IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2HL only~12 min read
Peak Splitting in Proton NMR
Turn up the resolution and each single line breaks into a small cluster. This looks like a complication and is actually a gift: the number of lines in a cluster tells you how many hydrogens are on the neighbouring carbon. That is the piece of information that lets you connect fragments into a full structure.
📘 What you need to know
Splitting happens because a neighbouring proton’s own tiny magnetic field slightly changes the field felt by the proton you are looking at.
A neighbouring proton can align with or against the applied field, so it either raises or lowers the effective field a little.
The n+1 rule: a proton with n equivalent neighbouring protons gives a signal split into n+1 lines.
The relative heights within a cluster follow Pascal’s triangle: 1:1 for a doublet, 1:2:1 for a triplet, 1:3:3:1 for a quartet.
Neighbours means protons on the adjacent atom — usually the next carbon along.
The multiplicity of one signal tells you about a different group. Splitting is how you join the pieces together.
In practice an alcohol O–H usually appears as a singlet and does not split its neighbours.
Why one line becomes several
Every proton is a tiny magnet, and it does not just respond to the instrument’s field — it also feels the field of any proton sitting next door. That neighbour can be pointing with the applied field or against it, and the two situations are almost equally likely.
If the neighbour points with the field, our proton feels a slightly stronger field, so it resonates at a slightly higher shift.
If the neighbour points against the field, our proton feels a slightly weaker field, so it resonates at a slightly lower shift.
Half the molecules in the sample are in each situation, so instead of one line at the average position you get two lines of equal height either side of it — a doublet. Add a second neighbour and there are four combinations of the two spins, but two of them cancel out to give the same field, so you get three lines in the ratio 1 : 2 : 1.
The ratios are not arbitrary. With two neighbours, “up down” and “down up” cancel each other and land at the same place, which is why the middle line of a triplet is twice as tall as the outer two.
The four patterns you have to recognise
The rule to write in every answer
a proton with n equivalent neighbouring protons gives n + 1 lines
The total area of a cluster is what the integration measures, not the height of its tallest line. A 3H triplet and a 3H singlet have the same total area even though the singlet looks much taller.
A triplet does not mean three hydrogens. It means three lines, which means two neighbouring hydrogens. How many hydrogens the signal itself represents comes from the integration, and the two numbers are completely independent. In ethanol the CH3 signal is a triplet worth 3H, and the CH2 signal is a quartet worth 2H — each one is reporting on the other group.
Splitting joins the pieces together
Here is why this matters. Ethyl ethanoate contains three hydrogen environments, and every one of them tells you something about a different part of the molecule.
Learn the quartet-plus-triplet pair as a single unit. Whenever you see a 2H quartet and a 3H triplet in the same spectrum, an ethyl group is present, and that is often half the answer.
Notice how the reasoning runs sideways. The CH2 signal is a quartet, but the “3” in that comes from the CH3. Students often try to explain a signal using its own hydrogens, which never works. Always look at the atom next door.
One exception you will meet. An alcohol O–H almost always shows up as a plain singlet, and it does not split the protons beside it either. The reason is that OH hydrogens swap between molecules extremely fast, so on the NMR timescale the neighbouring protons see an average and no splitting survives. When you apply the n+1 rule, ignore any OH.
Worked examples
WORKED EXAMPLE
Predict the number of signals, the integration and the splitting pattern in the proton NMR spectrum of 1-chloropropane, CH3CH2CH2Cl.
Step 1: find the environmentsThree different carbons, none of them equivalent, so three signals.CH3 (3H), middle CH2 (2H), CH2Cl (2H)Step 2: for each signal, count the hydrogens on the neighbouring carbonsThe CH3 has only the middle CH2 next to it, so 2 neighbours. The CH2Cl also has only the middle CH2 next to it, so 2 neighbours. The middle CH2 has both the CH3 and the CH2Cl next to it, so 3 + 2 = 5 neighbours.Step 3: apply n + 1CH3: 2 + 1 = 3 lines, a tripletCH2Cl: 2 + 1 = 3 lines, a tripletmiddle CH2: 5 + 1 = 6 lines, a sextet3 signals: 3H triplet, 2H sextet, 2H tripletThe middle CH2 is the one to watch — it has neighbours on both sides, and you have to add them together.
