# How to determine the unknown molecule using the infrared spectroscopy?

An unknown sample was analysed using infrared spectroscopy. What is the compound?

So I am having difficulty in using the infrared data to determine the unknown compound. I keep on misinterpreting the information on the Infrared spectrum. According to the answer provided, it is ethanamide. But how? What does the strong and broad range tell you? Is it $$2730-3100$$ or $$3200-3600$$? Why can't it be $$O-H$$? What about the range between $$1500 - 2000$$, does that correspond to $$C$$ double bond $$C$$ or $$C$$ double bond $$O$$?

How can I interpret the diagram correctly? The range seems to overlap it selves?

• You can't identify an unknown compound by manually analyzing its IR spectra. Computers can do it by pattern matching the unknown spectra to a library of spectra, assuming that the unknown sample is pure. // You could perhaps choose the correct compound from say 4 choices.
– MaxW
Nov 3 '20 at 9:25
• With the information provided it is not possible to identify this moelcule as an amide as the crucial information about the amide C=O and N-H signals is not given. This will provide hte missing information personal.utdallas.edu/~scortes/ochem/OChem_Lab1/recit_notes/… Nov 3 '20 at 11:24
• @Waylander Sorry, it is a multiple choice question. Mind taking a look again? Thanks Nov 3 '20 at 11:46
• @MaxW Hi Max, thanks for commenting. It is a multiple choice question, see the update. Many thanks. Nov 3 '20 at 11:47
• @Negrawh In future please give us the whole question first time Nov 3 '20 at 20:30

First, eliminate ethylene (ethene) because the strong absroption from 3200 to 3600 requires $$-OH$$ or $$-NH$$.

Ethanol could be suggested by the 3200-3600, but lacks a strong absorption at 1050-1410, so eliminate ethanol.

The strong peak at 1700 suggests acid or amide $$C=O$$.

The lack of strong absorption between 2500-3000 eliminates carboxylic acid, so the answer must be: ethanamide!

This process is only possible because it is a multiple choice question. In the actual world, a sample would be analyzed thusly:

1. if it's ethylene, it will be in a gas sample tube, and has a recognizable odor.
2. if it is ethanol, it evaporates quickly, and has a recognizable odor.
3. if it is ethanamide, it evaporates slowly and has a fishy odor.
4. if it is ethanoic acid, it smells like vinegar.

In addition to James's answer, you can often tell primary amines and amides apart from OH groups because they will have two peaks in the 3300 - 3500 region. Your sample clearly shows this double peak, suggesting an NH2 group. Combined with the strong c=o absorbance at 1700 and you have an amide.