Isomers are compounds that have the same molecular formula but different structural formulae, so their atoms are joined in a different arrangement. Because the arrangement differs, isomers have different physical properties (such as boiling point), even though members of the same homologous series share similar chemical properties.
Butane, C4H10, is the smallest alkane with isomers: butane and 2-methylpropane.
This page covers content standard 10.5 of the Form 5 Carbon Compounds chapter: isomerism. Once you can write molecular formulae for alkanes and alkenes, the next skill the syllabus asks for is recognising that a single molecular formula can describe more than one compound. Drawing and naming these isomers correctly is a reliable source of marks, and it trains exactly the careful, systematic thinking the exam rewards.
What is isomerism?
Isomers are two or more compounds that have the same molecular formula but different structural formulae. The molecular formula counts the atoms; the structural formula shows how those atoms are actually joined together. Isomers contain the same number of each kind of atom, but the atoms are arranged differently, the carbon chain may be straight in one isomer and branched in another, or a functional group may sit in a different position along the chain.
Definition to memorise. Isomerism is the existence of two or more compounds with the same molecular formula but different structural formulae. Each such compound is called an isomer.
Because the atoms are joined differently, isomers are genuinely different substances with their own names and their own physical properties. However, isomers that belong to the same homologous series (for example, two isomers of an alkane) have similar chemical properties, because they contain the same functional grouping.
Why do carbon compounds form so many isomers?
Carbon is unusual: each carbon atom forms four covalent bonds and can bond to other carbon atoms to build straight chains, branched chains and rings. The more carbon atoms a molecule has, the more ways there are to arrange them, so the number of possible isomers rises quickly as the chain gets longer. This is one reason there are millions of known carbon compounds.
Isomers of the alkanes
The first three alkanes, methane (CH₄), ethane (C₂H₆) and propane (C₃H₈), have only one possible structure each, so they have no isomers. Isomerism begins at butane, C₄H₁₀, which has two isomers:
- butane, an unbranched (straight) chain of four carbon atoms;
- 2-methylpropane, a three-carbon chain with a methyl (CH₃) branch on the middle carbon.
Both have the molecular formula C₄H₁₀, but the atoms are joined differently, so they are isomers. Pentane, C₅H₁₂, has three isomers: pentane, 2-methylbutane and 2,2-dimethylpropane.
Naming the isomers
To name an isomer using the IUPAC system:
- Find the longest continuous carbon chain, this gives the stem name (for example, four carbons → but-).
- Number the chain from the end that gives the branch the lowest locant.
- Name each branch as an alkyl group (CH₃ = methyl, C₂H₅ = ethyl) and put its position number in front.
- Combine: position + branch + stem, for example 2-methylpropane.
Worked example
Question. Draw the structural formulae of all the structural isomers of pentane, C₅H₁₂, and name each one.
Step 1, Start with the straight chain. Arrange all five carbon atoms in one continuous chain and fill the remaining bonds with hydrogen. The longest chain has 5 carbons, so this isomer is pentane:
CH₃, CH₂, CH₂, CH₂, CH₃Step 2, Shorten the main chain to four carbons and add one branch. Take one carbon off the end and attach it as a methyl branch. The longest chain now has 4 carbons (but-), with a methyl group on carbon 2. This isomer is 2-methylbutane:
CH₃, CH(CH₃), CH₂, CH₃Step 3, Shorten the main chain to three carbons and add two branches. Attach two methyl groups to the middle carbon of a three-carbon chain. The longest chain is 3 carbons (prop-), with two methyls on carbon 2. This isomer is 2,2-dimethylpropane:
CH₃, C(CH₃)₂, CH₃Step 4, Check. Each structure has exactly 5 carbon and 12 hydrogen atoms, matching C₅H₁₂. No two structures are the same when you follow the longest chain, so there are exactly three isomers.
Answer. C₅H₁₂ has three structural isomers: pentane, 2-methylbutane and 2,2-dimethylpropane.
Practice question
Question. State how many structural isomers the molecular formula C₄H₁₀ has. Draw each one and give its name.
Answer. C₄H₁₀ has two structural isomers. The first is butane, a straight chain of four carbon atoms: CH₃, CH₂, CH₂, CH₃. The second is 2-methylpropane, a three-carbon chain with a methyl branch on the middle carbon: CH₃, CH(CH₃), CH₃. Both have the molecular formula C₄H₁₀ (four carbon and ten hydrogen atoms), but the carbon atoms are joined in a different arrangement, so they are isomers.
How branching affects physical properties
Isomers are different substances, so their physical properties differ. The clearest trend is with boiling point: a branched isomer usually boils at a lower temperature than its straight-chain isomer with the same molecular formula. A branched molecule is more compact and has a smaller surface area, so the forces of attraction between neighbouring molecules (van der Waals forces) are weaker and less energy is needed to separate them. For example, 2-methylpropane boils lower than butane, and 2,2-dimethylpropane boils lower than pentane. Being able to explain this difference, not just state it, is what earns the higher marks.
Exam tip
The single most common slip in this topic is drawing the same isomer twice, for instance, drawing a branch on carbon 2 and then again on carbon 3 of a four-carbon chain, which is really the same molecule flipped over. Always identify the longest chain and number from the end giving the lowest locant before you decide two structures are different. Check every isomer against the molecular formula by counting carbon and hydrogen atoms: if the count does not match, you have added or lost an atom. Remember that isomers of the same series have similar chemical properties but different physical properties, a favourite compare-and-explain question.
Where this fits
This is content standard 10.5 of the Carbon Compounds chapter, and it builds directly on the naming rules from homologous series and nomenclature. Practise drawing full and condensed structures in the chapter worked examples, and use the common mistakes page to avoid double-counting the same isomer. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers work through drawing and naming isomers step by step until the systematic method is automatic. Because drawing and naming isomers recurs across SPM Chemistry, the systematic habit you build here pays back in every organic question.
Quick recap
- Isomers = same molecular formula, different structural formula.
- Isomerism in alkanes begins at butane, C₄H₁₀ (2 isomers); pentane, C₅H₁₂ has 3.
- Name by the longest chain, numbering to give branches the lowest locant.
- Same series → similar chemical properties but different physical properties.
- Branched isomers have lower boiling points than straight-chain isomers (weaker van der Waals forces).
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