Chemistry • Year 12 • Module 8 • Lesson 11
Drug Classification & Functional Groups
Build HSC band 5–6 extended-response technique by evaluating data, critiquing sources, and applying structure–activity reasoning to real drug molecules.
1. Extended response — functional groups, solubility and drug design (Band 5–6)
8 marks Band 5–6
Scenario & data
A pharmaceutical research team is designing a new analgesic intended to relieve pain without causing gastric irritation. They are comparing three candidate molecules (A, B and C) to ibuprofen as a reference compound. The table below summarises their key structural features and aqueous solubility at pH 7.4 (physiological pH).
| Molecule | Key functional groups | Aqueous solubility at pH 7.4 | Gastric irritation (animal model) |
|---|---|---|---|
| Ibuprofen (reference) | −COOH, aromatic ring, branched alkyl | Low (0.021 mg/mL) | Moderate |
| Molecule A | −OH (alcohol), aromatic ring, branched alkyl | Very low (0.008 mg/mL) | Minimal |
| Molecule B | −COOH, −NH2, aromatic ring | High (12.4 mg/mL) | Low |
| Molecule C | −CONH− (amide), aromatic ring, branched alkyl | Moderate (1.3 mg/mL) | Low |
Data: hypothetical research scenario, functional group consequences consistent with HSC Chemistry Module 8 content.
Q1. Evaluate which candidate molecule (A, B or C) is the most promising replacement for ibuprofen as an analgesic with reduced gastric side-effects. In your response you must:
- Define the term pharmacophore and explain its relevance to designing a replacement for ibuprofen.
- Compare candidates A, B and C on at least three criteria drawn from the data (functional groups, solubility, gastric irritation) and explain the chemical reasoning behind each comparison.
- Identify which functional group in ibuprofen is most likely responsible for its moderate gastric irritation and explain the mechanism.
- Reach an evidence-based judgement about which candidate is most promising and what trade-off it involves.
2. Source critique — evaluating a media claim about drug molecules (Band 5–6)
7 marks Band 5–6
“Paracetamol and aspirin are essentially the same drug — they both relieve pain, they are both inexpensive, and the difference is just branding. The molecules are almost identical because they are both small organic compounds containing an aromatic ring and oxygen-containing groups. Chemists could easily make one from the other by a simple reaction step. Any doctor who recommends one over the other is just influenced by advertising.”
Source: paraphrased from an online health forum post, 2023.
Q2. Evaluate this claim. Identify the specific scientific flaw(s) in the statement, explain the correct chemistry, and discuss how the difference between paracetamol and aspirin could be demonstrated experimentally.
3. Evaluate this claim about drug classification (Band 5–6)
6 marks Band 5–6
“The TGA’s approval process is unnecessary for common medicines like paracetamol and ibuprofen because these drugs have been used safely for decades. Requiring manufacturing registration and clinical evidence just drives up the price for Australian patients and achieves nothing that basic chemistry cannot already guarantee. A molecule that has a known functional group profile cannot be harmful.”
Source: paraphrased from a consumer advocacy blog, 2022.
Q3. Evaluate this claim. Identify which parts, if any, are scientifically defensible, and explain using your knowledge of functional groups, structure–activity relationships, and the TGA registration framework why the claim contains significant scientific errors. Reformulate the claim into a biologically and chemically defensible statement.
Q1 — Sample Band 6 response (8 marks), with marking criteria
A pharmacophore is the specific arrangement of features in a drug molecule responsible for the key interactions that produce biological activity. For ibuprofen as an analgesic, the carboxylic acid group (−COOH) and the adjacent aromatic ring are critical features of the pharmacophore, as they interact with the COX enzyme target. Any replacement must retain sufficient pharmacophore characteristics to maintain binding, but ideally without the side-effects linked to −COOH. [1 — pharmacophore definition + relevance]
Comparing the three candidates on functional groups, solubility and gastric irritation: Molecule A replaces −COOH with −OH (alcohol), eliminating the acidic group — this reduces gastric irritation to minimal, but the resulting very low aqueous solubility (0.008 mg/mL) is a serious concern for bioavailability (a drug must dissolve to be absorbed). Molecule A also no longer has an ionisable group, meaning it cannot form the carboxylate anion that aids solubility at physiological pH. [1 — Molecule A analysis] Molecule B retains −COOH and adds −NH2, achieving high solubility (12.4 mg/mL) due to the amine group’s basic character and capacity for ionisation — however, it still contains −COOH, so gastric irritation is only “low” rather than “minimal”. [1 — Molecule B analysis] Molecule C replaces −COOH with an amide (−CONH−), which is polar and capable of hydrogen bonding but is not acidic. Gastric irritation is low, and moderate solubility (1.3 mg/mL) is considerably better than Molecule A, suggesting adequate bioavailability. The amide group can still interact with polar residues in a target binding site, offering pharmacophore continuity. [1 — Molecule C analysis]
The functional group in ibuprofen most likely responsible for moderate gastric irritation is the carboxylic acid (−COOH). At gastric pH (pH 1–2), −COOH is largely protonated (unionised), allowing the molecule to penetrate gastric mucosal cells by passive diffusion; once inside the more neutral intracellular environment it ionises, trapping the carboxylate ion inside the cell and causing irritation. This “ion-trapping” mechanism is well-documented for NSAIDs. [1 — −COOH identified + mechanism]
Overall judgment: Molecule C is the most promising candidate. It achieves minimal gastric irritation (by removing −COOH), maintains moderate aqueous solubility (better than Molecule A, acceptable for oral dosing), and the amide group preserves some capacity for polar interactions relevant to a target binding site. The trade-off is that the amide pharmacophore is not identical to the carboxylic acid pharmacophore, so biological potency against the original COX target may differ and would need to be confirmed by in vivo testing. [1 — evidence-based judgement; 1 — trade-off identified; 1 — link to TGA or further testing context (accept if mentioned)]
Marking criteria (8 marks):
- 1 mark — Correct definition of pharmacophore and its relevance to replacing ibuprofen’s activity.
