Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

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Since fluorescein is coloured it seems that the quinonoid formula
on the right is the probable one even for the free compound, and that
the lactone formula is doubtful.
In eosin the four bromine atoms are grouped in pairs, in the opositions
indicated by asterisks in the formula. Eosin, also, must be
regarded as quinonoid, especially because its reduction product, leucoeosin,
is colourless.
Experiment;—Dissolve some eosin in sodium hydroxide solution
and boil with zinc dust until the mixture is colourless. Decant the
solution. Acidify one portion of the decanted liquid and leave the
other portion to stand in an open dish.
Like quinone itself, its derivatives are also converted by reducing
agents into colourless benzenoid hydrogenated products (in the case of
the dyes they are called " leuco-compounds "). The following scheme
expresses this process for eosin as well as for the other dye :
-OH.
Many leuco-compounds are reconverted into the dye even by the
oxygen of the atmosphere ; leuco-indigo (p. 373) and leuco-eosin are
examples of this.
The most magnificent dyes, which are chiefly used in the dyeing of
silk, are relatives of eosin obtained from di- and tetra-chlorophthalic
anhydride (phloxine, rose bengale). The (basic) rhodamines also belong
to this class. They are produced by condensation of phthalic anhydride
with m-aminophenols (in place of resorcinol); the dye containing
the diethylated NH2-group, in particular, is of great technical
importance.
Finally gallein may be mentioned ; in it pyrogallol is the phenolic
component.
The conversion of the phthaleins into anthracene derivatives, the
so-called phihalideins, need not be detailed here.