Describe and explain structural isomerism (chain, position and functional group isomers) and stereoisomerism (cis-trans / geometric isomerism in alkenes) in organic compounds

What this dot point is asking

QCAA wants you to identify isomers from a molecular formula, classify them as chain, position, functional-group or geometric isomers, and explain the structural conditions that produce each type. Isomerism is a high-yield IA3 and EA topic because it links naming, structure, properties, and reactivity in a single question.

The answer

Isomers are different compounds that share the same molecular formula. Two broad classes appear in QCE Chemistry Unit 4: structural (constitutional) isomers, and stereoisomers (specifically cis-trans / geometric isomers).

Structural isomers

Structural isomers have the same molecular formula but different atom connectivity. There are three sub-types in the Unit 4 syllabus.

Chain isomers. Same functional group and substituents, different carbon skeleton (branching).

Example. C5H12 has three chain isomers:

• pentane (CH3-CH2-CH2-CH2-CH3): straight chain
• 2-methylbutane: one branch
• 2,2-dimethylpropane: two branches on a central carbon

All three are alkanes; only the skeleton differs. Properties differ measurably (boiling points 36, 28 and 9 degrees C respectively).

Position isomers. Same carbon skeleton and same functional group, but the functional group sits at a different position.

Example. C4H10O alcohols:

• butan-1-ol (OH on C1)
• butan-2-ol (OH on C2)

Both four-carbon straight chains, both alcohols; only the OH locant differs. Reactivity differs: butan-1-ol is a primary alcohol (oxidises to butanal then butanoic acid); butan-2-ol is secondary (oxidises to butan-2-one only).

Functional-group isomers. Same molecular formula, different functional group (and different homologous series).

Common Unit 4 pairs:

• alcohols and ethers (CnH2n+2O)
• aldehydes and ketones (CnH2nO, from n = 3)
• carboxylic acids and esters (CnH2nO2)

Example. C2H6O.

• ethanol (CH3-CH2-OH): alcohol.
• methoxymethane (CH3-O-CH3): ether.

Ethanol boils at 78 degrees C; methoxymethane boils at -24 degrees C. The functional group changes hydrogen-bonding capability, which dominates the property difference.

Stereoisomers: cis-trans (geometric) isomerism in alkenes

Stereoisomers have identical connectivity but different spatial arrangement. The Unit 4 syllabus covers cis-trans isomerism in alkenes.

Conditions for cis-trans isomerism:

1. A C=C double bond (rotation around C=C is restricted by the pi bond).
2. Each carbon of the double bond must carry two different groups.

If either condition fails (no double bond, or one end has two identical groups), there is no cis-trans pair.

Naming convention. When the higher-priority group on each sp2 carbon sits on the same side of the double bond, the isomer is "cis"; opposite sides is "trans".

Example. But-2-ene (CH3-CH=CH-CH3):

• cis-but-2-ene: both methyl groups on the same side.
• trans-but-2-ene: methyl groups on opposite sides.

Both are C4H8, both are but-2-ene by IUPAC numbering, both have identical connectivity. They differ only in geometry around the C=C.

Property differences. Cis isomers usually have a small net dipole; trans isomers are usually symmetrical and so non-polar. Cis tends to have slightly higher boiling point (dipole-dipole adds to dispersion) but lower melting point (less efficient crystal packing). The cis vs trans melting and boiling difference is a common QCAA stimulus.

Worked counter-example. But-1-ene (CH2=CH-CH2-CH3): the terminal carbon has two H atoms. Both groups on one end of C=C are identical, so condition 2 fails. No cis-trans pair exists. The same logic explains why prop-1-ene, 2-methylpropene and isobutene do not have cis-trans isomers.

