NITRO DERIVATIVES OF PHENOL 475DINITRO DERIVATIVES OF PHENOL
There are six isomers of dinitrophenol: α or 2,4- (m. p. 113-114°C), β or 2,6-
(m. p. 62-63°C), γ or 2,5- (m. p. 104-108°C), δ or 3,4- (m. p. 134°C), ε or 2,3-
(m. p. 114°C) and θ or 3,5- (m. p. 122°C).
α β γ δε θOf these only the 2,4-isomer is used in explosive compositions or as a starting
material for the preparation of picric acid by one of the methods described later.
The 2,6-isomer on nitration also gives picric acid but it is not used for this purpose
on a commercial scale. Both these isomers may be obtained by the nitration of
phenol with nitric acid. All the other isomers are prepared by indirect methods.
Laurent [8] was the first to obtain dinitrophenol by nitrating phenol. Investi-
gations that followed revealed that Laurent’s dinitrophenol was not a chemical
individual, but a mixture of the 2,4- and 2,6-isomers. Kiirner [17] obtained pure
2,4-dinitrophenol by the nitration of p- nitrophenol and Armstrong [18] prepared
2,6-dinitrophenol along with some 2,4-isomer, starting from o- nitrophenol. Clemm
[19] determined the constitution of 2,4-dinitrophenol, which was later confirmed
by Salkowski [20].
Finally Hübner and W. Schneider [21] defined the conditions under which the
formation of the two isomeric dinitrophenols must proceed and pointed out the
difference in their structures. They proved the p- nitrophenol was nitrated to 2,4-
dinitrophenol when using 54% nitric acid. o- Nitrophenol is slightly more difficult
to nitrate and yields a mixture of 2,6- and 2,4-dinitrophenols. The two isomers
may be separated by steam distillation, since the 2,4- is more volatile than the
2,6-isomer. In consequence the residue in the retort becomes enriched in the
2,6-isomer which can finally be obtained in a pure form. Another method of sepa-
ration is based on taking advantage of the difference in solubility of the barium salts
of the two isomers, the 2,4-isomer salt being readily soluble in water, while the salt
of the 2,6-isomer dissolves only with difficulty.