HSC Exam Practice

Sample response. A pharmacophore is the specific arrangement of atoms and functional groups within a drug molecule that is responsible for the key interactions (e.g. hydrogen bonding, ionic, hydrophobic) with a biological receptor, producing the drug’s therapeutic effect. Medicinal chemists must identify it because any modification to the pharmacophore could alter binding affinity, potency or selectivity; and because any part of the molecule outside the pharmacophore can potentially be modified to improve solubility, reduce side-effects, or enhance delivery without losing activity.

Marking notes. 1 mark for a correct definition of pharmacophore (the specific structural feature responsible for biological activity / receptor binding). 1 mark for linking pharmacophore to receptor binding and biological effect. 1 mark for explaining why medicinal chemists must identify it (so they know what must be preserved during molecular modification / drug optimisation).

Sample response. Aspirin contains (1) a carboxylic acid group (−COOH): this gives aspirin its weakly acidic character because −COOH can donate a proton to water, and it also increases the molecule’s polarity. (2) An ester group (−COO−): this linkage is susceptible to hydrolysis in aqueous acidic or basic conditions, which is why aspirin breaks down in the stomach or intestine to give salicylic acid and acetic acid; it also contributes polarity to the molecule. (An aromatic ring is also present but is not a key “characteristic” functional group in the same sense; accept if mentioned with correct property.)

Marking notes. 2 marks for carboxylic acid: 1 for correct name/notation, 1 for correct chemical property (weak acid behaviour / proton donation / polarity). 2 marks for ester: 1 for correct name/notation, 1 for correct chemical property (hydrolysable in aqueous acid or base / susceptible to hydrolysis). Do not award marks for simply naming the groups without a chemical property.

Sample response. An analgesic is a medicine that relieves pain; examples include aspirin, paracetamol, ibuprofen and morphine. An antibiotic is a medicine that kills or inhibits the growth of bacteria; examples include penicillin and amoxicillin. They differ in therapeutic target: analgesics act on pain pathways (typically COX enzymes or opioid receptors), while antibiotics act on bacterial cellular processes (e.g. cell-wall synthesis).

Marking notes. 1 mark for correct definition of analgesic. 1 mark for correct definition of antibiotic. 1 mark for one named example of each (2 examples total; accept any correct named example per class). Award the 3rd mark if at least one example is given for each class.

Sample response. Paracetamol contains two polar functional groups: a phenolic −OH (hydroxyl attached directly to the aromatic ring) and an amide (−CONH−). Both groups are capable of forming hydrogen bonds (the −OH as both donor and acceptor; the amide N−H as donor and C=O as acceptor), making paracetamol highly polar overall and relatively water-soluble. Ibuprofen also contains a polar carboxylic acid (−COOH), but it has a large branched isobutyl alkyl chain that is non-polar, giving the molecule significant hydrophobic character. As a result, ibuprofen has much lower aqueous solubility than paracetamol and exhibits “mixed polarity”, whereas paracetamol is more uniformly polar.

Marking notes. 1 mark for correctly identifying paracetamol’s functional groups (phenol/phenolic −OH and amide). 1 mark for explaining hydrogen bonding from these groups. 1 mark for correctly noting ibuprofen’s large non-polar alkyl chain alongside its −COOH. 1 mark for a clear comparison stating paracetamol is more uniformly polar than ibuprofen due to the absence of a large non-polar region.

Sample response. The TGA (Therapeutic Goods Administration) is the Australian Government’s regulatory body responsible for assessing the safety, quality and efficacy of medicines before they are approved for sale in Australia. It administers the Australian Register of Therapeutic Goods (ARTG). Before a new drug can be legally sold, the manufacturer must have the drug listed on the ARTG (i.e. submit evidence from clinical trials and manufacturing quality assessments for TGA evaluation and registration). Manufacturers must also hold a Manufacturing Licence from the TGA.

Marking notes. 1 mark for identifying TGA as the Australian body that regulates medicine safety, quality and efficacy before market authorisation. 1 mark for correctly stating one required step (e.g. registration on the ARTG, submission of clinical evidence, holding a Manufacturing Licence). 1 mark for explaining what the TGA assesses (safety + quality + efficacy), not just naming the body.

Sample response. Although both morphine and codeine contain hydroxyl groups, pharmacological activity depends not just on which functional groups are present, but on the three-dimensional shape and overall structure of the pharmacophore and how it fits the opioid receptor binding site. Codeine has a methoxy group (−OCH₃) in place of one phenolic −OH found in morphine. This structural difference changes the geometry and polarity of the pharmacophore, resulting in codeine binding to the opioid receptor with lower affinity than morphine. The lock-and-key model explains why shape and functional group arrangement, not just group identity, determine potency. (Codeine is also a prodrug; it is metabolised to morphine in vivo — accept this as additional context if provided.)

Marking notes. 1 mark for establishing that functional group identity alone does not determine potency — three-dimensional shape/pharmacophore also matters. 1 mark for identifying a specific structural difference between morphine and codeine (e.g. −OCH₃ vs −OH; or note that codeine’s methyl-capped hydroxyl changes pharmacophore geometry). 1 mark for explaining the consequence: altered binding affinity at the opioid receptor = different potency.

