VCE Chemistry Unit 4 deep-dive: how are organic compounds categorised, analysed and used? (2026 guide)

How Unit 4 closes the year

Unit 4 of the VCAA Chemistry Study Design 2023-2027 covers organic chemistry and food chemistry. It contributes the second SAC and a major share of the end-of-year exam.

Area of Study 1: organic chemistry

Hydrocarbon families. Alkanes (saturated, CnH2n+2, single bonds), alkenes (one C=C, CnH2n), alkynes (one C triple bond C). Naming follows IUPAC: longest carbon chain, lowest locants for substituents.

Functional groups summary.

• Haloalkane: R-X (X = F, Cl, Br, I).
• Alcohol: R-OH.
• Aldehyde: R-CHO (terminal carbonyl).
• Ketone: R-CO-R' (internal carbonyl).
• Carboxylic acid: R-COOH.
• Ester: R-COO-R'.
• Amine: R-NH2 (primary), R2NH (secondary), R3N (tertiary).
• Amide: R-CO-NH-R'.

Isomerism. Structural isomers have the same molecular formula but different connectivity. Chain (different carbon skeleton), positional (different position of substituent), functional group (different group entirely). Stereoisomers have the same connectivity but different spatial arrangement: cis-trans (E/Z) in alkenes; optical (R/S) at chiral centres.

Reactions.

Substitution. Alkane plus halogen with UV light: $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$. Haloalkane plus nucleophile: $CH_3Br + OH^- \rightarrow CH_3OH + Br^-$.

Addition. Alkene plus hydrogen halide: $CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br$. Markovnikov's rule: the H adds to the carbon already bearing more hydrogens. Alkene plus water (with H2SO4 catalyst): hydration gives an alcohol.

Oxidation. Primary alcohol oxidises to aldehyde then carboxylic acid. Secondary alcohol oxidises to ketone. Tertiary alcohol does not oxidise. Acidified dichromate or permanganate is the typical oxidant.

Esterification. Carboxylic acid plus alcohol with concentrated H2SO4 catalyst gives ester plus water. Reversible: $CH_3COOH + CH_3CH_2OH \rightleftharpoons CH_3COOCH_2CH_3 + H_2O$.

Hydrolysis. Ester plus water (acid or base catalysed) returns acid and alcohol.

Reaction pathways. Multi-step syntheses combine these reactions. Map the carbon skeleton from starting material to product; identify the bond changes; choose reagents to effect them.

Analytical techniques

Mass spectrometry. Sample ionised, accelerated, deflected by magnetic field, detected at the detector. Spectrum plots relative abundance against m/z. The molecular ion peak gives molecular mass. Fragmentation reveals structural features.

Infrared spectroscopy. Bonds vibrate at characteristic frequencies. The VCAA data book lists ranges. Broad 3200 to 3550 cm-1: O-H alcohol. Sharp 1670 to 1750 cm-1: C=O. Broad 2500 to 3300 plus C=O: carboxylic acid OH.

Nuclear magnetic resonance. Proton (1H) NMR. Three pieces of information per signal: chemical shift (electronic environment), integration (relative number of protons), splitting (n+1 neighbours). Carbon-13 NMR shows one peak per unique carbon environment.

Chromatography. Separation by differential affinity to a stationary phase. HPLC, gas chromatography. Retention time identifies; peak area quantifies.

Worked example: structure determination

Compound C4H8O. Degrees of unsaturation = (2*4 + 2 - 8) / 2 = 1.

IR shows a strong band at 1715 cm-1 (C=O), no broad OH peak. So a carbonyl, not an alcohol or acid.

1H NMR shows three signals: a singlet at 2.1 ppm (integration 3, three H), a quartet at 2.4 ppm (integration 2, two H) and a triplet at 1.05 ppm (integration 3, three H). The quartet and triplet are the ethyl group (CH3 next to CH2). The singlet at 2.1 ppm is the methyl alpha to carbonyl.

Structure: CH3-CO-CH2-CH3 (butan-2-one).

Mass spectrum confirms: M+ at 72, fragments at 43 (CH3CO+) and 57 (loss of CH3).

Area of Study 2: food chemistry

Proteins. Amino acids have the general structure H2N-CHR-COOH. Peptide bonds form between the carboxyl of one and the amino of the next, releasing water. Primary structure is the sequence. Secondary structure is alpha helix (intra-chain hydrogen bonds) and beta sheet. Tertiary is the 3D fold (hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds, hydrophobic core). Quaternary is multiple subunits.

Denaturation. Heat, pH change, or organic solvents disrupt the higher-order structure. Primary structure (covalent bonds) is preserved.

Enzymes. Biological catalysts. Active site has specific shape and chemistry. Substrate binds (induced fit). Catalyses the reaction (lower activation energy). Products released, enzyme unchanged. Optimum pH and temperature; denatures outside the range.

Carbohydrates. Monosaccharides (glucose, fructose, galactose, all C6H12O6 but different connectivity). Disaccharides linked by glycosidic bonds (sucrose = glucose + fructose; lactose = glucose + galactose; maltose = glucose + glucose). Polysaccharides: starch (plants, alpha-1,4 with alpha-1,6 branches in amylopectin), glycogen (animals, similar to amylopectin but more branched), cellulose (plants, beta-1,4 linkages, indigestible by humans).

Lipids. Triglycerides: glycerol esterified with three fatty acids. Saturated (no C=C, animal fats, solid at room temperature); monounsaturated (one C=C, olive oil); polyunsaturated (multiple C=C, fish oils). Trans fats from partial hydrogenation. Phospholipids: amphipathic, form bilayers.

Vitamins. Water-soluble (B, C) versus fat-soluble (A, D, E, K).

Energy content of food. Calorimetry: known mass burned, temperature rise of water measured. Specific energy (J per g) and energy density (J per mL).

Common VCAA Unit 4 examiner traps

• Mis-naming organic compounds (wrong longest chain, wrong locant).
• Confusing aldehyde with ketone (terminal versus internal carbonyl).
• Confusing primary, secondary, tertiary alcohols.
• Quoting IR ranges from memory rather than the data book.
• Confusing alpha helix and beta sheet.

In one sentence

Unit 4 rewards systematic organic chemistry (functional groups plus reactions plus IUPAC naming) plus integrated structure determination (combining IR plus NMR plus MS) plus food chemistry (the four biomolecule families with their structures and reactions).