Ch 4 · Proton (1H) NMR spectroscopy · AS & A Level Chemistry 9701

Topic 4 of 4 in Analytical techniques — AS & A Level Chemistry 9701 — 68 lessons.

Introduction

Proton (^1H) NMR spectroscopy is the most structurally powerful of the A2 spectroscopic techniques: the number, position, area and splitting of its peaks together reveal how many types of proton a molecule contains, how many of each, what is attached nearby, and even which protons are exchangeable with water.

Key ideas — Proton environments and chemical shift

Protons in the same proton environment — related to each other by molecular symmetry and bonded to equivalent carbon or heteroatom environments — give a single peak at a characteristic chemical shift, delta, in ppm. From the Cambridge data booklet, typical ranges include: mathrmC-CH_3 (alkyl), 0.7–1.6; mathrmC-CH_2-C, 1.2–1.4; mathrmCH-C=O, 2.0–2.9; mathrmO-CH, 3.3–4.3; aromatic mathrmAr-H, 6.5–8.5; aldehyde -CHO, 9.0–10.0; carboxylic acid -COOH, 10.0–12.0, variable; and alcohol -OH, 1.0–5.5, variable and broad.

🧑‍🏫 Teacher's explanation

Imagine a group of people standing in a room. If everyone is standing in an open space, they all hear the same background noise similarly. But if one person stands behind a thick curtain, the sound changes for them. In NMR, the 'sound' is the magnetic field. Protons (hydrogen nuclei) act like tiny spinning magnets. When placed in a strong external magnetic field, they experience a local environment. If a proton is surrounded by electrons, those electrons 'shield' it from the external field. If it is near an electronegative atom like Oxygen, the electrons are pulled away, 'deshielding' the proton and making it feel a stronger magnetic field.

📖 Definitions

🔍 Worked example — Easy

A molecule has two types of protons. Type A has a chemical shift of 1.2 ppm and Type B has a chemical shift of 3.5 ppm. Identify which type is likely bonded to an electronegative atom like Oxygen.

🔍 Worked example — Standard

Calculate the ratio of protons in a molecule where the area under the peak at 1.3 ppm is 12 units and the area under the peak at 3.7 ppm is 6 units.

🔍 Worked example — Exam-style

A compound with molecular formula ceCH3CH2Cl shows two signals. One is a triplet and one is a quartet. Predict the chemical shift ranges for the ceCH3 group and the ceCH2 group.

⚠️ Common errors

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🎯 Exam question

Key ideas — TMS and deuterated solvents

Tetramethylsilane (TMS), Si(CH_3)_4, is used as the standard reference: its twelve equivalent protons give one strong, sharp peak set at delta = 0, and because silicon is less electronegative than carbon, no other common proton absorbs at a lower shift, so every other peak is measured relative to it. The sample is dissolved in a deuterated solvent, such as CDCl_3, in which ordinary hydrogen atoms are replaced by deuterium; deuterium is not detected at proton NMR frequencies, so the solvent itself produces no interfering peaks in the spectrum.

🧑‍🏫 Teacher's explanation

Imagine you are trying to measure the speed of several different runners on a track. To make sure everyone is using the same scale, you place a 'marker' at the starting line and call that speed 'zero'. In NMR, we need a universal 'zero' point so that chemists across the world can agree on where a peak sits. We use Tetramethylsilane (TMS) for this because its protons are very 'shielded'—they are surrounded by a dense cloud of electrons—making them appear at a very low frequency, effectively our zero point.

📖 Definitions

🔍 Worked example — Easy

Explain why Si(CH_3)_4 is used as a standard in ^1H NMR spectroscopy.

🔍 Worked example — Standard

A sample is dissolved in CDCl_3. Why is this preferred over CHCl_3 for ^1H NMR?

🔍 Worked example — Exam-style

A spectrum shows a broad singlet that disappears when a drop of D_2 extO is added. Identify the functional group.

