How are organic compounds named and how do isomers arise?
IUPAC nomenclature of organic compounds, functional groups and homologous series, general, structural, displayed and skeletal formulae, and structural isomerism including chain, position and functional group isomers.
A CCEA A-Level Chemistry answer on organic nomenclature and isomerism, covering IUPAC naming, functional groups and homologous series, the types of formulae used in organic chemistry, and structural isomerism including chain, position and functional group isomers.
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What this dot point is asking
CCEA wants you to name organic compounds using IUPAC rules, recognise functional groups and homologous series, use general, structural, displayed and skeletal formulae, and identify and draw structural isomers (chain, position and functional group isomers).
IUPAC nomenclature
For example, is propan-2-ol: a three-carbon chain (prop), an alcohol group (-ol) on carbon 2.
When a compound has a principal functional group, the chain must be numbered so that group gets the lowest locant, and that takes priority over giving substituents low numbers. Multiple identical substituents take the prefixes di, tri and tetra, and each one keeps its own locant (for example 2,2-dimethylpropane).
Functional groups and homologous series
Types of formula
- General formula: for the whole series, such as for alkanes.
- Molecular formula: the actual number of each atom, such as .
- Structural formula: shows how atoms are grouped, such as .
- Displayed formula: shows every atom and bond.
- Skeletal formula: lines for the carbon skeleton, with hydrogens on carbon omitted.
Structural isomerism
Worked through with , the two chain isomers are butane (a straight chain) and 2-methylpropane (a branched chain). With , the two position isomers are propan-1-ol and propan-2-ol, differing only in where the OH sits. With , ethanol (an alcohol) and methoxymethane (an ether) are functional group isomers: same formula, completely different chemistry. Distinguishing these three types is a frequent CCEA exam task, and the key is to compare the carbon skeleton, the position of the group, and the identity of the group in turn.
Examples in context
Example 1. Why isomers have different boiling points. Butane and 2-methylpropane share the formula , yet butane boils a few degrees higher. The straight chain of butane packs together more closely and has a larger surface of contact, so its van der Waals forces are stronger than those of the more compact, branched 2-methylpropane. This connects nomenclature directly to physical properties: knowing the structure from the name lets you predict the trend, a favourite CCEA application.
Example 2. Isomers in drug design. Many drugs have isomers with very different effects, because a small change in the arrangement of atoms changes how the molecule fits a biological target. Even at AS level the principle is visible: an alcohol and its functional group isomer the ether react completely differently (the alcohol oxidises and esterifies; the ether is largely unreactive), so naming a compound correctly and recognising its isomers is the first step to predicting its behaviour.
Try this
Q1. Name the compound . [1 mark]
- Cue. Propan-1-ol.
Q2. State what is meant by structural isomers. [2 marks]
- Cue. Compounds with the same molecular formula but a different arrangement of atoms.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20205 marksA hydrocarbon has the molecular formula . Draw and name three structural isomers of this formula, and state the type of structural isomerism shown by each pair you have drawn.Show worked answer →
Markers want three correctly drawn and named isomers and the correct classification of the relationships.
has one degree of unsaturation, so it can be an alkene or a cyclic alkane. Three valid isomers are:
But-1-ene, (double bond on carbon 1).
But-2-ene, (double bond on carbon 2).
2-methylpropene, (branched chain).
But-1-ene and but-2-ene are position isomers (same chain and functional group, different position of the double bond). But-1-ene and 2-methylpropene are chain isomers (different carbon skeletons). A candidate could also offer cyclobutane or methylcyclopropane, which would be functional group isomers of the alkenes (a ring instead of a double bond).
Markers reward three correct structures with correct names and the correct isomer-type labels for the relationships described.
CCEA 20183 marksName the following compound and explain how you decided on the position numbers: CH3CH(CH3)CH2CH2OH.Show worked answer →
A naming question testing the lowest-locants rule.
The longest carbon chain that includes the functional group is four carbons (butane), and the principal functional group is the alcohol (-ol), so the parent name is butanol. There is a methyl branch.
Numbering must give the functional group the lowest locant. Numbering from the end nearer the OH puts the OH on carbon 1 and the methyl on carbon 3, giving 3-methylbutan-1-ol. Numbering from the other end would put the OH on carbon 4, which is higher, so it is rejected.
The name is therefore 3-methylbutan-1-ol.
Markers reward identifying the four-carbon parent chain, the alcohol suffix, and numbering to give the OH the lowest locant (1, not 4), with the methyl correctly placed on carbon 3.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)