Ch 3 · Carbon-13 NMR spectroscopy · AS & A Level Chemistry 9701
Topic 3 of 4 in Analytical techniques — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
Carbon-13 NMR spectroscopy detects the small proportion of naturally occurring ^13C nuclei in a sample, giving one peak for each chemically distinct carbon environment in a molecule and, from its chemical shift, information about the groups attached to that carbon.
Key ideas — What a carbon-13 NMR spectrum shows
A carbon-13 NMR spectrum plots signal intensity against chemical shift, delta, measured in parts per million (ppm) relative to the reference compound tetramethylsilane (TMS), set at delta = 0. Each peak corresponds to one carbon environment: a carbon atom, or set of carbon atoms, chemically equivalent to every other carbon in that same environment but distinguishable from carbons in a different environment.
🧑🏫 Teacher's explanation
Imagine you are in a crowded room where everyone is wearing a different colored shirt. If you want to identify groups of people, you look for people wearing the same color. In Carbon-13 NMR, the 'color' is the electronic environment surrounding a carbon atom. Each unique 'color' produces a signal (a peak) on the spectrum.
📖 Definitions
🔍 Worked example — Easy
How many signals would you expect to see in the ^13C NMR spectrum of propan-1-ol, CH_3CH_2CH_2OH?
🔍 Worked example — Standard
A compound has the molecular formula C_2H_4Cl_2. It shows only one peak in its ^13C NMR spectrum. Deduce the structure.
🔍 Worked example — Exam-style
A molecule with formula C_4H_10 shows two signals in its ^13C NMR spectrum. Identify the possible isomer.
⚠️ Common errors
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🎯 Exam question
Key ideas — Predicting the number of peaks
The number of peaks in a ^13C spectrum equals the number of chemically distinct carbon environments in the molecule, found by identifying which carbon atoms are related by symmetry, and so equivalent, and which are not. Propan-1-ol, CH_3CH_2CH_2OH, has three different carbon environments and gives three peaks; propan-2-ol, (CH_3)_2CHOH, has only two environments, since the two identical CH_3 groups are equivalent by symmetry, giving two peaks.
🧑🏫 Teacher's explanation
Imagine you are looking at a group of identical twins wearing different colored shirts. Even though they are the same person (same element), they look different to an observer because of their surroundings. In ^13C NMR, the 'observer' is a magnetic field. If two carbon atoms are in the exact same chemical environment—meaning they are bonded to the same types of atoms and are at the same distance from functional groups—they are 'equivalent' and will produce a single signal.
📖 Definitions
🔍 Worked example — Easy
How many peaks would be expected in the ^13C NMR spectrum of ethane, CH_3CH_3?
🔍 Worked example — Standard
Predict the number of peaks for propan-1-ol, CH_3CH_2CH_2OH.
🔍 Worked example — Exam-style
A compound with the formula mathrmC_4H_10 shows only 2 peaks in its ^13C NMR spectrum. Deduce the structure.
⚠️ Common errors
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🎯 Exam question
Key ideas — Using chemical shift to deduce structure
Each range of chemical shift, from the Cambridge data booklet, is characteristic of carbon in a particular bonding environment: mathrmC-C aliphatic carbon with no adjacent electronegative atom, 0–50 ppm; mathrmC-Cl/mathrmC-Br, 10–70 ppm; mathrmC-N of amines, 25–60 ppm; mathrmC-O of alcohols and ethers, 50–90 ppm; aromatic ring carbon, 110–165 ppm; nitrile carbon, 110–125 ppm; ester/carboxylic acid carbonyl carbon, 160–185 ppm; and aldehyde/ketone carbonyl carbon, 190–220 ppm. Matching the number and shift of the observed peaks to a proposed structure, and checking that no peak is unaccounted for, allows possible structures to be deduced or confirmed.
🧑🏫 Teacher's explanation
Imagine you are in a crowded room where everyone is wearing a different colored shirt. If you want to know how many different 'types' of people are there, you wouldn't count every single person; you would count how many different colors are present. Carbon-13 NMR works exactly like this for molecules.
📖 Definitions
🔍 Worked example — Easy
A compound with the molecular formula CH_3 extCH_2 extOH is analyzed. Predict the number of peaks expected in its ^13C NMR spectrum and identify the approximate chemical shift range for the C-O carbon.
🔍 Worked example — Standard
An unknown molecule has a ^13C NMR spectrum with two peaks: one at 25 ppm and one at 205 ppm. Suggest a possible functional group for each peak.
🔍 Worked example — Exam-style
A molecule has the formula CH_3 extCOOCH_2 extCH_3. How many peaks will appear in the ^13C NMR spectrum, and what are the approximate chemical shift ranges for the carbonyl carbon and the C-O carbon?
⚠️ Common errors
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🎯 Exam question
Key ideas — Why 13C NMR gives sharp, well-separated peaks
A carbon-13 NMR spectrum is normally recorded with the attached hydrogen nuclei decoupled, their spin effects removed electronically, so a carbon's signal is not split by neighbouring protons and each carbon environment gives a single, sharp peak rather than a group of overlapping lines. Because only about 1.1\% of carbon atoms are the NMR-active ^13C isotope — the common ^12C isotope has no nuclear spin and is invisible to NMR — the chance of two ^13C nuclei being directly bonded in the same molecule is very small, so carbon–carbon spin–spin splitting is negligible and the peaks remain sharp and well separated.
🧑🏫 Teacher's explanation
Imagine you are in a crowded stadium trying to hear a single person shout. If everyone is shouting at once, it is just noise. If everyone is shouting different things at different times, it is chaos. In ^13C NMR, we want to hear each person (each carbon atom) clearly without them being drowned out by the crowd or interrupted by their neighbors.
📖 Definitions
🔍 Worked example — Easy
How many signals should be observed in the ^13C NMR spectrum of propan-1-ol, ceCH3CH2CH2OH, assuming all carbons are in unique environments?
🔍 Worked example — Standard
A molecule has the formula ceCH3CH2CH2CH3. How many signals will appear in its ^13C NMR spectrum?
🔍 Worked example — Exam-style
Explain why ^13C NMR spectra do not show ^13C-^13C spin-spin splitting, even though ^13C has a nuclear spin.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. The carbon-13 NMR spectrum of compound Y, molecular formula C_3H_6O_2, shows three peaks at delta = 14, delta = 60 and delta = 172 ppm. (a) State the number of carbon environments in Y. [1] (b) Using the chemical shift data, identify the type of carbon responsible for each peak. [3] (c) Hence suggest a structure for Y. [1]
🔍 Worked example — Easy
A molecule has the formula CH_4. Predict the number of peaks in its ^13C NMR spectrum.
🔍 Worked example — Standard
The ^13C NMR spectrum of a compound shows two peaks at delta = 25 ppm and delta = 135 ppm. If the molecule is C_2H_4O, suggest a possible structure.
🔍 Worked example — Exam-style
Compound Y has molecular formula C_3H_6O_2. Its ^13C NMR spectrum shows peaks at delta = 14 ppm, delta = 60 ppm, and delta = 172 ppm. Identify the structure.
⚠️ Common errors
Common mistakes
Practice
Answers