Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

332 FLUOKBSCBIN
be isolated, loses water between the COOH- and OH-groups, which are
favourably situated with respect to each other; the acid is thus converted
into the lactone, phenolphthalein :
C=(C6H4OH)2
The colourless lactone is hydrolysed by alkalis and the intensely
red alkali salts, well known in volumetric analysis, are formed. In
them one benzene ring has assumed the quinonoid form with elimination
of water in the manner expressed in the following equation :
JOOH
OH
The red salts are the di-alkali salts of the quinonoid phenolcarboxylic
acid formulated. This acid is not stable in the free state, but undergoes
immediate isomerisation to the colourless lactone.
Phenolphthalein is a triphenylmethane derivative and can easily
be connected with fuchsone, the parent substance of the dyes which
belong to this series. Fuchsone is diphenylquinomethane and is
obtained from y-hydroxytriphenylcarbinol by elimination of water
(Bistrzycki) :
OH
C 6 H 5\J.
C6H/
In complete accordance with Willstatter's theory, fuchsone is only
yellowish-orange in colour. But if, in addition, one of the two free
benzene rings has an OH-group in the y-position, we have the dye
benzaurin already mentioned on p. 328. The o-carboxylic acid of this
dye is the quinonoid form of phenolphthalein. In fact, the colour tones
of these two substances are very similar. Phenolphthalein is decolorised
by concentrated alkali; NaOH (KOH) is added on and the
trisodium (or potassium) salt of the benzenoid carbinol form is produced.
(Try these reactions with phenolphthalein.)
Fluorescein.—In this case the reaction undergoes an extension.
The two OH-groups of the resorcinol molecule which are oriho to the
position of condensation together form an oxygen bridge and hence
a new ring (the xanthane ring) results by elimination of water :