← Unit 4: Structure, synthesis and design
Topic 2: Chemical synthesis and design
Describe the principles and apply proton (1H) nuclear magnetic resonance (NMR) spectroscopy to identify the number and types of hydrogen environments, peak ratios (integration) and splitting patterns to determine the structure of organic compounds
A focused answer to the QCE Chemistry Unit 4 dot point on proton NMR spectroscopy. Explains chemical environments, chemical shifts (with the QCAA reference table), the n+1 splitting rule, and integration. Walks through the 1H NMR of ethanol and ethyl ethanoate, the canonical IA3 / EA spectra.
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What this dot point is asking
QCAA wants you to describe the principles of 1H NMR spectroscopy, interpret the three pieces of information a 1H NMR spectrum gives (number of environments, integration, splitting pattern), and use these to determine the structure of an organic compound. The dot point is the highest-information-density spectroscopy item in Unit 4; in IA3 and EA Paper 2 the 1H NMR question typically determines a full structure where MS and IR alone are ambiguous.
The answer
Nuclear magnetic resonance (NMR) spectroscopy measures the magnetic environment of nuclei. For protons (1H NMR), each chemically distinct hydrogen in a molecule absorbs radiofrequency energy at a slightly different frequency, giving a peak whose position, intensity and shape encode structural information.
What a 1H NMR spectrum shows
A 1H NMR spectrum has three diagnostic features:
| Feature | What it tells you |
|---|---|
| Number of peaks (signals) | Number of chemically distinct hydrogen environments |
| Chemical shift (ppm, x-axis) | Type of environment (functional group neighbourhood) |
| Integration (peak area or curve) | Number of hydrogens in each environment (relative ratio) |
| Splitting (multiplicity) | Number of hydrogens on adjacent (n) carbons |
The x-axis is chemical shift in parts per million (ppm), referenced to tetramethylsilane (TMS) at 0 ppm. The scale runs from about 0 to 12 ppm, with the right side (low ppm) being shielded (more upfield) and the left side (high ppm) being deshielded (more downfield).
Counting hydrogen environments
Two hydrogens are in the same environment if they are equivalent by symmetry (interchangeable by a molecular symmetry operation or by free rotation of an attached methyl group). They give a single combined signal.
Example. Ethanol, CH3-CH2-OH, has three environments: CH3 (3H equivalent), CH2 (2H equivalent), OH (1H). The 1H NMR shows three peaks.
Example. Methoxymethane, CH3-O-CH3, has one environment: both methyl groups are equivalent by the C2 symmetry of the molecule. The 1H NMR shows one peak (a singlet).
The 1H NMR is an immediate test for symmetry. A compound with high symmetry (few environments) shows few peaks; a compound with low symmetry shows many.
Chemical shift: where each peak appears
Each environment has a characteristic chemical shift range, determined by the inductive effects of attached groups. The QCAA syllabus uses the following reference ranges (the QCAA data booklet table for Unit 4 spectroscopy):
| Hydrogen environment | Chemical shift (ppm) |
|---|---|
| R-CH3 (alkyl, no nearby heteroatom) | 0.5 to 1.5 |
| R-CH2-R (alkyl) | 1.0 to 2.0 |
| R-CHR-R (alkyl, tertiary) | 1.5 to 2.5 |
| R-CH2-Cl or R-CH2-Br | 3.0 to 4.0 |
| R-CH2-O- (alpha to oxygen) | 3.3 to 4.5 |
| R-CHO (aldehyde H) | 9.5 to 10.5 |
| R-CH=CR2 (alkene H) | 5.0 to 6.5 |
| Ar-H (aromatic) | 6.5 to 8.5 |
| R-OH (alcohol) | 1.0 to 6.0 (variable) |
| R-COOH (acid OH) | 9.0 to 13.0 |
| R-NH2 (amine) | 1.0 to 5.0 (variable) |
| R-CO-NH-R (amide NH) | 5.0 to 9.0 |
| R-CO-CH3 (alpha to C=O) | 2.0 to 2.5 |
The further to the left (higher ppm) a peak appears, the more deshielded the proton is, typically because it is near an electron-withdrawing group (O, N, halogen, C=O, aromatic ring).
Integration: relative number of hydrogens
The area under each peak is proportional to the number of hydrogens in that environment. NMR spectrometers display this as a step curve or as a number near each peak. Integrations are relative ratios, not absolute counts; if a molecule has 6 hydrogens distributed 3 : 2 : 1, the spectrum shows three peaks with integration ratio 3 : 2 : 1.
The simplest assignment strategy is to scale the smallest integration to 1 and read the others as multiples.
