When the dianil is boiled for some time with concentrated hydrochloric
acid, the yellow color of the substance disappears, and the dianil is
converted into a colorless substance without, however, passing into
solution. The substance so obtained was filtered off, and crystallized
from alcohol. It crystallized in small colorless needles, which melted
at 183°, and possessed all the properties of the anil, which, in fact,
it proved to be. The reaction was therefore
C=N.C₆H₅ CO
/ \ / \
/ N.C₆H₅ / N.C₆H₅
/ / / /
C₆H₃——SO₂ + HCl + H₂O = C₆H₃——SO₂ + C₆H₅NH₃Cl
\ \
NO₂ NO₂
This reaction also explains the fact that some anil was always obtained
in making the dianil from the anilid. Hydrochloric acid is formed
in the reaction, and in turn acts on the dianil in the sense of the
equation just given.