Topic 21.1 - Spectroscopic identification of organic compounds

Question 1

¹H NMR

Answer 1

Proton nuclear magnetic resonance

Hydrogen’s nuclei can have two spins, allowing them to act as magnets. No magnetic fields leads to the spins having equal energy and having a random orientation.

If there is a magnetic field, the hydrogens can either align with the field or not. If the hydrogens align, there is low energy and the hydrogens are stable; however if they don’t align, there is high energy.

The differences between the energy levels corresponds to an energy level in the EMS (electromagnetic spectrum). The energy difference is based on the chemical environment of the hydrogens.

Question 2

How ¹H NMR is measured

Answer 2

Measured as chemical shifts, signals, and splitting patterns:

Chemical shift -> the position of the NMR in comparison to the standard: tetramethylsilane (Si(CH₃)₄)

TMS peaks at 0 and it is used as a reference.

TMS is used because its signal is away from the others, it gives one signal, it is not toxic, it is inert, and it has a low boiling point (easy to remove)

Signals -> peaks where a hydrogen environment is shown

Splitting pattern -> singlet, triplet, etc

Question 3

Low-resolution NMR

Answer 3

Low resolution gives one peak for each environmentally different proton groups.

Question 4

High-resolution NMR

Answer 4

High-resolution NMR gives more complex signals - doublets, triplets, quartets, multiplets. The number of protons in an environment can be measured by the n-rule. The n-rule states that there will be n+1 peaks for a carbon, based on the number of hydrogens on adjacent carbons.

Question 5

Calculating peaks for high-resolution NMR when the number of hydrogens on adjacent carbons is 5

Answer 5

n ->
5

n+1 ->
6

Ratio ->
1:5:10:10:5:1

Appearance ->
    l l
    l l
  l l l l 
l l l l l l