1. C2.8 Analytical Techniques
NMR spectroscopy
C2.8 Analytical Techniques
01 December 2018

2. NMR Spectroscopy Specification statement:
I can use 13C and low resolution 1H NMR spectra in the identification of chemical structures
You don’t need to know anything about how NMR works for the exam. HOWEVER in order to identify structures using NMR you need some of the background knowledge.
Complete the reading task to get an idea of the background of NMR spectroscopy.

3. Knowledge Check
What is the standard reference material chemical for chemical shift measurements?
What is meant by chemical shift?
What causes differences in chemical shift?

4. 13C Spectroscopy
Naturally occurring carbon atoms contain a very small percentage of the isotope 13C.
The presence of this isotope means organic compounds will absorb energy and a 13C NMR spectra can be produced.
The spectrum will have chemical shifts, δ, on the x-axis and absorption on the y-axis.
All the chemical shifts can be found in your data booklet.

5. 13C Spectroscopy
The chemical environment of a carbon atom is determined by the position of the atom within a molecule.
Carbon atoms that are bonded to different atoms or groups of atoms have different environments and will absorb at different chemical shifts.
If two carbon atoms are positioned symmetrically within a molecule then they are equivalent and will contribute to the same chemical peak.

6. Worked examples
Identify the carbon environments in the following molecules: Propan-2-ol Butanal Hexane

7. Knowledge Check
Draw the structure of the following molecules and identify the different carbon environments present:
Pentan-1-ol
Butan-2-ol
Methyl propan-2-ol
Methyl propan-1-ol

8. Interpreting 13C spectra
The spectrum gives two pieces of information:
The number of different carbon environments (number of peaks)
The type of carbon environment (from the chemical shifts)
The number of carbons in that environment (height of peaks)

9. Examples 3 2 1

10. Examples 2 1

11. Examples 2 1

12. Worked example

13. Worked Example

14. Knowledge Check
1. For each compound below, predict the number of 13C peaks and the chemical shift of each peak:
2. Draw the structures of the following and label each carbon environment. Predict the number of peaks in the carbon-13 NMR spectra of each isomer.
Pentane
Pentan-2-ol
Pentan-3-ol
Methyl benzene
CH3COOH
CH3COOCH2CH3
CH3CHO
CH3CH=CHCH3
CH3NHCH3
HOCH2CH2CHO

15. Past Paper Question
An alcohol Z has the molecular formula C3H8O. Identify the two possible isomers of Z and draw the 13C NMR spectrum of these molecules. (4 marks)