Splitting: the n+1 rule
A peak is split into (n+1) sub-peaks (multiplicity) by the n equivalent hydrogens on the adjacent carbon(s). The pattern of split heights is given by Pascal's triangle:
| Multiplicity | n | Relative intensities |
|---|---|---|
| Singlet (s) | 0 | 1 |
| Doublet (d) | 1 | 1 : 1 |
| Triplet (t) | 2 | 1 : 2 : 1 |
| Quartet (q) | 3 | 1 : 3 : 3 : 1 |
| Quintet | 4 | 1 : 4 : 6 : 4 : 1 |
Worked example. Ethanol (CH3-CH2-OH):
- The CH3 has 2 neighbours on the adjacent CH2; n = 2; multiplicity = 3 (triplet).
- The CH2 has 3 neighbours on the adjacent CH3; n = 3; multiplicity = 4 (quartet). (The OH is normally exchanging and does not couple to the CH2 in practice.)
- The OH has 2 neighbours on the adjacent CH2 in principle, but exchange with traces of water decouples it; appears as a (broad) singlet.
The triplet-quartet pattern is the canonical fingerprint of an ethyl group (-CH2-CH3). Spotting it in a spectrum is often the first step in identification.
Combining the three pieces: structure determination
The systematic approach for an unknown 1H NMR:
- Count the peaks. That is the number of H environments.
- Read the integrations. Scale to whole numbers; should sum to the total H count from the molecular formula.
- Read the chemical shifts. Compare each to the reference table; identify the environment type (alkyl, near-oxygen, near-carbonyl, aromatic, etc.).
- Read the splittings. Use n+1 backwards to identify the number of H on each adjacent carbon.
- Piece together the connectivity. Each environment links to its neighbours by the n value of its splitting.
- Check against the molecular formula. Total H and atom count must match.
For ethyl ethanoate (CH3-CO-O-CH2-CH3, the canonical ester spectrum):
- Three environments: -OCH2- (2H), -OCO-CH3 (3H), -CH3 of ethyl (3H).
- Chemical shifts: -OCH2- around 4.1 ppm (deshielded by O and by C=O), -OCO-CH3 around 2.0 ppm (alpha to C=O), -CH3 around 1.3 ppm (alkyl).
- Splittings: -OCH2- splits into a quartet by 3 neighbours on the adjacent CH3; -OCO-CH3 is a singlet (no adjacent H, separated by C=O); -CH3 of ethyl splits into a triplet by 2 neighbours on the adjacent CH2.
- Integration ratio: 2 : 3 : 3.
The (1H singlet, 2H quartet, 3H triplet) pattern with chemical shifts in this range is unambiguous for ethyl ethanoate.
NMR vs IR vs MS: when to use each
| Question | Best technique |
|---|---|
| What is the molecular mass? | MS (M+) |
| Are these functional groups present? | IR (O-H, C=O, N-H) |
| How many hydrogen environments? Where? Splitting? | 1H NMR |
| Distinguishing isomers with same MS and IR? | 1H NMR (usually decisive) |
A typical QCAA IA3 / EA stimulus provides all three spectra. The expected workflow is MS for Mr -> molecular formula candidates; IR to narrow functional groups; 1H NMR to fix the structure.
Common traps
Confusing chemical shift with integration. Chemical shift identifies the type of H. Integration counts how many. They are read off different parts of the spectrum.
Applying n+1 to non-adjacent hydrogens. Splitting comes only from hydrogens on directly bonded carbons (or atoms attached to those carbons). Hydrogens separated by C=O or oxygen do not couple in basic 1H NMR; the singlet for -OCO-CH3 in ethyl ethanoate is the classic example.
Forgetting that OH and NH protons exchange. These often appear as broad singlets at variable chemical shift, regardless of their nominal n+1 neighbours, because exchange with solvent water decouples them.
Treating equivalent hydrogens as separate peaks. A methyl group's 3 hydrogens are equivalent by free rotation and appear as a single peak. Drawing 3 separate peaks for them is wrong.
Skipping the integration check. Total integration must match the molecular formula. If it does not, you have miscounted environments or assigned the wrong peak.
In one sentence
Proton (1H) NMR spectroscopy gives four pieces of structural information from a single spectrum: the number of peaks (number of hydrogen environments), the chemical shift in ppm (type of environment from a reference table), integration (relative number of hydrogens per environment), and multiplicity via the n+1 rule (number of hydrogens on adjacent carbons); together they almost always determine the structure of an unknown Unit 4 organic compound when combined with MS and IR.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2023 QCAA-style5 marksThe 1H NMR spectrum of an unknown C4H8O2 shows three peaks: a singlet at 3.7 ppm (3H), a quartet at 2.3 ppm (2H), and a triplet at 1.1 ppm (3H). (a) Determine the structure of the compound. (b) Justify the structure by assigning each peak to a specific hydrogen environment.Show worked answer →
A 5-mark answer needs the structure plus assignment of all three peaks.
