How do chemists name, classify and represent organic molecules, and what is isomerism?
Functional groups and homologous series, IUPAC nomenclature, empirical, molecular, structural, displayed and skeletal formulae, general formulae, and structural and stereoisomerism.
An Eduqas A-Level Chemistry C3.1 answer on functional groups and homologous series, IUPAC naming, types of formula, general formulae, and structural and stereoisomerism.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
Eduqas topic C3.1 covers the language of organic chemistry: functional groups and homologous series, IUPAC nomenclature, the various ways of representing molecules, general formulae, and the meaning of isomerism. It is the grammar that every later organic topic depends on.
Functional groups and homologous series
Members of a series have similar chemical properties because they share a functional group, and a smooth gradation in physical properties (such as boiling point) because the chain length changes.
IUPAC nomenclature
The systematic name has three parts: a stem for the longest carbon chain (meth, eth, prop, but, pent...), a suffix for the principal functional group (-ane, -ene, -ol, -al, -one, -oic acid), and prefixes for substituents (chloro-, methyl-) with locant numbers. Number the chain from the end that gives the functional group, then the substituents, the lowest possible numbers.
Representing molecules
Structural isomerism
Stereoisomerism (introduction)
Stereoisomers have the same structural formula but a different spatial arrangement. (geometric) isomerism arises from restricted rotation about a double bond, and optical isomerism arises from a chiral carbon. The full treatment is in the Organic Chemistry and Analysis section; here you only need to recognise the categories.
Examples in context
Example 1. Why isomers matter in pharmacology. Two structural isomers can have completely different properties; for example, ethanol (a drinkable alcohol) and dimethyl ether (a gas) share the formula but behave entirely differently because of their different functional groups.
Example 2. Skeletal formulae in research. Chemists draw complex natural products as skeletal formulae because displaying every C and H would be unreadable; the convention lets a reader see the functional groups and ring systems at a glance.
Try this
Q1. Give the general formula of the alkenes and use it to deduce the molecular formula of the alkene with six carbon atoms. [1 mark]
- Cue. ; for , .
Q2. Name the compound . [1 mark]
- Cue. Butanoic acid (a four-carbon carboxylic acid).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20184 marksA hydrocarbon has the molecular formula . (a) Draw the displayed formulae and give the names of two chain isomers that are alkenes. (b) Explain why these are described as structural isomers.Show worked answer →
(a) But-1-ene, , and but-2-ene, (or methylpropene). Award 1 mark for each correct displayed formula and name (2).
(b) They have the same molecular formula () but different structural formulae, because the position of the double bond (or the carbon skeleton) differs (2).
Eduqas 20213 marks(a) Name the compound . (b) State the general formula of the alcohols and use it to give the molecular formula of the alcohol with five carbon atoms.Show worked answer →
(a) 2-methylpropan-1-ol (1).
(b) The general formula of the alcohols is (or ) (1). For : , i.e. (1).
Related dot points
- Alkanes from crude oil, combustion and free-radical substitution, alkenes and their electrophilic addition reactions, Markownikoff's rule, and addition polymerisation.
An Eduqas A-Level Chemistry C3.2 answer on alkanes (combustion and free-radical substitution), alkenes (electrophilic addition and Markownikoff's rule) and addition polymerisation.
- Nucleophilic substitution of halogenoalkanes, the mechanisms and the effect of the C-X bond, elimination to form alkenes, the relative rates of hydrolysis, and the environmental impact of halogenoalkanes.
An Eduqas A-Level Chemistry C3.3 answer on nucleophilic substitution of halogenoalkanes, elimination to alkenes, the relative rates of hydrolysis, and the environmental impact of CFCs.
- Classification of alcohols, oxidation of primary and secondary alcohols, dehydration and esterification, the acidity and reactions of carboxylic acids, and ester formation and hydrolysis.
An Eduqas A-Level Chemistry C3.4 answer on classifying and oxidising alcohols, dehydration, the acidity and reactions of carboxylic acids, and the formation and hydrolysis of esters.
- E/Z (geometric) isomerism from restricted rotation about a C=C bond, optical isomerism from a chiral centre, the meaning of enantiomers and optical activity, and the priority rules for naming.
An Eduqas A-Level Chemistry OA1.1 answer on E/Z (geometric) isomerism, optical isomerism and chirality, enantiomers and optical activity, and the priority rules for naming stereoisomers.
Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)