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

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.