Carbon-13 NMR Spectroscopy

Specification Reference Organic Chemistry, Analytical techniques 37.3

Quick Notes

13C NMR shows the environments of carbon atoms in a molecule.
Each peak = a different carbon environment.
- Chemically equivalent carbon atoms produce a single peak.
- Chemical shift (δ) (in ppm) indicates the type of environment (e.g. alkane, ester, carbonyl).
Number of peaks = number of unique carbon environments.
We can use chemical shift data to deduce the structure or confirm possible structures.

Full Notes

The background theory of carbon and hydrogen NMR has been outlined in more detail here.
This page is just what you need to know for CIE A-level Chemistry :)

Carbon-13 Nuclear Magnetic Resonance (13C NMR) is a type of spectroscopy used to identify the different carbon environments in an organic molecule. Graphs (spectra) are produced that are analysed.

Key Features of a 13C NMR Spectrum

Each peak represents a unique carbon environment in the molecule.
Equivalent carbon atoms produce one signal.
The number of peaks = number of different carbon environments.
The chemical shift (δ, in ppm) gives you information about the environment of the carbon atom and what it may be bonded to.

Matt’s exam tip

The most important thing to be able to do with NMR is to recognise ‘unique’ carbon and hydrogen environments in a molecule, before worrying about more advanced areas such as peak splitting and the n+1 rule. Remember it isn’t just the immediate atoms bonded to a carbon or hydrogen you need to look at. It is also what those atoms are themselves bonded to.

CIE A-Level Chemistry diagram showing unique carbon environments in molecules for 13C NMR interpretation.

Chemical Shift Values (Typical Ranges)

We can use chemical shift data (from the Data Booklet) to identify types of carbon atoms in a sample:

CIE A-Level Chemistry data table showing typical chemical shift ranges for 13C NMR environments.

Interpreting 13C NMR Spectra

To interpret a spectrum:

Count the peaks: This gives the number of distinct carbon environments.
Use symmetry: If the molecule has symmetry, some carbon atoms will be equivalent and produce only one peak.
Match chemical shifts: Use the δ values to deduce which functional groups might be present.

Example: Propanol (CH₃CH₂CH₂OH)

CIE A-Level Chemistry 13C NMR spectrum of propanol showing three peaks for three carbon environments.

3 Peaks

One for CH₃ (between 0–50 ppm)
One for CH₂ (between 0–50 ppm)
One for CH₂OH (between 50–70 ppm)

Predicting 13C NMR Peaks

You can be asked to predict the number of peaks for a given structure.

Example: Butane (CH₃CH₂CH₂CH₃)

CIE A-Level Chemistry 13C NMR carbon environments for butane showing two types due to symmetry.

Has 2 carbon environments due to symmetry (two sets of carbons are bonded to the exact same groups).
13C NMR: 2 peaks

Example: Butanone (CH₃COCH₂CH₃)

CIE A-Level Chemistry 13C NMR carbon environments for butanone showing four distinct environments.

4 distinct carbon atoms (no symmetry across all carbons and all carbons are bonded to different groups).
13C NMR: 4 peaks

Summary

Carbon-13 NMR is a powerful technique for identifying different carbon environments in organic molecules.
It is especially useful when used alongside other techniques (like IR or mass spectrometry) to deduce the full structure.
Focus on the number of peaks, chemical shift ranges, and molecular symmetry when analysing spectra.