Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

250 KOLBB'S SYNTHESIS
not be connected to the superheater until the oil bath and the steam
have reached the temperature mentioned. Both the connecting
tube and the condenser tube must be exceptionally wide. When the
acid removed from the condenser tube is dissolved in the aqueous
distillate from the receiver and the solution is cooled, the acid
crystallises in long, completely colourless needles. Melting point
156°. Yield 10-12 g.
The first stage of Kolbe's synthesis is analogous to the well-known aliphatic
synthesis of alkyl carbonates from alkoxides and carbon dioxide :
H5C2.ONa + CO2 —> H5C2—0—c/ ;
X)Na
J)
>Na.
The sodium phenylcarbonate so produced then undergoes rearrangement
on heating ; the carboxy-sodium group wanders to the nucleus :
/\0.c/° /\
A relatively small amount of the y-compound is formed at the same
time. It is remarkable that when potassium phenoxide is used the
y-compound is the main product.
Since, in Kolbe's synthesis, as here described, the mono-sodium
salicylate reacts to some extent with unchanged sodium phenoxide,
producing the di-sodium salt, part of the phenol is liberated and excluded
from the reaction. The reaction proceeds to completion if the
sodium phenoxide is heated to about 150° for a long time, with carbon
dioxide under pressure in the autoclave. This is the technical method
of Schmitt.
In the case of polyhydric phenols the synthesis of the carboxylic
acid can be carried out even in aqueous solution.
Ortho- and yaro-hydroxycarboxylic acids lose CO2 when heated to
high temperatures, and the ease with which this occurs increases with
the number of OH-groups. (Preparation of pyrogallol from gallic acid.)
How is w-hydroxybenzoic acid prepared? Discuss the reduction
of salicylic acid to pimelic acid (Einhorn).
Experiment.—A few drops of ferric chloride solution are added to
an aqueous solution of salicylic acid. The colour reaction characteristic
of phenols is obtained.