Chemistry • Year 12 • Module 7 • Lesson 2

IUPAC Nomenclature II: Functional Group Classes & Isomers

Build HSC Band 5–6 extended-response technique — evaluate competing naming claims, synthesise data about isomers, and assess source accuracy in pharmaceutical and fuel contexts.

Master · Band 5–6

1. Extended response — evaluate isomer type claims using data (Band 5–6)

8 marks Band 5–6

Stimulus: boiling point data for C₄H₈O isomers

Compound	Condensed structural formula	Functional group	Boiling point (°C)
Butanal	CH₃CH₂CH₂CHO	Aldehyde	75
2-methylpropanal	(CH₃)₂CHCHO	Aldehyde	61
Butan-2-one	CH₃COCH₂CH₃	Ketone	80

All three compounds have the molecular formula C₄H₈O. Data: NIST Standard Reference Database.

Q1. Analyse and evaluate the structural relationships between butanal, 2-methylpropanal, and butan-2-one using the stimulus data and your understanding of IUPAC nomenclature and structural isomers.

In your response you must:

Define structural isomers and confirm with evidence from the data that all three compounds qualify.
Classify the structural relationship between butanal and 2-methylpropanal; justify using their condensed structural formulas.
Classify the structural relationship between butanal and butan-2-one; justify using their condensed structural formulas.
Use the boiling-point data to explain the difference between butanal and 2-methylpropanal (same functional group, different skeleton) AND between butanal and butan-2-one (different functional group), linking to intermolecular forces in each case.
Reach an evidence-based judgement: which structural feature (skeleton branching or functional group type) has a larger influence on boiling point for C₄H₈O, and why?

Plan first: definition check → classify each pair (chain / position / functional group) → link each bp difference to intermolecular forces → compare the two bp differences numerically to reach a judgement. Butanal vs 2-methylpropanal: same aldehyde, chain isomers (bp differ by 14°C). Butanal vs butan-2-one: functional group isomers (bp differ by 5°C). Your judgement should reflect which difference is larger and why.

2. Source critique — evaluate a media claim about pharmaceutical naming

7 marks Band 5–6

Source: extract from a general science blog (2025)

“The Therapeutic Goods Administration (TGA) requires all pharmaceutical compounds to be named using the IUPAC system so that chemists and doctors know exactly what the molecule is. For example, the painkiller paracetamol and the solvent ethyl acetate are both esters — they have the same type of –COO– linkage. Since they are made from the same kind of reaction (an alcohol reacting with an acid), they are functional group isomers of each other. Knowing the IUPAC name is enough to tell whether two drugs will have the same biological effect, because the name always specifies the full three-dimensional shape of the molecule.”

Q2. Evaluate the scientific accuracy of the extract above. Your response must:

Identify at least two distinct scientific errors or misleading claims in the extract.
For each error, state the correct chemistry using precise IUPAC or biochemical terminology.
Explain why one of the errors could have practical consequences in a TGA pharmaceutical context.
Assess whether the overall claim that “IUPAC names are sufficient for identifying biological activity” is valid, drawing on your understanding of structural isomers and functional groups.

Locate each error: (1) Is paracetamol actually an ester? Look at its functional groups — it contains an amide linkage (–NHCO–) and a phenol (–OH), not –COO–. (2) Can two compounds with different molecular formulas be functional group isomers? (3) Does IUPAC nomenclature encode 3D shape? Think about stereoisomers (not covered yet, but worth mentioning). (4) Is sharing a “type of reaction” the same as being functional group isomers?

Answers — Do not peek before attempting

Q1 — Marking guide (8 marks)

Definition + confirmation (1 mark): Structural isomers share the same molecular formula but have different structural arrangements of atoms. All three compounds have the formula C₄H₈O (confirmed from the table), so all three qualify as structural isomers of one another.

Butanal vs 2-methylpropanal — chain isomers (2 marks): Both are aldehydes (same functional group, –CHO at C1 in each). Both have the molecular formula C₄H₈O. They differ only in carbon skeleton: butanal has a straight 4-carbon chain (CH₃CH₂CH₂CHO); 2-methylpropanal has a branched 3-carbon chain with a methyl group at C2 ((CH₃)₂CHCHO). Same functional group + same formula + different skeleton = chain isomers. [1 for correct type; 1 for justification referencing both condensed formulas.]

