I'm studying for my MCAT currently and I stumbled upon this question. It gives the structure of a folic acid molecule and asks what functional groups can be found within the molecule. I understand that there is an amine, amide, imine, and carboxylic acid group, but I don't understand why the benzene ring in the center of the molecule does not count as phenyl or benzyl group. If anyone could explain this to me I would really appreciate it. folic_acid_functional_groups

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    $\begingroup$ Similarly as there is ethyl CH3-CH2- and ethylene -CH2-CH2-, there is phenyl C6H5- and phenylene -C6H4-. ( E.g. either (o,m,p-)diaminobenzene either phenylenediamin ) $\endgroup$ – Poutnik Jun 10 at 10:58
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    $\begingroup$ Did you look up, e.g., in Wikipedia about benzyl bromide, benzyl chloride for their motif of structure in common vs. phenol, phenylamine (equally known as aniline)? $\endgroup$ – Buttonwood Jun 10 at 11:59
  • $\begingroup$ It does have a phenyl group. if you search up the IUPAC name of folic acid this turns up: (2S)-2-[(4-{[(2-amino-4-hydroxypteridin-6-yl)methyl]amino} phenyl )formamido]pentanedioic acid $\endgroup$ – Manav Jun 10 at 12:45

Does folic acid contain a benzyl or a phenyl group?

This is the question asked in the title. At the first glance to the structure, one would say folic acid consists of phenyl function but not benzyl function because the question did not define what is the phenyl group. In reality, phenyl group $(\ce{Ph})$ is $\ce{C6H5}$. As Poutnik pointed out in the comment, folic acid really contains a phenylene $(\ce{C6H4})$ function. On the other hand, the Benzyl function is strictly $\ce{C6H5-CH2}$ or $\ce{Ph-CH2}$. It is debatable to call $\ce{X-C6H4-CH2}$ a benzyl group, unless a definition is given upperhand. Nonetheless, folic acid does not contain either $\ce{C6H5-CH2}$ or $\ce{X-C6H4-CH2}$, and therefore no argument there for the presence of benzyl group. What it contain is $\ce{X-C6H4-C=O}$ group, which can tentatively called benzoyl (strictly when $\ce{X = H}$).

The more generic term for a substituted benzene ring, without restrictions on subatituents, is aryl.

Albeit, without giving definitions for the function naming, the mCAT question assigned $\bf{(A)}$ as a correct answer. However, there are clear-cut carboxylic and amine groups visible, the imine group is a part of Pteridine ring system:

mCAT Answer

Thus, it is a part of aromatic nucleus and therefore not a clear-cut isolated imine function. If the authors considered it as an imine, then phenylene system can be considered as a phenyl function. Accordingly, the mCAT question has possible two answers: Answers $\bf{(A)}$ and $\bf{(C)}$ as depicted in the image.

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    $\begingroup$ I tried to post an answer but found it makes no new contributions except mentioning "aryl" as a more generic term. So I deleted that answer and added the allusion to "aryl" here. Feel free to roll back. $\endgroup$ – Oscar Lanzi Jun 10 at 18:48
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    $\begingroup$ @Oscar Lanzi : I glad you did. I think it is a great addition. I didn't think about that. $\endgroup$ – Mathew Mahindaratne Jun 10 at 19:28
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    $\begingroup$ I would argue that the imine functionality you have assigned is incorrect as they are part of an aryl system. The imine I would say is in the other half of the pteridine within the 'guanidine' moiety, but honestly none of them are good representations. $\endgroup$ – Tragamor Jun 11 at 11:08
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    $\begingroup$ @Tragamor: That was my argument in my answer. None represents a clear cut imine. If they are considering one of them are imine, they should have consider the aryl nucleus as phenyl. On guanidine, carbin with two geminal nitrogen can't be a imine (are you saying enamine?) $\endgroup$ – Mathew Mahindaratne 2 days ago
  • $\begingroup$ It's not an enamine but I'm referring to the double bonded nitrogen within the 'guanidine' as that would have more "imine" like character than the pyridazine ring candidates. $\endgroup$ – Tragamor 2 days ago

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