IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2Core idea~12 min read
Structural Isomerism
Two compounds can be built from exactly the same atoms and still be completely different substances. One might be a gas, the other a liquid; one might react and the other refuse to. All that changed was the order the atoms were joined up in. Counting those possibilities is a standard exam question, and it is much easier with a system.
📘 What you need to know
Isomers have the same molecular formula but a different arrangement of atoms.
Structural isomers have their atoms joined together in a different order.
There are three kinds: branched chain, positional and functional group isomerism.
Branched chain: the longest carbon chain is a different length.
Positional: same chain and same group, but the group is on a different carbon.
Functional group: the atoms rearrange into a completely different class of compound.
Alcohols and halogenoalkanes are primary, secondary or tertiary depending on how many carbons are attached to the carbon carrying the group. For amines you count the carbons on the nitrogen instead.
Bending or rotating a drawing does not create an isomer. If two structures give the same IUPAC name, they are the same compound.
The three types
Every structural isomerism question is one of these three, and you can usually tell which by comparing the two structures side by side.
The bottom two rows both use C4H10O, which shows the difference clearly. Butan-1-ol and butan-2-ol are both alcohols and behave similarly. Ethoxyethane is an ether and behaves nothing like either of them.
Branched chain isomerism
The longest chain gets shorter and the leftover carbons hang off it as branches. You need at least four carbons before this is possible — with three you can shuffle atoms all you like and still only get propane.
Positional isomerism
Same chain, same functional group, but the group has moved. Butan-1-ol and butan-2-ol are the standard pair. Because the class is unchanged, positional isomers have similar chemical properties but slightly different physical ones.
Functional group isomerism
The atoms reorganise so thoroughly that you end up in a different class. These isomers have very different chemical properties, because the reactive group is not the same. Three pairs of classes are worth memorising:
alcohols and ethers — both CnH2n+2O
aldehydes and ketones — both CnH2nO
alkenes and cycloalkanes — both CnH2n
The alkene and cycloalkane pair catches people out because there is no oxygen to spot. C3H6 can be propene, with a double bond, or cyclopropane, a three-carbon ring. Closing a ring costs you two hydrogens, exactly like making a double bond does.
Primary, secondary and tertiary
Once you can spot positional isomers, you need the vocabulary for describing them. For an alcohol or a halogenoalkane, look at the carbon that carries the group and count how many other carbon atoms are attached to it.
This classification decides how the molecule reacts later on. Primary alcohols oxidise all the way to carboxylic acids, secondary alcohols stop at ketones, and tertiary alcohols resist oxidation because there is no hydrogen left on that carbon to remove.
Amines use a different rule. For an amine you count the carbons attached to the nitrogen, not to a carbon. So CH3NH2 is a primary amine because the nitrogen has one carbon on it, and (CH3)3N is tertiary because the nitrogen has three. Mixing this up with the alcohol rule is a classic mistake.
The trap: same molecule, different drawing
Molecules are three-dimensional and free to rotate, so the same compound can be drawn in dozens of ways that look different on paper. Bending a chain through 90 degrees, flipping it left to right, or drawing a branch pointing down instead of up all produce the same molecule.
The test that never fails
Name both structures. Same name means the same compound. Different names means they really are isomers.
This is why the naming page came first. Counting isomers by eye leads to double-counting; counting them by name does not. If you end up with two structures you both want to call 2-methylbutane, you have drawn the same thing twice.
Worked examples
WORKED EXAMPLE
How many structural isomers does C4H10 have?
Step 1: check which types of isomerism are possibleNo functional group, so no functional group isomerism and nothing to reposition. That leaves branched chain isomerism only.Step 2: start with the longest possible chainchain of 4: CH3CH2CH2CH3 = butaneStep 3: shorten the chain by one and branch the spare carbonchain of 3 with a methyl on carbon 2: CH3CH(CH3)CH3 = 2-methylpropaneYou cannot put the methyl on carbon 1 or 3 — that just makes butane again, which the name check confirms.Step 4: shorten againA chain of 2 cannot hold two extra carbons as separate branches without becoming a longer chain, so we are finished.2 isomers: butane and 2-methylpropaneFour carbons is the smallest alkane that has any isomers at all. C1, C2 and C3 have exactly one each.
