A molecular formula tells you which atoms are present, but not how they are joined. Very often the same set of atoms can be assembled in more than one way — and the results can be genuinely different substances, with different boiling points and sometimes completely different chemistry.
📚 What you need to know
Structural isomers have the same molecular formula but different structural formulas.
Chain isomers differ in how the carbon skeleton branches.
Position isomers have the same group on a different carbon.
Functional group isomers have completely different groups, and so different chemical properties.
Alcohols and halogenoalkanes are classed primary, secondary or tertiary by the number of carbons attached to the carbon carrying the group.
For amines, you count the carbons attached to the nitrogen instead.
What counts as a different isomer?
Two structures are isomers only if they are genuinely different molecules. Rotating a drawing, or bending the chain round a corner, does not make a new compound — the atoms are still joined in the same order.
The reliable test is to name them. If both structures give the same IUPAC name, they are the same compound drawn two ways. Different names mean different isomers. It never fails.
The three types
Same formula in all three rows. The skeleton changes, then the position of the group, then the group itself.
Chain isomerism
Same formula, different skeleton. The carbons can run in one long chain or be broken up with branches. C4H10 gives butane (a straight chain of four) and 2-methylpropane (a chain of three with a methyl branch).
Chain isomers have similar chemistry but noticeably different physical properties. Branched molecules are more compact, so they touch each other over a smaller surface area, which means weaker dispersion forces and a lower boiling point. Butane boils at –1 °C; 2-methylpropane boils at –12 °C.
Four carbons is the smallest chain that can branch at all — with three, moving a carbon to the middle just recreates the same chain.
Position isomerism
Same formula, same functional group, but the group is attached to a different carbon. Butan-1-ol and butan-2-ol both have the formula C4H10O and both are alcohols — the OH has simply moved along.
Because the functional group is unchanged, position isomers have similar chemical properties and similar boiling points.
Functional group isomerism
Same formula, different functional group, and therefore a different family altogether. Butan-1-ol (an alcohol) and ethoxyethane (an ether) are both C4H10O, but one hydrogen bonds and the other cannot.
These are the isomers with very different chemical properties. Three pairings are worth remembering:
Alkenes and cycloalkanes — both CnH2n
Alcohols and ethers — both CnH2n+2O
Aldehydes and ketones — both CnH2nO
Quick check: butan-1-ol boils at 118 °C but ethoxyethane, its functional group isomer, boils at just 35 °C. Same atoms, same mass — but only the alcohol can hydrogen bond to itself.
Primary, secondary and tertiary
Some position isomers get a further label, based on how crowded the carbon carrying the functional group is.
Circle the carbon holding the group, then count its carbon neighbours. One, two or three gives primary, secondary or tertiary.
🧩 How to classify
Find the carbon that the functional group is attached to.
Count how many other carbon atoms are bonded to that carbon.
For amines, count the carbons on the nitrogen instead of on a carbon.
The same system applies to halogenoalkanes: 1-bromobutane is primary, 2-bromobutane is secondary, and 2-bromo-2-methylpropane is tertiary. This classification matters later, because primary, secondary and tertiary compounds react at very different rates.
Class
Count what?
Primary
Secondary
Tertiary
Alcohols
Carbons on the C–OH carbon
propan-1-ol
propan-2-ol
2-methylpropan-2-ol
Halogenoalkanes
Carbons on the C–X carbon
1-bromobutane
2-bromobutane
2-bromo-2-methylpropane
Amines
Carbons on the nitrogen
methylamine
dimethylamine
trimethylamine
Finding every isomer
Questions often ask how many isomers a formula has. Working randomly will lose you one; working systematically will not.
🧩 A system that finds them all
Draw the longest possible chain first, and place the functional group at every distinct position along it.
Shorten the chain by one carbon and add the spare carbon as a branch, in every distinct position.
Repeat until the chain is too short to branch.
Finally, ask whether a different functional group would fit the same formula.
Name every structure. Duplicate names mean you have drawn the same isomer twice.
WORKED EXAMPLE
Deduce the number of structural isomers with the formula C4H9Cl, and name them.
Start with the 4-carbon chain and move the ClCl on C1 → 1-chlorobutane. Cl on C2 → 2-chlorobutane. C3 and C4 just repeat these from the other end.Now shorten to a 3-carbon chain with a methyl branchCl on the end → 1-chloro-2-methylpropane. Cl on the branched middle carbon → 2-chloro-2-methylpropane.4 isomersTwo primary, one secondary, one tertiary — a useful check that you have them all.
WORKED EXAMPLE
C3H6O can be an aldehyde or a ketone. Draw both, name them, and state the type of isomerism.
Aldehyde — C=O at the end, carrying an HCH₃CH₂CHO — propanalKetone — C=O in the middle, between two carbonsCH₃COCH₃ — propanoneDifferent functional groups, same formulafunctional group isomerism
💡 Exam tip
Always name every structure you draw. It is the only reliable way to spot accidental duplicates.
Say which type of isomerism when asked — chain, position or functional group. “They are isomers” is not enough.
Explaining a boiling point difference? Chain isomers → branching and surface area. Functional group isomers → type of intermolecular force.
Count the atoms in every structure to confirm the formula really matches.
⚠️ Common mix-up
Rotating or bending a molecule does not create an isomer. Name it and see.
Position vs chain isomerism. 2-methylpropan-2-ol is a chain isomer of butan-1-ol, not a position isomer — the skeleton changed too.
Primary, secondary and tertiary count carbons, not hydrogens.
For amines, count on the nitrogen. Applying the alcohol rule gives the wrong answer.
Branched isomers have lower boiling points, not higher — less contact means weaker forces.
That completes Classifying Organic Compounds by Functional Group. You can now take a molecular formula and work out what could exist, draw each possibility, name it unambiguously, and predict how it will behave — which is the whole foundation for the organic reactions that follow.
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