 1 mark — Identifies Error 1 (atom economy does not determine E-factor).
  • 1 mark — Explains why Error 1 is wrong: E-factor captures process waste (solvents, auxiliaries) invisible to atom economy.
  • 1 mark — Identifies Error 2 (sustainability requires more than atom economy — names additional green chemistry principles).
  • 1 mark — Explains how the errors would be detected (measure E-factor directly / conduct a PMI or LCA audit).
  • 1 mark — Reformulates the claim into a scientifically accurate, nuanced statement.
  • 1 mark — Uses precise lesson terminology throughout (atom economy, E-factor, green chemistry principles, process metrics) with no factual errors.

HSCScience • Chemistry • Year 12 • Module 8 • Lesson 15 • Worksheet 3 (Master)