I came across a question where the following compound except with two methyls, ortho and meta to the nitro group, is dissolved in methylene chloridde, and then treated with trifluoroacetic acid.

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It is given that $\ce{CO2}$ gas is given off in a decarboxylation reaction. I cannot find any sources about this. The solution explains that:

Treatment of a carboxylic acid with acid results in decarboxylation, and the evolution of $\ce{CO2}$, especially if the resulting compound contains a benzylic or allylic carbon, as is the case here.

First of all, what mechanism would allow for acid catalyzed decarboxylation? I know how decarboxylation of carboxylic acids with beta-carbonyls with heat works, and that acid can help that but that doesn't seem to have relevance here.

Secondly, why does having a benzylic or allylic carbon confer any special stability to promote the reaction?

  • $\begingroup$ With a beta carbonyl you delocalise the negative charge from decarboxylation into the second carbonyl group, here you delocalise it into the nitro group. $\endgroup$
    – orthocresol
    Jun 4 '17 at 6:19
  • $\begingroup$ Do you have a source for the question (name of a book, website etc.). $\endgroup$
    – NotEvans.
    Jun 4 '17 at 12:24
  • 3
    $\begingroup$ Acid-catalysis is activating the nitro group to make it more electron-withdrawing. $\endgroup$
    – Zhe
    Jun 4 '17 at 13:34
  • $\begingroup$ @NotEvans it was an MCAT general study app. It provides no sources of its own to back itself up. I can see how electron delocalization stabilizes the decarboxylate - and I guess I can see how the stoichiometry works to protonate at the allylic carbon at the end to produce para-nitromethylbenzene. $\endgroup$
    – user36847
    Jun 6 '17 at 1:20
  • $\begingroup$ What I still don't understand is why the app says that having a benzylic or allylic carbon, in particular, stabilizes the final compound. Should I just ignore that and move on? $\endgroup$
    – user36847
    Jun 6 '17 at 1:21

The decarboxylation of carboxylic acids having allylic or benzylic carbons can also be explained by what may be called the vinylog concept. One of the most common decarboxylations is that of β-keto acids as illustrated by the decarboxylation of β-keto acid 1 to afford cyclohexanone 2. Hagemann's ester (3) is a γ-keto ester, which upon aqueous acid hydrolysis gives acid 4. This acid suffers decarboxylation to afford 3-methylcyclohex-2-en-1-one (5). The red double bond in 4 simply extends the properties of a β-keto ester to the γ-position in a vinylogous (allylic?) fashion.

Nitrobenzene 6, which may be the compound in question (if not, ignore the methyl groups), is doubly vinylic (benzylic?) as indicated by the two red double bonds in structure 6. While this compound does not have a keto group in conjugation with the aromatic ring, the nitro group functions as a viable substitute for a keto group. Protonation of the N=O bond on oxygen leads to decarboxylation and rearomatization to nitrobenzene 7.

The green σ-bonds in structures 1, 4 and 6 must have overlap with the π-framework of the carbonyl (in 1), the γ-position of enone 4 and the aromatic ring of nitrobenzene 6 for decarboxylation to occur.

Finally, nitroacetic acid, as its magnesium salt, decarboxylates at 80 oC to provide nitromethane and CO2.

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