Organic Analysis

Question 1

Summarise the theory of infrared spectroscopy

Answer 1

Infrared energy is absorbed by bond which make this bonds vibrate
The bonds vibrate at the same frequency as the infrared energy

Question 2

The process which occurs inside an IR spectrometer is as follows:

Answer 2

1. A beam of IR light conatining a wide range of frequencies is passed through a sample.
2. Some of the frequencies of IR light are absorbed by the molecule. Many of the frequencies are not absorbed, and so pass straight through the sample. The light that emerges through the other side of the sample is missing the frequencies which are absorbed.
3. Once detected, an computer plots a graph of the intencity of the light which emerges from the sample. This is called TRANSMITTANCE.
4. The frequency is plotted on the x-axis with the units ‘wavenumbers’ which are cm-1.

Question 3

A typical IR spectrum:

Answer 3

This spectrum shows that IR radiation around 2950cm-1 and 1450cm-1 has been absorbed by the bonds in the sample molecule. The peaks on the spectra are called absorptions. Different bonds have different absorptions.

Question 4

Exam technique

Answer 4

Whenever answering an IR question, always quote the absorption range for the peak, and the type of bond it represents.

Example: “The spectrum represents a ketone because it has an aborption matching the C=O bond from 1680-1750cm-1”

Question 5

What bonds must be present in the molecule?

Answer 5

This spectrum shows that IR radiation around 1000 to 1300 cm-1 is absorbed due to the C-O bond, and 1680 to 1750cm-1 has been absorbed by the C=O bond.

Question 6

Molecular Ions Peaks KEY FACTS

Answer 6

• The peak with the largest m/z value is called the ‘Molecular Ion’ peak.
• The m/z ratio of the molecular ion peak is equal to the Mr of the molecule. In many spectroscopy questions this is the way you would find the Mr of an unknown molecule.