Butanal vs butan-2-one — functional group isomers (2 marks): Same formula C₄H₈O, but butanal has a terminal C=O with H (aldehyde, suffix –al) and butan-2-one has an internal C=O between two carbons (ketone, suffix –one). Entirely different functional groups = functional group isomers. [1 for correct type; 1 for justification using structural feature of C=O position.]

Boiling point analysis using intermolecular forces (2 marks):

Butanal (75°C) vs 2-methylpropanal (61°C): both are aldehydes with the same polar C=O group, so hydrogen-bonding capacity and dipole–dipole forces are similar. The straight-chain butanal has greater molecular surface area → stronger London dispersion forces → higher bp (Δbp = 14°C). [1]

Butanal (75°C) vs butan-2-one (80°C): functional group isomers; the ketone (butan-2-one) has a slightly higher bp. Both have C=O; the ketone’s internal C=O is flanked by two electron-donating alkyl groups, increasing its polarity slightly compared with the aldehyde (one alkyl + one H). Stronger dipole–dipole forces in the ketone → marginally higher bp. [1]

Judgement (1 mark): Skeleton branching has a larger influence on boiling point for this C₄H₈O set: the bp difference due to chain branching (butanal vs 2-methylpropanal, Δ = 14°C) is greater than the bp difference due to functional group change (butanal vs butan-2-one, Δ = 5°C). This is because skeleton branching substantially reduces molecular surface area and London dispersion forces, whereas switching from an aldehyde to a ketone produces only a modest change in dipole–dipole interactions. Accept alternative evidence-based judgements if correctly supported.

Q2 — Source critique marking guide (7 marks)

Error 1: Paracetamol is not an ester (2 marks). The extract claims paracetamol contains a –COO– ester linkage. This is scientifically incorrect. Paracetamol (4-acetamidophenol) contains an amide linkage (–NHCO–) and a phenol –OH group — it has NO ester group. Identifying a drug as an ester when it is in fact an amide is a critical error in pharmaceutical chemistry. [1 for identifying the error; 1 for the correct chemistry: amide + phenol in paracetamol.]

Error 2: Functional group isomers require the same molecular formula (2 marks). The extract implies that paracetamol and ethyl acetate are functional group isomers. Structural isomers (of any kind, including functional group isomers) must have the same molecular formula. Paracetamol (C₈H₉NO₂) and ethyl acetate (C₄H₈O₂) have completely different molecular formulas — they are not isomers of any kind. Sharing a “type of reaction” (esterification or amide bond formation) does not make compounds isomers. [1 for identifying the error; 1 for the correct definition with molecular formula criterion.]

Error 3: IUPAC names do not encode 3D shape (1 mark). The extract claims that knowing the IUPAC name is sufficient to determine biological activity because it “always specifies the full three-dimensional shape.” Standard IUPAC nomenclature (as taught in this lesson) encodes the connectivity of atoms (carbon skeleton, functional groups, locants) but does NOT encode stereochemistry (chirality, cis/trans geometry) unless stereo-descriptors (R/S, E/Z) are added. Two compounds with identical IUPAC connectivity names but different stereochemistry (e.g. (R)- and (S)-enantiomers) can have completely different biological effects. [1]

Practical consequences (1 mark): If a pharmacist or regulatory chemist misidentified an amide drug as an ester (or vice versa), they could mistake the compound’s chemical stability, reactivity, and metabolic fate. Esters are hydrolysed more easily by body esterases (prodrug strategy); amides are significantly more stable under physiological conditions. Confusing the two classes could lead to incorrect dosing, toxicity assessments, or formulation decisions in TGA drug approval processes. [1]

Assessment of “IUPAC names sufficient for biological activity” (1 mark): This claim is not valid. Structural isomers (position isomers, functional group isomers, chain isomers) share the same formula but can have profoundly different biological activities — for example, propan-1-ol and propan-2-ol (position isomers) have different metabolic fates; propanal and propanone (functional group isomers) are in different reactivity classes entirely. Additionally, IUPAC names do not inherently specify stereochemistry, and biological receptors are often highly stereospecific. The TGA requires full structural characterisation, not just connectivity names. [1]