Why the distinction matters

Isomerism is not just a naming exercise. QCAA past papers use isomerism to test:

• Property prediction. Why does butan-1-ol boil higher than methoxypropane (functional-group isomers, same Mr)?
• Reactivity contrast. Why does butan-2-ol form a ketone on oxidation but butan-1-ol forms an aldehyde then a carboxylic acid (position isomers, different oxidation outcome)?
• Spectroscopic interpretation. Why does the 1H NMR of methoxymethane show a single peak while ethanol shows three (functional-group isomers, different chemical environments)?
• Polymer design. Why does trans-polyisoprene (gutta-percha) behave as a rigid plastic while cis-polyisoprene (natural rubber) is elastic?

Each of these connects an isomerism category to a measurable Unit 4 property.

Drawing isomers efficiently

For a question asking for "all isomers of C4H10O":

1. Start with the longest straight-chain alcohol. Move OH along the chain to generate position isomers.
2. Generate chain isomers (branch the carbon skeleton, then position OH on each non-equivalent carbon).
3. Generate functional-group isomers. For CnH2n+2O, the other series is ether.
4. Check each for duplication (drawing the same skeleton twice is a common slip).

For C4H10O, the seven isomers are: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol, methoxypropane, 2-methoxypropane, and ethoxyethane. QCAA typically asks for "any four" with names.

Common traps

Drawing rotated versions as new isomers. Rotating a structural formula in 2D does not produce a new isomer. Check connectivity, not appearance.

Forgetting to check the second cis-trans condition. "It has a C=C double bond so it must have cis-trans isomers" is wrong. Both ends of the double bond need two different substituents.

Calling functional-group isomers "structural isomers" without qualifying. All three subtypes are structural isomers; QCAA marking guides usually want the specific subtype named.

Confusing E/Z notation with cis/trans. The Unit 4 syllabus uses cis/trans. E/Z is more rigorous (uses Cahn-Ingold-Prelog priority) and is the only valid notation when each sp2 carbon has three different substituents. For Unit 4, the simpler cis/trans suffices.

Examples in context

Example 1. Octane isomers in Gladstone refinery petrol blending. Caltex/Ampol Gladstone refinery blends petrol from straight-chain heptane and branched 2,2,4-trimethylpentane (iso-octane). Both have formula $\text{C}_8 \text{H}_{18}$ but iso-octane has highly branched structure giving a research octane number of $100$; straight n-octane gives $-20$. Branching disrupts knocking by reducing pre-ignition. These are structural isomers (chain isomers). Refiners run a catalytic reformer at $500^{\circ}\text{C}$ to convert straight-chain alkanes into branched isomers and cyclic aromatics, raising the octane rating of mainstream unleaded fuel to $91$.

Example 2. Geometric isomerism in cane-juice unsaturated lipids. Cane-derived oils contain oleic acid (cis-9-octadecenoic acid) and elaidic acid (trans-9-octadecenoic acid). Both have identical molecular formula $\text{C}_{18} \text{H}_{34} \text{O}_2$ but the cis isomer has a kinked chain (the $\text{C} = \text{C}$ double bond locks both alkyl groups on the same side) while the trans is straight. The kink prevents close packing: cis oleic acid melts at $14^{\circ}\text{C}$ (liquid at room temp); trans elaidic at $44^{\circ}\text{C}$ (solid). Industrial hydrogenation that produces trans fats was banned in many countries because trans fats raise LDL cholesterol.

Try this

Q1. Define structural isomers and give two structural isomers of $\text{C}_4 \text{H}_{10}$. [3 marks]

• Cue. Same molecular formula different connectivity. Butane (straight) and 2-methylpropane (branched).

Q2. Identify the type of isomerism shown by cis- and trans-2-butene and state which has the lower boiling point. [3 marks]

• Cue. Geometric (cis-trans / E-Z) isomerism. trans-2-butene lower boiling point: less polar, dipoles cancel.

Q3. Consider 2-bromobutane. (a) Identify the chiral centre. (b) Draw both enantiomers in 3-D. (c) Explain how plane-polarised light could distinguish them. [2+3+2 marks]

• Cue. (a) C2 with H, CH$_3$, CH$_2$CH$_3$, Br. (b) Mirror images at C2. (c) Rotate polarised light equal but opposite directions.