# The decarboxylation of phenylacetic acid via addition of acid

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.

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?

• With a beta carbonyl you delocalise the negative charge from decarboxylation into the second carbonyl group, here you delocalise it into the nitro group. – orthocresol Jun 4 '17 at 6:19
• Do you have a source for the question (name of a book, website etc.). – NotEvans. Jun 4 '17 at 12:24
• Acid-catalysis is activating the nitro group to make it more electron-withdrawing. – Zhe Jun 4 '17 at 13:34
• @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. – user36847 Jun 6 '17 at 1:20
• 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? – user36847 Jun 6 '17 at 1:21