⚠️ Common errors

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🎯 Exam question

Key ideas — Integration: relative peak areas

The area under each peak is proportional to the number of protons responsible for it, recorded electronically as an integration trace or a ratio printed above each peak. Comparing the areas, or integration ratio, of all the peaks in a spectrum gives the relative numbers of each type of proton present, which, combined with the molecular formula, gives the actual number of each type.

🧑‍🏫 Teacher's explanation

Imagine you are looking at a crowd of people through a tinted window. You can't see individual faces clearly, but you see colored blobs. If the red blob is twice as large as the blue blob, you can assume there are twice as many people wearing red shirts as there are people wearing blue shirts. In ^1H NMR, the 'color' is the chemical shift (position on the x-axis) and the 'size' is the area under the peak.

📖 Definitions

🔍 Worked example — Easy

A ^1H NMR spectrum of a compound with the molecular formula C_2H_6O shows two peaks. The first peak has an integration value of 3, and the second peak has an integration value of 5. Determine the ratio of protons in the two environments.

🔍 Worked example — Standard

A compound with molecular formula CH_3CH_2Cl shows two signals. The first signal has an area of 30 units and the second signal has an area of 20 units. What is the simplest whole-number ratio of the protons in these two environments?

🔍 Worked example — Exam-style

An unknown compound with molecular formula C_3H_8O shows two peaks in its ^1H NMR spectrum. The integration ratio of the peaks is 3:5. Determine the actual number of protons in each environment.

⚠️ Common errors

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🎯 Exam question

(a) Explain why only one peak is observed. [1] (b) A second compound, C_3H_6Cl_2, shows two peaks in its ^1H NMR spectrum with an integration ratio of 1:2. Suggest a possible structure for this compound. [2] [3 marks]

Key ideas — Splitting patterns and the n+1 rule

Each proton's signal is split into a multiplet by the magnetic effect of non-equivalent protons on the immediately adjacent carbon atom(s); protons on the same carbon, or too far away, do not cause splitting. The n+1 rule states that a proton with n equivalent neighbouring protons is split into n+1 lines: one neighbouring proton gives a doublet, two give a triplet, three give a quartet, and no neighbouring protons give a singlet. Reading a splitting pattern therefore deduces directly the number of protons on the adjacent carbon.

🧑‍🏫 Teacher's explanation

Imagine you are in a crowded room trying to listen to a friend speak. If your friend is alone, you hear their voice clearly (a singlet). If there is one person standing right next to them, their voice might seem slightly muffled or split into two distinct tones because of the interference (a doublet). If there are two people, the interference becomes more complex (a triplet).

📖 Definitions

🔍 Worked example — Easy

A proton in a molecule is adjacent to 2 equivalent protons. Predict the splitting pattern using the n+1 rule.

🔍 Worked example — Standard

A molecule has a signal that appears as a quartet. How many protons are on the carbon atom adjacent to this group?

🔍 Worked example — Exam-style

A compound has the formula CH_3CH_2Cl. Predict the splitting pattern for the CH_3 group and the CH_2 group.

⚠️ Common errors

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🎯 Exam question

(a) Determine the number of protons on the carbon atom adjacent to the quartet. [1] (b) Draw the structure of the molecule. [2] [3 marks]

Key ideas — Identifying O-H and N-H by D2O exchange

The -OH and -NH protons exchange rapidly with the labile deuterium of heavy water, D_2O: shaking the sample with a few drops of D_2O and re-running the spectrum causes the -OH or -NH peak to disappear, replaced by -OD or -ND, which is invisible to proton NMR, while every mathrmC-H peak is unaffected, since carbon-bonded protons do not exchange. This confirms which peak, of several candidates, is due to an exchangeable -OH or -NH proton.

🧑‍🏫 Teacher's explanation

Imagine you are at a crowded party where everyone is wearing a name tag. Proton NMR is like a camera that takes a photo of everyone's name tag. However, some people are 'social butterflies' (the - extOH and - extNH groups) and they keep swapping name tags with the people around them. If you suddenly introduce a 'special guest' (heavy water, $ ext{D}_2 ext{O}$), these social butterflies will immediately swap their name tags for a different type that the camera cannot see.

