Addition to Aromatic Rings Facilitated by Two Nitro Groups

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The hydroxy group adds into the aromatic ring at the position para to one of the nitro groups and the negative charge is stabilized by the nitro group. The additional nitro group ortho to the attack allows delocalization of the negative charge in the intermediate which gives rise to the characteristic purple colour of these intermediates.

See mechanism of addition-elimination

M. Mąkosza, Russ. Chem. Bull., 1996, 45, 491–504.