Interpret a proton (1H) NMR spectrum using chemical shift, peak area (integration), and spin-spin splitting (the n+1 rule), and use D2O exchange to identify OH and NH protons

What this dot point is asking

SEAB wants you to interpret a proton ($^1\text{H}$) NMR spectrum using chemical shift, integration (peak area), and spin-spin splitting (the n+1 rule), and to use D2O exchange to identify OH and NH protons. Reading a proton NMR spectrum is the most information-rich structure-determination skill in the syllabus.

The answer

What proton NMR measures

Hydrogen nuclei (protons) behave like tiny magnets. In a strong magnetic field they absorb radio-frequency radiation, and the exact frequency depends on the electronic environment of each proton. The spectrum plots absorption against chemical shift ($\delta$, in ppm), referenced to TMS at $\delta = 0$.

Chemical shift: the proton environment

Different chemical environments give different chemical shifts (read from the data-booklet table). Rough guide:

• $\text{CH}_3$, $\text{CH}_2$, CH (alkyl): $\delta \approx 0.9$ to $1.7$
• protons next to C=O or a halogen: $\delta \approx 2$ to $4$
• protons on a carbon bearing O ($\text{O-CH}$): $\delta \approx 3.3$ to $4.5$
• aromatic ring protons: $\delta \approx 6.5$ to $8$
• aldehyde CHO: $\delta \approx 9$ to $10$
• carboxylic acid OOH: $\delta \approx 10$ to $12$

The number of distinct peaks (or groups of peaks) equals the number of different proton environments.

Integration: how many protons

The area under each peak (the integration trace) is proportional to the number of protons in that environment. A ratio of areas $3:2:1$, for example, suggests a $\text{CH}_3$, a $\text{CH}_2$ and one other proton.

Spin-spin splitting: the neighbours

A proton's signal is split by the protons on neighbouring carbon atoms, because their spin states slightly alter the local magnetic field. The n+1 rule: a proton with $n$ equivalent neighbouring protons is split into $(n+1)$ peaks:

• 0 neighbours: singlet
• 1 neighbour: doublet
• 2 neighbours: triplet
• 3 neighbours: quartet

So in $\text{CH}_3\text{CH}_2\text{Br}$, the $\text{CH}_3$ (2 neighbours) is a triplet and the $\text{CH}_2$ (3 neighbours) is a quartet.

D2O exchange: spotting OH and NH

Protons on O-H and N-H groups can exchange with deuterium when D2O is added. After shaking with D2O, the O-H or N-H peak disappears (the proton is replaced by a non-absorbing deuterium). So a peak that vanishes on adding D2O identifies an exchangeable OH or NH proton.

Examples in context

Example 1. Distinguishing isomers. Proton NMR readily separates isomers that share a molecular formula. For example, the splitting patterns and chemical shifts distinguish methyl ethanoate from ethyl methanoate, or propan-1-ol from propan-2-ol. SEAB uses this to test whether candidates can move from a spectrum to a unique structure.

Example 2. Checking the purity of a synthesised product. After a synthesis, chemists run a proton NMR spectrum to confirm the expected number of environments and integration ratios; unexpected peaks reveal impurities or by-products. This quality-control use shows why NMR is central to modern organic chemistry and is increasingly tested in Paper 3.

Try this

Q1. Predict the splitting pattern of each proton environment in 1,1,2-trichloroethane, $\text{CHCl}_2\text{CH}_2\text{Cl}$. [2 marks]

• Cue. The CH (one neighbour set of 2) is a triplet; the $\text{CH}_2$ (one neighbour) is a doublet.

Q2. Explain how you would use D2O to identify the O-H proton in a spectrum. [2 marks]

• Cue. Add D2O and re-run; the O-H peak disappears because the proton exchanges for a non-absorbing deuterium.

Q3. A spectrum shows integration ratio $9:1$. Suggest a structural feature consistent with the 9-proton peak. [1 mark]

• Cue. A $(\text{CH}_3)_3\text{C}-$ (tert-butyl) group, three equivalent methyls giving nine protons.