WORKED EXAMPLE
A compound with molecular formula C3H8O gives three signals: a 6H doublet near 1.2 ppm, a 1H multiplet near 4.0 ppm and a 1H singlet. Deduce its structure.
Step 1: check the hydrogen total6 + 1 + 1 = 8, which matches C3H8O ✓Step 2: interpret the 6H doublet6H in one signal means two equivalent CH3 groups. A doublet means 1 neighbouring hydrogen, so both methyls are attached to a carbon carrying exactly one H.a (CH3)2CH– groupStep 3: interpret the 1H singletA lone hydrogen that does not split and is not split is almost always an OH. That also uses up the oxygen.Step 4: assemble and check the middle signal(CH3)2CH–OH. The single CH has 6 neighbours, so it is a 7-line multiplet, and being next to an oxygen it sits near 4 ppm.propan-2-ol, (CH3)2CHOHPropan-1-ol is ruled out at step 2. It would give a 3H triplet, not a 6H doublet, because its two methyls do not exist — it only has one.
WORKED EXAMPLE
Explain how proton NMR distinguishes propanal from propanone, given that both have the molecular formula C3H6O.
Step 1: work out the spectrum of propanoneCH3COCH3 is symmetrical, so both methyls are equivalent and neither has any neighbouring hydrogens.1 signal: a 6H singlet near 2.1 ppmStep 2: work out the spectrum of propanalCH3CH2CHO has three environments. The CHO proton has 2 neighbours, the CH2 has 3 + 1 = 4, and the CH3 has 2.3 signals: 1H near 9.7, 2H near 2.4, 3H near 1.1Step 3: identify the decisive evidenceThe number of signals alone separates them: one versus three. On top of that, the aldehyde proton near 9.7 ppm is unmistakable, because almost nothing else appears that far to the left.propanone gives one signal; propanal gives three, including one near 9.7 ppmInfrared cannot do this, because both compounds have a C=O in the same range. This is a good example of why the techniques are used together.
Multiplicity summary
Neighbouring H atoms (n)
Lines you see (n+1)
Name
Height ratio
Typical cause
0
1
singlet
1
next to a C=O, an O, or a carbon with no H
1
2
doublet
1 : 1
next to a CH
2
3
triplet
1 : 2 : 1
next to a CH2
3
4
quartet
1 : 3 : 3 : 1
next to a CH3
5
6
sextet
1 : 5 : 10 : 10 : 5 : 1
a CH2 between a CH3 and a CH2
💡 Exam tip
Count neighbours on the atom next door, never on the group itself. This is the one idea the whole topic rests on.
Add up neighbours from both sides for a CH2 in the middle of a chain. Forgetting one side turns a sextet into a quartet.
Keep splitting and integration separate. Splitting counts the neighbours; integration counts the hydrogens in the signal.
Learn quartet-plus-triplet as “ethyl group”. It appears constantly and saves you a lot of thinking.
Treat a 3H singlet as a methyl with no hydrogen neighbours — so it is attached to a C=O, an oxygen, or a fully substituted carbon.
Ignore OH protons when applying n + 1. They exchange too fast to split anything.
⚠ Common mix-up
Reading a triplet as three hydrogens. Three lines means two neighbours. The hydrogen count comes from the integration.
Applying n + 1 to the hydrogens in the signal itself. A CH3 is not split by its own three hydrogens; they are equivalent to each other.
Counting neighbours on one side only. A CH2 with a CH3 on one side and a CH2 on the other has five neighbours, not three.
Measuring the tallest line as the integration. Integration is the area of the whole cluster.
Expecting an OH to split. It normally appears as a singlet regardless of its neighbours.
Forgetting that equivalent protons do not split each other. The six hydrogens of propanone give a singlet, not a multiplet.
Treating each line of a cluster as a separate signal. A quartet is one signal, not four.
Up next: Structural Analysis of Molecules — time to put mass spectrometry, infrared and NMR together and identify an unknown compound from scratch.
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