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OTHER NITRO DERIVATIVES OF PHENOLS^541

Aubertein and Emeury [29] made a detailed examination of the causes of foaming.

Oxidation of non-sulphonated resorcinol is the original cause of gas evolution
during nitration. When the product is in the form of very fine crystals the gas bubbles

attach themselves to the crystals and raise the product to the surface in the form

of a foam. This does not occur when the crystals of the product are large. There-

fore, the method of nitration should be such as to favour formation of large crystals

of styphnic acid. To achieve this, Aubertein and Emeury advise:
(1) keeping the temperature of nitration constant and avoiding any fall in
temperature;
(2) keeping the concentration of nitric acid during the nitration as constant
as possible and the temperature near to 55°C;
(3) avoiding the application of too vigorous stirring particularly just before
the styphnic acid begins to crystallize. Excessive stirring promotes foaming.
Aubertein and Emeury established the influence of various other factors upon

the fineness of crystals and subsequently on the foaming. For example a lower con-

centration of sulphuric acid (92-97%) used for sulphonation favours the formation

of fine crystals and hence of foam. This is the result of a low rate of nitration produced

by the lower concentration of the nitrating acid.

The yield of styphnic acid depends on the excess of nitric acid. It falls from

86 to 82% of theoretical when the excess is reduced from 45 to 14%.

Styphnic acid of high purity

Sulphonation. 100 parts of resorcinol are introduced in small increments during

a period of half an hour into 1000 parts of sulphuric acid (93-98%) under stirring.

The temperature rises to 50°C and is maintained then for 15 min, when the mixture

is ready to be nitrated.

Nitration. 200 parts of concentrated nitric acid (90% HNO 3 ) are added to the

sulphonation mixture in such a way that the first 110 parts are introduced relatively

quickly (within 30-50 min), and the temperature is kept between 50° and 55°C.

After that, the rate of stirring and the rate of introducing nitric acid is reduced (3-

5 times) to maintain the temperature at 55-60°C. Styphnic acid begins to crystal-

lize. From this moment the rate of adding nitric acid is gradually increased and

cooling is so arranged that the temperature finally reaches 60-65°C.

The whole is stirred for half an hour at this temperature, cooled, filtered, washed

with cold water until the wash water is neutral to Congo paper and dried at 60°C.

The yield of the product with m. p, 176°C is 180-185 parts.

Styphnic acid of low purity

Sulphonation. 10-20 parts of sodium nitrite are added to 1000 parts of sulphuric

acid (92-98%) and 100 parts of resorcinol are introduced as above.

Nitration. Initially 125 parts of nitric acid (instead of 110 parts) are added.

Otherwise the process is the same as described above.