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I have the following question, enter image description here The given answer is c but I have trouble understanding why a would not be a correct answer. Because from my understanding in part a, one is amide functional group and the other is aldehyde with a -Nh2 nitrile group as a substituent. Then it should be a functional group isomerism too, right?

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  • $\begingroup$ Nitrile is -CN. Other than that, I agree with your reasoning. These are compounds with the same molecular formula, but different functional groups, hence they must be f.gr. isomers. $\endgroup$ Commented Apr 11, 2017 at 10:42
  • $\begingroup$ What's wrong with choice b? I'm leaning towards All of the above. $\endgroup$ Commented Apr 11, 2017 at 11:20
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    $\begingroup$ Incidentally, the aminoaldehyde would probably react with itself almost immediately. Notwithstanding that I agree that "all of the above" is the correct answer. $\endgroup$ Commented Apr 11, 2017 at 12:42
  • $\begingroup$ @orthocresol I thought of that as well, but I figured classification of structure occured in the abstract, at an instantaneous moment not beholden to whatever reaction just occured to form form the compound nor by what route it may be about to decompose. But that aside, how likely is it to sacrifice the carbonyl for a three-membered ring? The aziridine wikipedia page mentions several syntheses, and I didn't see intramolecular attack of alpha- amino aldehyde (en.m.wikipedia.org/wiki/Aziridine?wprov=sfla1? $\endgroup$ Commented Apr 12, 2017 at 1:29

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I think choice D, All of the above may be the correct answer. All three pairs are structural isomers, with different functional groups, thus functional group isomers (FGIs).

This brief overview uses a carboxylic acid and ester pair as an example of FGI, which validates choice B. It also cites an aldehyde $\ce{CH3CH2(C=O)H}$ and a ketone $\ce{HCH2(C=O)CH3}$. If the methyl group is swapped out for an amino group (not called a nitrile), we have your compounds $\ce{NH2CH2(C=O)H}$ and $\ce{HCH2(C=O)NH2}$. Not only should this pair be FGIs by analogy with the aldehyde-ketone, but they satisfy the definition of functional group isomers: Structural isomers (same atoms, different connectivity) that have different functional groups. This validates choice A.

Wikipedia cites your exact ether-alcohol pair as example of FGIs, validating choice C.

Thus, all of the above must be the correct answer. If you are in a class, I would ask your instructor about this problem.

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