If you have followed this tutorial group-by-group, you will realize that in even rather simple, mono-functional molecules there are so many IR bands that it is not feasible to assign every band in an IR spectrum. Instead, look for tell-tale bands -- the region from 4000-1300 cm-1 is particularly useful for determining the presence of specific functional groups. You can rely on the IR correlation charts (linked in the left frame), but we recommend (in fact, in organic chem 1 lab, we require) that you memorize the distinctive bands of the common functional bands:
3500-3300 cm-1 NH stretch 1&Mac251;, 2&Mac251; amines 3500-3200 cm-1 OH stretch alcohols, a broad, strong band 3100-3000 cm-1 CH stretch alkenes 3000-2850 cm-1 CH stretch alkanes 1760-1665 cm-1 C=O stretch ketones, aldehydes, esters 1680-1640 cm-1 C=C stretch alkenes
Begin by looking in the region from 4000-1300. Look at the CH stretching bands around 3000:
Indicates: Are any or all to the right of 3000? alkyl groups (present in most organic molecules) Are any or all to the left of 3000? a C=C bond or aromatic group in the molecule
Look for a carbonyl in the region 1760-1690. If there is such a band:
Indicates: Is an OH band also present? a carboxylic acid group Is a CO band also present? an ester Is an aldehydic CH band also present? an aldehyde Is an NH band also present? an amide Are none of the above present? a ketone
(also check the exact position of the carbonyl band for clues as to the type of carbonyl compound it is)
Look for a broad OH band in the region 3500-3200 cm-1. If there is such a band:
Indicates: Is an OH band present? an alcohol or phenol
Look for a single or double sharp NH band in the region 3400-3250 cm-1. If there is such a band:
Indicates: Are there two bands? a primary amine Is there only one band? a secondary amine
Other structural features to check for:
Indicates: Are there CO stretches? an ether (or an ester if there is a carbonyl band too) Is there a C=C stretching band? an alkene Are there aromatic stretching bands? an aromatic Is there a C≡C band? an alkyne Are there -NO2 bands? a nitro compound
If there is an absence of major functional group bands in the region 4000-1300 cm-1 (other than CH stretches), the compound is probably a strict hydrocarbon.
Also check the region from 900-650 cm-1. Aromatics, alkyl halides, carboxylic acids, amines, and amides show moderate or strong absorption bands (bending vibrations) in this region.
As a beginning student, you should not try to assign or interpret every peak in the spectrum. Concentrate on learning the major bands and recognizing their presence and absence in any given spectrum.
In the organic chem teaching labs, you usually know what compound you started with and what compound you are trying to make. For instance, if you are oxidizing an alcohol to a ketone, your product should show a carbonyl band but no hydroxyl band. If no carbonyl band is present, the experiment was not successful. If both carbonyl and hydroxyl bands are present, the product is not pure.