The<unk>  is an organic reaction converting benzene to picric acid by a mixture of aqueous nitric acid and mercury(II) nitrate.
According to one series of studies the mercury nitrate first takes benzene to the corresponding nitroso compound and through the diazonium salt to the phenol.  The presence of nitrite is essential for the reaction; picric acid formation is prevented when urea, a trap for nitrous acid, is added to the mixture. From then on the reaction proceeds as a regular aromatic nitration.
A conceptually related reaction at one time of interest to the pigment industry is the Bohn-Schmidt reaction (1889) involving the hydroxylation of  hydroxyantraquinone with sulfuric acid and lead or selenium to a polyhydroxylated anthraquinone.
Wolffenstein-Böters reaction
Koebner phenomenon
Duroziez sign
Achmatowicz reaction
Sherman sign
Fleischner sign
Lighthouse sign
Westermark sign