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Given the following pairs of samples, how do I distinguish between them?

  1. benzamide vs ethanamide
  2. ethanoyl chloride vs ethanoic anhydride

My Approach:

  1. We could easily convert the amides to amines by reducing them with $\ce{LiAlH4}$. Now, the question reduces to identifying the methyl group against the benzene group.

  2. I am really lost here but my guess would be that we are looking for some characteristic reaction for Anhydrides.

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  • $\begingroup$ This seems like a homework question. We ‎have a policy which states that you should show your thoughts and/or efforts into solving the ‎problem. It'll make us certain that we aren't doing your homework for you. Otherwise, this ‎question may get closed. $\endgroup$ – bon Oct 26 '15 at 14:46
  • $\begingroup$ @bon I have updated the question to show my efforts but otherwise, I'm reallly lost here. $\endgroup$ – Agnishom Chattopadhyay Oct 26 '15 at 15:01
  • $\begingroup$ Well what reactions do benzene rings undergo that methyl groups don't? $\endgroup$ – bon Oct 26 '15 at 15:04
  • $\begingroup$ Flame Test: Sooty Flame appears when Aromatic Compounds are put on the bunsen flame $\endgroup$ – Agnishom Chattopadhyay Oct 26 '15 at 17:20
  • $\begingroup$ This is hardly a very accurate test. Can you think of any other reactions that might work. $\endgroup$ – bon Oct 26 '15 at 17:32
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Since this sounds like an assignment, I am not going to give it all away.

Does you have to use qualitative chemical tests? I don't even teach those anymore. They were supplanted by spectroscopic techniques in most professional labs decades ago.

IR spectroscopy would answer both questions quickly.

  1. Benzamide would have peaks characterizing an aromatic ring, for example: $\ce{C}_{sp^2}\ce{-H}$ stretches around $3050-3100\ \mathrm{cm^{-1}}$. Ethanamide will not have these peaks, but will instead have $\ce{C}_{sp^3}\ce{-H}$ peaks $2800-3000\ \mathrm{cm^{-1}}$. There are other differences as well.
  2. There are differences in the position (and sometimes number!) of carbonyl peaks between an anhydride and an acid halide. What are they?

So would mass spectrometry. Benzamide and ethanamide have different molar masses and different fragmentation patterns. Ethanoyl chloride... chlorine has two major isotopes. How can that help?

NMR spectroscopy would also be useful. However, IR is cheap, fast, and easy.

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