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

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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?

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    $\begingroup$ 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. $\endgroup$ – MaxW Nov 3 '20 at 9:25
  • $\begingroup$ 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/… $\endgroup$ – Waylander Nov 3 '20 at 11:24
  • $\begingroup$ @Waylander Sorry, it is a multiple choice question. Mind taking a look again? Thanks $\endgroup$ – Negrawh Nov 3 '20 at 11:46
  • $\begingroup$ @MaxW Hi Max, thanks for commenting. It is a multiple choice question, see the update. Many thanks. $\endgroup$ – Negrawh Nov 3 '20 at 11:47
  • $\begingroup$ @Negrawh In future please give us the whole question first time $\endgroup$ – Waylander Nov 3 '20 at 20:30
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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.
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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.

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