WORKED EXAMPLE
Deduce all the structural isomers of C4H9Cl and classify each one as primary, secondary or tertiary.
Step 1: sort out the possible carbon skeletons firstFour carbons gives two skeletons: a straight chain of 4, and a chain of 3 with a methyl branch. Do the chlorine positions on each skeleton in turn.Step 2: put the Cl on the straight chainCl on carbon 1: 1-chlorobutane, primaryCl on carbon 2: 2-chlorobutane, secondaryCarbon 3 is the same as carbon 2 counted from the other end, and carbon 4 is the same as carbon 1. Both would repeat a name we already have.Step 3: put the Cl on the branched skeletonCl on an end CH3: 1-chloro-2-methylpropane, primaryCl on the central carbon: 2-chloro-2-methylpropane, tertiaryStep 4: count them4 isomers — two primary, one secondary, one tertiaryWorking skeleton by skeleton stops you missing any. Doing it at random almost always loses one of the branched pair.
WORKED EXAMPLE
Name three compounds with the molecular formula C3H6O and state the type of isomerism between them.
Step 1: see which general formula it fitsC3H6O fits CnH2nO, so aldehydes and ketones are both in playStep 2: write those twoCH3CH2CHO = propanal, an aldehydeCH3COCH3 = propanone, a ketoneStep 3: look for a third optionA C=C plus an OH also uses up two hydrogens, and that fits too.CH2=CHCH2OH = prop-2-en-1-ol, an unsaturated alcoholpropanal, propanone and prop-2-en-1-ol — functional group isomersThree different classes from one formula. Their chemical properties are completely different, which is the hallmark of functional group isomerism.
WORKED EXAMPLE
Classify each of these as primary, secondary or tertiary: butan-2-ol, 2-methylpropan-2-ol, and the amine (CH3)2NH.
Step 1: butan-2-ol — find the carbon with the OHThat is carbon 2. It has carbon 1 on one side and carbon 3 on the other.2 carbons attached, so secondaryStep 2: 2-methylpropan-2-ol — same questionCarbon 2 carries the OH, and it is joined to carbon 1, carbon 3 and the methyl branch.3 carbons attached, so tertiaryStep 3: the amine — switch rules and count on the nitrogenThe nitrogen has two methyl groups and one hydrogen.2 carbons on the N, so secondarysecondary, tertiary, secondaryNote that (CH3)2NH has no branched carbon at all, yet it is still a secondary amine. The rule really does move to the nitrogen.
Comparing the three types at a glance
Type
What changes
Chemical properties
Example pair
branched chain
length of the longest chain
similar
pentane and 2-methylbutane
positional
which carbon carries the group
similar
propan-1-ol and propan-2-ol
functional group
the class of compound
very different
propanal and propanone
💡 Exam tip
Work skeleton by skeleton when counting isomers. Fix the carbon framework first, then move the functional group along it. Random guessing loses isomers.
Name every structure you draw. Two identical names means you have drawn the same compound twice, and that is the only reliable way to spot it.
Check both ends before adding a new isomer. Carbon 3 of a four-carbon chain is carbon 2 from the other end.
For primary, secondary and tertiary, count carbons on one specific atom — the carbon holding the group, or the nitrogen in an amine.
If a question gives you a molecular formula with an oxygen, always test the functional group isomer pairs: alcohol with ether, aldehyde with ketone.
Remember the ring option. CnH2n could be an alkene or a cycloalkane, and cycloalkanes are easy to forget.
⚠ Common mix-up
Counting a bent or flipped drawing as a new isomer. Rotation is free, so it is the same molecule. Name it and check.
Confusing positional with branched chain isomerism. Positional keeps the same chain length and moves the group; branched chain changes the chain length itself.
Saying functional group isomers have similar chemical properties. They do not — that is the whole point of them.
Using the alcohol rule on an amine. For amines you count carbons on the nitrogen, not on a carbon.
Calling methanol something other than primary. Zero attached carbons still counts as primary.
Thinking 2-methylpropan-2-ol is a positional isomer of butanol. Its longest chain is three carbons instead of four, so it is a branched chain isomer.
Forgetting that three carbons cannot branch. Any “branch” on propane just extends the chain into butane.
Up next: Cis-Trans Isomers — so far the atoms have been joined in a different order. Next we look at isomers where the connections are identical and only the shape in space differs.
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