Question

How can you tell the difference between an ester, ketone, carboxylic acid, aldehyde, amines, amides and phenol using infrared spectroscopy?

Answer 1

Possibly you can't use IR spectroscopy to differentiate between them.

Explanation:

Older chemists might refer to IR spectroscopy as a "sporting method of analysis". They refer to the fact, when you go shooting, it is UNSPORTING to shoot a bird on the ground, rather than on the wing. Modern methods of analysis, X-ray crystallography and NMR spectroscopy, are definitely NON-sporting means of analysis, in that they give you a direct idea of connectivity without much guesswork.

IR spectroscopy readily identifies the carbonyl group `C=O` of organic compounds: of amides, or esters, or ketones, of acids as a strong sharp absorption at around `1900-1700` `cm^-1`. The problem is while they occur as a strong absorption between `1800` and `1600` `cm^-1`, which can be automatically "tagged" as a carbonyl, they could be amides `C(=O)NR_2`, or ketones, `RC(=O)R`, or aldehydes, or anhydrides, or acids, all of which contain the carbonyl group. Sometimes you can infer an ester if you see a carbonyl, and note strong absorption at approx. `1050` `cm^-1` due to the `C-O` stretch.

Phenols and amines of course do not contain the carbonyl absorption. Sometimes, the `O-H` or `N-H` bonds may be inferred on the IR spectrum. Anyway, IR spectroscopy will give you an idea of the functional groups present; they will not identify the molecule. Look at various IR spectra to distinguish between `C(=O)` absorptions.