- 1 mark each — Chemically reasoned comparison of each candidate (A, B, C) on at least one criterion from the data, using functional group knowledge. (3 marks total)
- 1 mark — Identifies −COOH as the source of gastric irritation and explains the mechanism (ion-trapping or direct mucosal effect).
- 1 mark — Evidence-based judgement identifying one candidate as most promising with data justification.
- 1 mark — Identifies a genuine trade-off or limitation of the chosen candidate (e.g. potency uncertainty, bioavailability concern).
Q2 — Sample Band 6 source critique (7 marks), with marking criteria
The claim contains several significant scientific flaws. [1 — overall evaluative judgement]
Flaw 1: “Essentially the same drug.” Paracetamol and aspirin have different functional groups, different molecular frameworks, different pharmacophores, and different biological targets. Aspirin contains an ester (−COO−) and a carboxylic acid (−COOH); paracetamol contains a phenol (−OH on aromatic ring) and an amide (−CONH−). The pharmacophore of aspirin irreversibly acetylates the COX enzyme; paracetamol’s mechanism is distinct and still partially debated. They are not equivalent therapeutically: aspirin is anti-inflammatory; paracetamol is not an effective anti-inflammatory. [2 marks: 1 for identifying different functional groups; 1 for correct pharmacological distinction]
Flaw 2: “Easily made from one another.” The two molecules require completely different synthetic routes and starting materials. Aspirin is synthesised by esterification of salicylic acid; paracetamol is synthesised by acetylation of 4-aminophenol with acetic anhydride. Converting one to the other would involve breaking and forming multiple bonds and would not be a simple reaction step. [1 mark]
Flaw 3: “A molecule with a known functional group profile cannot be harmful.” This is incorrect: functional group identity alone does not determine safety. Dose, stereochemistry (chirality), impurities, metabolic activation, and interaction with other drugs all determine whether a drug is harmful in a given patient. Structure–activity relationships (SAR) confirm that very small structural changes can dramatically alter toxicity. The TGA’s manufacturing assessment ensures batch-to-batch consistency, correct dose, absence of harmful impurities — none of which are guaranteed by functional group knowledge alone. [1 mark]
Experimental detection of the difference: The two drugs can be distinguished by (a) pH testing in aqueous solution — aspirin is more acidic than paracetamol due to its −COOH; (b) hydrolysis testing in dilute acid — aspirin’s ester hydrolyses to give salicylic acid (detected by FeCl3 colourimetric test), whereas paracetamol’s amide is much more resistant to hydrolysis; (c) infrared (IR) spectroscopy — the two molecules show distinct absorption patterns, especially in the C–O–C ester stretch for aspirin vs the N–H amide stretch for paracetamol. [1 mark for any two valid experimental methods]
Marking criteria (7 marks): 1 — overall evaluative judgement; 1 — functional groups of each drug correctly identified as different; 1 — pharmacological difference correctly stated (different targets/mechanisms); 1 — “easily made from one another” refuted with reference to synthesis; 1 — functional group profile alone does not guarantee safety, with at least one supporting reason; 1 — at least two experimental methods described that would detect the difference; 1 — response uses correct chemical terminology throughout (pharmacophore, functional group names, SAR).
Q3 — Sample Band 6 response (6 marks), with marking criteria
The claim is largely flawed, though one element is partially defensible. [1 — overall judgement]
What is defensible: Long-term widespread use of a drug does generate real-world safety and efficacy data, which is valuable information in regulatory decision-making. It is reasonable to state that an established safety record is evidence the TGA can use. [1 — concedes defensible element]
What is wrong: Functional group profile does not guarantee safety. SAR demonstrates that even molecules with the same functional groups can have dramatically different biological effects depending on molecular geometry, stereochemistry, substitution pattern and metabolic pathway. The ester group in aspirin, for example, is hydrolysed in vivo to acetic acid and salicylic acid — products with their own biological effects not predictable from the ester group alone. A molecule’s functional group profile tells you about reactivity tendencies, not about toxicity in a complex biological system. [1 — SAR / functional groups alone insufficient for safety guarantee]
What is wrong: Manufacturing and formulation matter. The TGA does not just assess molecular identity. It assesses manufacturing quality, excipient safety, dissolution profiles, batch consistency, labelling and dosing accuracy. An impure batch of paracetamol containing trace residues from synthesis could cause liver damage even at nominally “safe” doses. The “basic chemistry” argument ignores process chemistry and quality assurance. [1 — manufacturing / TGA role correctly stated]
Defensible reformulation: “Well-established drugs like paracetamol and ibuprofen have extensive real-world safety data that informs TGA decisions. However, the TGA registration process remains necessary because functional group identity alone does not guarantee safety — dose, manufacturing quality, stereochemistry, metabolic products and drug interactions must all be assessed. The cost of regulation should be weighed against the cost of harm from unregulated pharmaceutical manufacturing.” [1 — defensible reformulation]
Marking criteria (6 marks): 1 — overall evaluative judgement stated; 1 — defensible element correctly identified; 1 — explains why functional group profile alone is insufficient for safety (e.g. SAR, metabolic products, stereochemistry); 1 — explains TGA’s manufacturing / formulation / quality role; 1 — uses at least one correct lesson term (pharmacophore, SAR, functional group, TGA, metabolite) accurately; 1 — biologically and chemically defensible reformulation.