Sample response. Paracetamol has an aqueous solubility of 14.0 mg/mL, while ibuprofen is only 0.021 mg/mL — approximately 670 times less soluble [1 — data quoted]. Ibuprofen has a large branched isobutyl alkyl chain attached to the aromatic ring, creating a significant non-polar (hydrophobic) region [1]. This non-polar region reduces interaction with water molecules and decreases aqueous solubility substantially. Paracetamol, by contrast, contains an amide (−CONH−) and a phenol (−OH), both polar groups that increase intermolecular interactions with water via hydrogen bonding and dipole interactions [1], giving it much higher aqueous solubility despite also having an aromatic ring. The data confirm that it is not the aromatic ring alone that determines solubility, but the overall balance of polar and non-polar functional groups [1].

Marking notes. 1 mark for quoting or comparing relevant data values. 1 mark for identifying ibuprofen’s large non-polar alkyl chain as the cause of its low solubility. 1 mark for identifying paracetamol’s polar groups (−OH and amide) as enabling hydrogen bonding with water. 1 mark for the explicit conclusion that the net balance of polar vs non-polar functional groups determines the solubility difference.

Sample response. Aspirin and ibuprofen both contain a carboxylic acid group (−COOH), which acts as a Brønsted–Lowry acid by donating a proton (H⁺) to water: −COOH ⇌ −COO⁻ + H⁺. This produces an excess of H⁺ ions in solution, giving acidic character. Morphine, by contrast, does not contain a carboxylic acid. Its acid–base behaviour is dominated by its tertiary amine group (−N<, listed in the table). An amine is a Brønsted–Lowry base: the lone pair on nitrogen accepts a proton from water, forming an ammonium-type ion and releasing OH⁻: −N: + H₂O ⇌ −NH⁺ + OH⁻. This produces excess hydroxide, giving basic character in solution. The table therefore reflects the direct relationship between functional group type and acid–base behaviour: −COOH → acidic; tertiary amine → basic.

Marking notes. 1 mark for identifying −COOH as the source of acidic character in aspirin and ibuprofen and stating that it donates H⁺. 1 mark for identifying the tertiary amine in morphine as the source of basic character. 1 mark for correctly describing the proton-donation mechanism for −COOH (or equivalent Brønsted–Lowry language). 1 mark for correctly describing the proton-acceptance mechanism for the amine.

Sample response. The claim is partially correct but significantly overstated. Identifying functional groups does allow useful predictions about general chemical properties such as polarity, acid–base behaviour, hydrogen-bonding capacity and likely solubility. For example, knowing that aspirin contains −COOH correctly predicts it will be weakly acidic and capable of proton donation to water. Knowing that paracetamol contains an amide and a phenol correctly predicts stronger hydrogen bonding and greater aqueous solubility than ibuprofen. To this limited extent, functional group identification has genuine predictive value. However, the claim fails when applied to pharmacological properties because pharmacological effect — including potency, receptor selectivity and biological mechanism — depends on the three-dimensional structure of the pharmacophore, not just which functional groups are present. Structure–activity relationships (SAR) demonstrate this clearly: morphine and codeine both contain hydroxyl groups, yet their potencies at the opioid receptor differ substantially because the three-dimensional arrangement of the pharmacophore, and the fit with the receptor binding site, differ between the two molecules. The misconception in the lesson — “drugs with the same functional group always have the same pharmacological effect” — is precisely this error. Furthermore, chirality is a structural feature that cannot be detected by functional group identification alone: the two enantiomers of ibuprofen have the same functional groups but different pharmacological activities. SAR also means that simply replacing or shifting one functional group can alter potency dramatically, even if another group of the same type remains. In summary: functional group identification is a necessary starting point for understanding drug chemistry, but it is insufficient on its own to predict pharmacological properties, which require three-dimensional pharmacophore analysis and SAR reasoning.

Marking criteria (7 marks):

• 1 mark — Provides an overall evaluative judgement (e.g. “partially correct but insufficient”) and does not simply agree or disagree without reasoning.
• 1 mark — Concedes that functional group identification does allow prediction of some chemical properties (polarity, acid–base behaviour, solubility trends) with a correct example.
• 1 mark — Correctly states that pharmacological effect depends on pharmacophore 3D shape, not just group identity.
• 1 mark — Uses a named drug example to illustrate that the same functional group in different molecules can give different pharmacological effects (e.g. morphine vs codeine −OH; or aspirin vs ibuprofen both having −COOH but different mechanisms).
• 1 mark — Correctly defines or applies the concept of structure–activity relationships (SAR) in the context of the argument.
• 1 mark — Identifies at least one structural factor beyond functional group identity that affects pharmacological activity (chirality / stereochemistry; 3D pharmacophore geometry; receptor complementarity; molecular shape).
• 1 mark — Response is structured as a coherent argument with a clear conclusion, uses precise lesson terminology (pharmacophore, SAR, functional group, binding affinity, receptor) and does not contain contradictions.