HSC Exam Practice

Sample response. A chiral centre is a carbon atom that is bonded to four different groups (substituents). Because the four groups are all different, two non-superimposable mirror-image three-dimensional arrangements are possible.

Marking notes. 1 mark for “carbon bonded to four different groups/substituents”; 1 mark for consequence (two non-superimposable mirror-image arrangements possible / gives rise to enantiomers).

Sample response. Structural isomers have the same molecular formula but differ in how the atoms are connected (different bonding arrangements). Enantiomers have the same molecular formula and the same connectivity but differ in three-dimensional spatial arrangement around a chiral centre; they are non-superimposable mirror images of each other.

Marking notes. 1 mark — structural isomers differ in connectivity (bonding arrangement); 1 mark — enantiomers have the same connectivity; 1 mark — enantiomers are non-superimposable mirror images differing in 3D spatial arrangement.

Sample response. A racemic mixture is a 50:50 mixture of both enantiomers of a chiral compound. Its optical rotation is zero (no net rotation) because the two enantiomers rotate plane-polarised light in equal but opposite directions, and the rotations cancel each other out exactly.

Marking notes. 1 mark — 50:50 mixture of both enantiomers; 1 mark — optical rotation is zero / no net rotation; 1 mark — explains why (equal and opposite rotations cancel).

Sample response. A polarimeter passes plane-polarised light through the drug sample and measures the angle by which the plane of polarisation is rotated. If the sample is a pure enantiomer (or an unequal mixture of enantiomers), a non-zero rotation is measured and the sample is described as optically active. If the rotation is zero, the sample is either achiral or a racemic mixture. Polarimetry therefore allows a chemist to distinguish between a pure enantiopure preparation and a racemic mixture of a chiral drug.

Marking notes. 1 mark — plane-polarised light is passed through the sample and the angle of rotation is measured; 1 mark — a non-zero rotation indicates the sample is optically active (single enantiomer or unequal mixture); 1 mark — zero rotation indicates achiral compound or racemic mixture (rotations cancel).

Sample response. Enzymes are themselves chiral macromolecules built from L-amino acids. This means their active sites have a specific three-dimensional shape that fits one enantiomer of a substrate but not its mirror image, so only one enantiomer can bind and react.

Marking notes. 1 mark — enzymes are chiral (built from L-amino acids / have chiral active sites); 1 mark — only one enantiomer fits the active site’s 3D shape.

Sample response. Thalidomide has a chiral centre, producing two enantiomers. The drug was formulated and sold as a racemic mixture, meaning each dose contained both the R- and S-enantiomers. The R-enantiomer produced the intended sedative effect. The S-enantiomer, however, was teratogenic — it interacted with chiral biological receptors in the developing foetus through a different mechanism and caused severe developmental defects (limb abnormalities in particular) in babies born to women who took the drug during pregnancy. Because the drug contained the S-form by definition as a racemate, every patient who received the drug also received the teratogenic enantiomer.

Marking notes. 1 mark — thalidomide has a chiral centre and exists as two enantiomers; 1 mark — sold as a racemic mixture so patients received both enantiomers; 1 mark — R-enantiomer = sedative, S-enantiomer = teratogenic (both named correctly); 1 mark — biological specificity: S-enantiomer interacted with chiral foetal receptors causing developmental defects.

Sample response (a). (R)-ibuprofen plasma concentration rises from 0 at time 0, reaches a maximum of approximately 10–12 μg/mL at about 1.5–2 hours after the dose, then falls progressively back toward zero by 6 hours. The curve is lower in peak and falls more rapidly than (S)-ibuprofen.

Marking notes (a). 1 mark — describes R-ibuprofen rising then falling; 1 mark — identifies approximate peak time (~1.5–2 h) or peak concentration (~10–12 μg/mL; accept ±2 μg/mL).

Sample response (b). The original racemic dose contained 50% (S)- and 50% (R)-ibuprofen. Initially, both enantiomers appear in plasma in roughly equal amounts. However, the liver enzyme 2-arylpropionic acid epimerase metabolically converts a significant proportion of the (R)-form to the (S)-form (in-vivo enantiomeric interconversion). This conversion is one-way under physiological conditions (R → S but not S → R). The result is that (R)-ibuprofen is progressively depleted from plasma as it is converted, while the (S)-ibuprofen pool is continuously replenished from the converted R-form, explaining why its plasma concentration is higher and more sustained than expected from the initial 50% dose alone.

Marking notes (b). 1 mark — original racemic dose was 50/50; 1 mark — (R)-ibuprofen undergoes in-vivo enzymatic conversion to (S)-ibuprofen (interconversion in the body); 1 mark — this explains the elevated and sustained (S)-ibuprofen concentration and the declining (R) concentration.

