It is possible to identify functional groups by figuring out whether the peaks in the infra-red spectrum are characteristic of bonds found in that group or not. Characteristic absorptions include:
Carbonyls (C=O): the sharp absorption at 1715 cm-1 is also present in esters and carboxylic acids.
Alcohols (C-O and O-H): the broad spectrum at 3100 – 3500 cm-1 is characteristic of an O-H bond found on an alcohol. The sharp peak at 1050 cm-1 is due to the C-O bond.
Carboxylic acids (C=O, C-O and O-H): the very broad spectrum at 2500-3000 cm-1 is characteristic of the O-H bond found in a carboxylic acid. The sharp peaks represent the C=O bond (at 1710 cm-1) and the C-O bond (at 3000 cm-1).
Esters (C=O and C-O): the sharp absorptions represent a C-O bond (at 1250 cm-1) and a C=O bond (at 1750 cm-1. The absence of the broad absorption between 2500 and 3500cm-1 indicates that there are no O-H bonds.
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Identifying impurities
A level Chemistry revision notes- most of the time impurities will be found in organic compounds. Infra-red spectroscopy can be used to identify the extent and nature of these impurities. The bonds which compose the impurity will also absorb the radiation and give peaks which would not be expected in the structure of the desired molecule.
In a known pure sample no impurities are present. Infra-red spectroscopy can, therefore, be used to test the purity of organic compounds and identify any impurities.
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Using the fingerprint region to identify functional groups
The majority of functional groups have characteristic absorptions that fall between 1500 and 3500 cm-1. As there are very few other peaks within this region of the spectrum it is usually used to identify functional groups (apart from the C-H peak which is easy to identify). Therefore, these peaks can be attributed to particular bonds.
The region of the spectrum which falls between 500 and 1500cm-1 is more complex as it tends to contain a wide array of peaks positioned close to one another. This is due to the molecule’s complete structure as opposed to specific bonds. It is known as the fingerprint region and for every molecule it is completely different, even if they share a functional group. Therefore, a molecule can be identified from this region in an infra-red spectrum. A fingerprint region can then be compared to pre-recorded infra-red spectra until a match is made, a process known as fingerprinting.
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Global warming
The fact that covalent bonds absorb infra-red radiation is the reason for global warming. The Sun’s radiation (which includes visible and ultra-violet light) is absorbed by the Earth. It is then emitted at a lower frequency which tends to sit within the infra-red region.
Certain molecules in the atmosphere, like carbon dioxide, methane and water, do not absorb either visible or ultra-violet light. This means that they allow radiation to travel to the Earth’s surface. If the light re-emitted by the Earth (which lies within the infra-red region) is a match to the natural frequency vibration or bond rotations in these molecules, the radiation is absorbed.
It is important for life on this planet that radiation is emitted by the Earth in order to keep it warm, a process called the greenhouse effect. However, an increase in the level of carbon dioxide in the atmosphere has led to too much heat being trapped which has caused global warming.