Amazingly enough, IR is not used much by professional organic chemists. This is because all IR can show is different functional groups. Thus, IR cannot tell the difference between any of the molecules shown below:
All of the molecules above will show an OH peak and various C-H stretches, but each spectra will look striking similar. Hence, we should recognize the limits of the instrument and not try to use it for more than it is intended. Further, there are really only four peaks in an IR that we look for to tell us something about our unknown molecule. Here, we show them in order of importance.
1) Carbonyl peak (1750-1650cm-1): This will be a very sharp, prominent peak and shows that a carbonyl is present in somewhere in your unknown molecule. What is less obvious is which type of carbonyl it is. It is not as easy to distinguish between ketones, esters, aldehydes, ect.
2) OH peak (3500-3200cm-1): This is a large, broad stretch which cannot be mistaken for any other functionality. One problem to be aware of is that the OH of water will also show up here, in the event that your unknown is not totally dry. Remember, that this can be from an alcohol OH or a carboxylic acid OH.
3) C-O peak (1300-1040cm-1): Usually a large, sharp peak, this can be from an alcohol, carboxylic acid, ether, or an ester.
4) Nitriles (2250-2230cm-1) and alkynes (2100-2280cm-1) peaks: Usually rather small peaks, but easy to spot as they are the only peaks in that area.
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