The decarboxylation of phenylacetic acid via addition of acid
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 &beta;-keto acids as illustrated by the decarboxylation of &beta;-keto acid 1 to afford cyclohexanone 2. Hagemann's ester (3) is a &gamma;-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 &beta;-keto ester to the &gamma;-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 &sigma;-bonds in structures 1, 4 and 6 must have overlap with the &pi;-framework of the carbonyl (in 1), the &gamma;-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.