The spontaneous decarboxylation of strong carboxylic acid − carboxylate mixtures and the use of carbon surfaces to trap the released free radicals

[Display omitted] •Spontaneous decarboxylation in carboxylate-carboxylic acid mixtures.•Hydrogen bonding complexes between carboxylates and carboxylic acids.•Decarboxylation of nitrophenylacetic acid to synthesize 4-nitrotoluene.•Electrochemical reduction of 4-nitrophenylacetic acid in acetonitrile.•Trapping 4-nitrobenzyl radicals on carbon surfaces. The mechanism of the spontaneous decomposition of a mixture of 4-nitrophenylacetic acid (NO2BzCOOH) with its respective tetrabutylammonium carboxylate in acetonitrile (NO2BzCOO−) was analyzed using cyclic voltammetry, atomic force microscopy, nuclear magnetic resonance and electronic structure calculations. This reaction is favoured by the high acidity of the carboxylic acid and occurs following a decarboxylation loop where the carboxylate anion has the function of reductant whereas the acidic proton of the carboxylic acid acts as oxidant. This redox reaction allows the generation of hydrogen as well as an acyloxy radical, that after cleavage, yields 4-nitrobenzyl radicals that have the possibility to react either in solution to produce 4-nitrotoluene or react with a carbon surface to produce a covalently modified layer with 4-nitrobenzyl groups.