Sample response (a). The optical rotation of a 50:50 racemic mixture of L-alanine and D-alanine would be zero degrees. The L-form rotates light by +14.7° and the D-form rotates it by −14.7°; in equal proportions the rotations cancel completely.

Marking notes (a). 1 mark — zero degrees / no net rotation; 1 mark — justification that equal and opposite rotations of the two enantiomers cancel.

Sample response (b). L-glucose and D-glucose are enantiomers: they have the same molecular formula and the same connectivity, but their three-dimensional arrangements around the chiral centres differ as non-superimposable mirror images. Human metabolic enzymes (such as hexokinase) have chiral active sites shaped specifically to bind and catalyse reactions with D-glucose. This is biological specificity: the enzyme’s three-dimensional active site has a precise steric complementarity to D-glucose. L-glucose, being the mirror image, cannot fit the active site correctly — the spatial arrangement of hydroxyl groups and hydrogen atoms around its chiral centres does not match the enzyme’s binding pocket — so the enzyme cannot bind or process it.

Marking notes (b). 1 mark — L-glucose and D-glucose are enantiomers (same formula, same connectivity, different 3D arrangement); 1 mark — enzymes have chiral active sites shaped to fit D-glucose; 1 mark — biological specificity: L-glucose’s mirror-image 3D arrangement means it cannot fit the enzyme’s active site, so it is not metabolised.

Sample response. Chirality is central to modern pharmaceutical development because biological systems are themselves chiral, and the three-dimensional arrangement of a drug molecule — not just its molecular formula or connectivity — determines how it interacts with biological targets. A chiral centre is a carbon bonded to four different groups, which gives rise to two non-superimposable mirror-image enantiomers. Enantiomers have the same bonds and the same connectivity, but their different spatial arrangements mean they interact differently with chiral biological macromolecules such as enzymes and receptors — this is called biological specificity.

The thalidomide case is the most significant historical example. Thalidomide was sold as a racemic mixture (50:50 R and S). The R-enantiomer was the intended sedative, but the S-enantiomer was teratogenic, causing severe birth defects in babies born to women who took the drug during pregnancy. The Australian TGA now operates a birth-defect monitoring registry for thalidomide use, and the drug is only dispensed under strict conditions. This case demonstrated that for chiral drugs, each enantiomer must be evaluated as if it were a separate compound, because chiral receptors in the body will respond to them differently.

Ibuprofen illustrates a more nuanced situation. (S)-ibuprofen is the active form, inhibiting COX-2 to produce its anti-inflammatory effect, while (R)-ibuprofen is largely inactive at the COX-2 receptor. Despite this, racemic ibuprofen remains on the Australian PBS as an over-the-counter analgesic because the body’s metabolic enzymes convert a significant proportion of R- to S-ibuprofen in the liver. The racemic formulation therefore still delivers effective pain relief without the safety catastrophe seen with thalidomide. This shows that the significance of chirality must be assessed drug-by-drug based on the specific biological activity, safety profile, and pharmacokinetics of each enantiomer.

The global trend toward enantiopure drugs (single-enantiomer formulations) reflects a growing regulatory and scientific consensus that when enantiomers differ in biological activity or safety, administering only the therapeutically relevant form is preferable. CSIRO chiral synthesis research has expanded the practical ability to produce enantiopure compounds. However, chiral synthesis is more expensive and technically demanding than producing a racemate, and for drugs like ibuprofen where in-vivo interconversion occurs and the inactive enantiomer is harmless, the racemic formulation remains justified.

In summary, chirality is significant in pharmaceutical development because biological specificity means enantiomers can have fundamentally different therapeutic effects, side effects and safety profiles. The thalidomide tragedy established that racemic formulations can be dangerous and drove regulatory change; ibuprofen demonstrates that the decision must be context-specific; and the industry’s overall shift toward enantiopure drugs reflects the scientific importance of three-dimensional molecular architecture in medicine.

Marking criteria (one mark each):

• Defines chiral centre correctly (carbon bonded to four different groups; gives rise to enantiomers).
• Defines or explains biological specificity (chiral enzymes/receptors distinguish enantiomers by 3D shape).
• Thalidomide: names R = sedative, S = teratogenic; sold as racemate; consequence = birth defects; Australian TGA context.
• Ibuprofen: names S = active COX-2 inhibitor, R = inactive; explains why racemic form still works (in-vivo R→S conversion in liver).
• Explains why the same molecular formula and connectivity is not sufficient to guarantee the same biological effect (difference is 3D arrangement / chirality).
• References the regulatory or industrial shift toward enantiopure drugs and a reason for it (OR a limitation such as cost/interconversion).
• Reaches an explicit, nuanced evaluative judgement: chirality is significant but context-specific; racemic vs enantiopure decision must be drug-by-drug; neither “always use racemate” nor “never use racemate” is correct as a blanket rule.