📖 Definitions

🔍 Worked example — Easy

A student observes a ^1 extH NMR spectrum of an unknown organic compound. There are three signals: one at delta = 1.2 ext ppm, one at delta = 3.5 ext ppm, and one at delta = 4.8 ext ppm. After adding $ ext{D}_2 ext{O}$ to the sample, the signal at delta = 4.8 ext ppm disappears. Identify the functional group responsible for the signal at delta = 4.8 ext ppm.

🔍 Worked example — Standard

An unknown molecule has the formula $ ext{C}_2 ext{H}_6 ext{O}$. The ^1 extH NMR spectrum shows two peaks: a triplet at delta = 1.2 ext ppm and a singlet at delta = 5.0 ext ppm. What happens to the signal at delta = 5.0 ext ppm when $ ext{D}_2 ext{O}$ is added?

🔍 Worked example — Exam-style

A spectrum shows a peak at delta = 3.4 ext ppm (quartet) and a peak at delta = 1.2 ext ppm (triplet). A second spectrum of the same sample with $ ext{D}_2 ext{O}$ shows only the peaks at delta = 3.4 ext ppm and delta = 1.2 ext ppm. What is the structure of the original compound?

⚠️ Common errors

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🎯 Exam question

(a) Identify the functional group responsible for the signal at delta = 4.5 ext ppm. [1] (b) Deduce the structure of the compound. [2] [3 marks]

Key ideas — Deducing structure from a full spectrum

Combining every piece of evidence — the number of peaks (number of environments), their chemical shifts (type of environment), their integration ratio (relative numbers of protons) and their splitting (number of adjacent protons) — together with the molecular formula, allows a structure to be built up systematically and checked for consistency with every peak observed, including confirmation by D_2O exchange of any exchangeable proton.

🧑‍🏫 Teacher's explanation

Imagine you are looking at a crowded party through a frosted window. You can't see individual faces clearly, but you can see groups of people. You notice one group of three people standing together, and another group of two people standing near them. By looking at how close they are to each other (splitting) and how much space they take up (integration), you can figure out who is talking to whom.

📖 Definitions

🔍 Worked example — Easy

A compound with molecular formula C_2H_4Cl_2 shows one singlet in its ^1H NMR spectrum at 3.7 ext ppm. Deduce the structure.

🔍 Worked example — Standard

An unknown compound with formula C_3H_7O shows two signals. Signal A is a triplet at 1.2 ext ppm with an integration of 3H. Signal B is a multiplet at 3.4 ext ppm with an integration of 2H. Signal C is a singlet at 4.0 ext ppm with an integration of 2H. Deduce the structure.

🔍 Worked example — Exam-style

A molecule has the formula C_2H_5Br. The ^1H NMR spectrum shows a quartet with an area of 2 and a triplet with an area of 3. Calculate the chemical shift difference if the quartet is at 1.7 ext ppm and the triplet is at 3.4 ext ppm.

⚠️ Common errors

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🎯 Exam question

(a) Deduce the structure of the compound. [2] (b) Explain why the signal at 3.4 ext ppm is a triplet. [1] [3 marks]

Worked example

Question. Compound Q, molecular formula C_4H_8O_2, gives the following proton NMR spectrum: delta = 1.3 (3H, triplet), delta = 2.1 (3H, singlet), delta = 4.1 (2H, quartet). No peak disappears on shaking with D_2O. (a) State what the absence of any change on adding D_2O shows about compound Q. [1] (b) Using the integration, chemical shift and splitting data, deduce the structure of Q. [4] (c) Explain the splitting pattern of the peak at delta = 4.1. [2]

Ch 4 · Proton (1H) NMR spectroscopy · AS & A Level Chemistry 9701

All chapters in Analytical techniques