Step 1: count environments and check H count. Three peaks; total H = 3 + 2 + 3 = 8, matching C4H8O2. Two oxygens with this H count suggests an ester or carboxylic acid.
Step 2: identify the structure. For methyl propanoate (CH3-CH2-CO-O-CH3): three environments. OCH3 (3H singlet, around 3.7 ppm, deshielded by adjacent O); CH2 (2H quartet, around 2.3 ppm, deshielded by C=O, split by adjacent CH3); CH3 (3H triplet, around 1.1 ppm, split by adjacent CH2). This matches the spectrum exactly.
Step 3: assign peaks.
- 3.7 ppm singlet (3H): -OCH3. Singlet because no neighbouring H (only -O- on one side, C=O carbon on the other). Chemical shift around 3.7 ppm matches methoxy attached to an ester oxygen.
- 2.3 ppm quartet (2H): -CH2-. Adjacent to a CH3 (n+1 rule: 3 + 1 = 4 peaks, quartet). Deshielded by the C=O at the other side; shift around 2.3 ppm matches alpha-to-carbonyl CH2.
- 1.1 ppm triplet (3H): CH3-. Adjacent to a CH2 (n+1 rule: 2 + 1 = 3 peaks, triplet). Shift around 1.1 ppm matches a CH3 with no electron-withdrawing neighbour.
Compound: methyl propanoate, CH3-CH2-CO-O-CH3.
Markers reward the systematic environment count, splitting analysis via n+1, chemical-shift consistency with the QCAA reference table, and a structure that explains all three peaks.
2022 QCAA-style3 marksPure ethanol gives a 1H NMR spectrum with three peaks: a triplet at 1.2 ppm (3H), a quartet at 3.7 ppm (2H), and a singlet at 2.6 ppm (1H, often broad). (a) Assign each peak to a hydrogen environment. (b) Explain the splitting patterns using the n+1 rule.Show worked answer →
A 3-mark answer needs the assignments and the n+1 reasoning.
(a) Assignment of peaks (ethanol, CH3-CH2-OH).
- 1.2 ppm triplet (3H): CH3. Three equivalent methyl hydrogens, adjacent to a CH2 (2 neighbours). Shielded (no electron-withdrawing groups attached).
- 3.7 ppm quartet (2H): CH2. Two methylene hydrogens, adjacent to a CH3 (3 neighbours). Deshielded by the adjacent oxygen.
- 2.6 ppm singlet (1H): OH. Hydroxyl proton; chemical shift varies (1 to 6 ppm depending on solvent and concentration). Appears as a singlet because exchange with traces of water (or D2O exchange) decouples it from the neighbouring CH2.
(b) Splitting via n+1 rule. A proton signal is split by n equivalent neighbouring protons into (n+1) peaks. The CH3 has 2 neighbours on the adjacent CH2, so 2 + 1 = 3 peaks (triplet). The CH2 has 3 neighbours on the adjacent CH3, so 3 + 1 = 4 peaks (quartet). The OH normally has 2 neighbours but appears as a singlet because of rapid exchange (a Unit-4-relevant exception that markers explicitly reward).
Markers reward the three assignments with chemical shifts justified, the n+1 application to both alkyl signals, and the explicit "OH appears as singlet due to exchange" caveat.
Related dot points
- Describe the principles and apply mass spectrometry and infrared (IR) spectroscopy to determine the molecular mass, molecular formula, structural features and functional groups of organic compounds
A focused answer to the QCE Chemistry Unit 4 dot point on mass spectrometry and IR spectroscopy. Explains the molecular ion (M+) and fragmentation peaks in MS, the diagnostic IR absorption ranges (O-H, N-H, C=O, C-O, C-H), and walks through identifying an unknown C3H6O2 from its MS and IR spectra. The standard QCAA IA3 / EA spectroscopy item.
- Describe the principles and apply chromatographic techniques (thin-layer chromatography (TLC), gas chromatography (GC) and high-performance liquid chromatography (HPLC)) to separate, identify and quantify the components of a mixture
A focused answer to the QCE Chemistry Unit 4 dot point on chromatography. Explains the stationary/mobile phase principle, Rf values in TLC, retention times in GC and HPLC, and the use of calibration curves for quantification. Includes the canonical food and pharmaceutical IA3 / EA contexts.
- Describe and explain structural isomerism (chain, position and functional group isomers) and stereoisomerism (cis-trans / geometric isomerism in alkenes) in organic compounds
A focused answer to the QCE Chemistry Unit 4 dot point on isomerism. Distinguishes chain, position and functional-group isomers, sets out the conditions for cis-trans isomerism in alkenes, and works through C4H8O3 and 1,2-dichloroethene examples. Highlights the property differences QCAA tests in IA3 secondary data.