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How do we name and represent organic molecules?

Functional groups, homologous series, IUPAC nomenclature, structural and displayed formulae, structural and E/Z isomerism, and reaction mechanism notation.

A focused answer to WJEC A-Level Chemistry Unit 2, covering functional groups and homologous series, IUPAC nomenclature, structural, displayed and skeletal formulae, structural and E/Z isomerism, and the curly-arrow notation for mechanisms.

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What this dot point is asking

WJEC wants you to recognise functional groups and homologous series, name organic compounds by IUPAC rules, draw structural, displayed and skeletal formulae, identify structural and E/Z isomers, and read the curly-arrow notation used in mechanisms.

The answer

Functional groups and homologous series

Key series include alkanes (CnH2n+2\text{C}_n\text{H}_{2n+2}), alkenes (CnH2n\text{C}_n\text{H}_{2n}), alcohols, halogenoalkanes, aldehydes, ketones and carboxylic acids.

IUPAC nomenclature

Isomerism

Structural isomers have the same molecular formula but different structural formulae (chain, position or functional-group isomers). E/Z (geometric) isomers arise when restricted rotation about a C=C\text{C}=\text{C} double bond locks two different groups on each carbon into fixed positions.

Mechanism notation

A curly arrow shows the movement of an electron pair, starting at a bond or lone pair and pointing to where the new bond forms. This notation underpins the substitution, addition and elimination mechanisms met throughout organic chemistry.

Types of structural isomerism

Structural isomers share a molecular formula but differ in how the atoms are connected, and they come in three kinds. Chain isomers differ in the branching of the carbon skeleton (butane and 2-methylpropane). Position isomers have the same functional group in a different place (propan-1-ol and propan-2-ol). Functional-group isomers have the same formula but different functional groups (ethanol and methoxymethane, both C2H6O\text{C}_2\text{H}_6\text{O}). Recognising the kind of isomerism is a common opening exam task and underpins later structure-determination work.

Empirical, molecular and the formula hierarchy

A molecule can be represented at several levels: the empirical formula (simplest ratio), the molecular formula (actual numbers), the structural formula (showing the arrangement, such as CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}), the displayed formula (every bond drawn) and the skeletal formula (lines for bonds, carbons implied). Being fluent in moving between these representations, and choosing the clearest one for a given purpose, is expected throughout the organic course.

Examples in context

E/Z isomerism in vision. The light-driven conversion of 11-cis-retinal to the all-trans form in the eye is the E/Z isomerism that triggers a nerve signal, showing how restricted rotation matters in biology. Naming in the chemical industry. Unambiguous IUPAC names let chemists worldwide order and synthesise the exact isomer needed, since different isomers can have very different properties and safety profiles.

Try this

Q1. Give the general formula of the alkenes. [1 mark]

  • Cue. CnH2n\text{C}_n\text{H}_{2n}.

Q2. Name the compound CH3CH2CH2COOH\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}. [1 mark]

  • Cue. Butanoic acid.

Q3. State why propene does not exhibit E/Z isomerism. [1 mark]

  • Cue. One carbon of the double bond carries two hydrogen atoms (two identical groups), so no E/Z isomers exist.

Q4. Name the type of structural isomerism shown by propan-1-ol and propan-2-ol. [1 mark]

  • Cue. Position isomerism.

Q5. State which two compounds are functional-group isomers of C2H6O\text{C}_2\text{H}_6\text{O}. [1 mark]

  • Cue. Ethanol and methoxymethane.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20193 marksGive the IUPAC name of the compound CH3CH(CH3)CH2CH2OH\text{CH}_3\text{CH(CH}_3\text{)CH}_2\text{CH}_2\text{OH} and state the homologous series to which it belongs.
Show worked answer β†’

Find the longest carbon chain containing the functional group. The OH\text{OH} is on a four-carbon chain, so the parent is butanol.

Number from the end nearest the OH\text{OH}, so the alcohol is on carbon 11 and a methyl branch is on carbon 33.

The name is 3-methylbutan-1-ol; it belongs to the alcohols.

Markers reward the longest chain, correct numbering toward the functional group, the branch position, and the homologous series.

WJEC 20204 marksBut-2-ene exhibits E/Z isomerism. Draw and name the two isomers and explain why this isomerism arises.
Show worked answer β†’

But-2-ene is CH3CH=CHCH3\text{CH}_3\text{CH}=\text{CHCH}_3. The C=C\text{C}=\text{C} double bond cannot rotate, so the two methyl groups are locked in fixed positions.

The Z (cis) isomer has both methyl groups on the same side; the E (trans) isomer has them on opposite sides.

E/Z isomerism arises because there is restricted rotation about the double bond and each carbon of the double bond carries two different groups.

Markers reward both structures, the E/Z labels, and the restricted-rotation-with-two-different-groups explanation.

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