IN a 12-l. round-bottom flask, fitted with a mechanical stirrer,
are placed 1045 cc. of concentrated commercial hydrochloric acid
(sp. gr. 1.138). The flask is surrounded with a freezing mixture
of ice and salt, and when the contents are at 0'0, stirring is started
and 500 g. of cracked ice are added; then 372 g. of aniline,
also cooled to 0'0, are run in during five minutes. The mixture
is treated with 500 g. more of cracked ice, and a cold solution
(0'0) of 290 g. of technical sodium nitrite dissolved in 600 cc.
of water are allowed to run in slowly (twenty to thirty minutes)
from a dropping funnel, the end of which is drawn to a small tip,
and reaches nearly to the bottom of the flask. During this addition,
the stirrer is operated rather vigorously, and the temperature is
kept as near 0'0 as possible by the frequent addition of cracked ice
(about 1 kg).
In the meantime, a sodium sulfite solution is prepared by dissolving
890 g. of sodium hydroxide, of about 90 per cent purity, in about 1 l.
of water and then diluting to 6 l. A few drops of phenolphthalein
solution are added and sulfur dioxide passed in, first until an acid
reaction is indicated and then for two or three minutes longer.
During the addition of the sulfur dioxide, the solution is cooled
with running water. On account of the strong alkaline solution,
the original color produced by the phenolphthalein is very faint,
but this slowly increases until it becomes deep just before the acid
point is reached. It is best to remove a small sample of the liquid
from time to time, dilute with three or four volumes of water
and add a drop more of phenolphthalein.
The sodium sulfite solution is placed in a 12-l. flask and cooled
to about 5'0. Approximately 500 g. of cracked ice are added, and then,
with mechanical stirring, the diazonium salt solution is run in as
rapidly as possible. The mixture becomes a bright orange-red. The
flask is now warmed to about 20'0 on a steam bath, until the solid
sodium sulfite, which has separated while cooling, redissolves.
The total amount of liquid is now about 10 l. One-half of this
is poured into another 12-l. flask, and both halves are warmed
on the steam bath to 60-70'0, until the color becomes quite dark
(thirty to sixty minutes). Sufficient hydrochloric acid (300-400 cc.)
is now added to each flask to make the solutions acid to litmus.
The heating is continued and the color gradually becomes lighter until,
after four to six hours, the solutions have become nearly colorless;
they may be heated overnight, if desired.
To the hot solutions are now added about one-third of their volume
of concentrated hydrochloric acid (2 l. to each portion) and the
mixtures cooled, first in running water, then in a freezing mixture,
to 0'0. The phenylhydrazine hydrochloride precipitates in the form
of slightly yellowish or pinkish crystals which may be filtered
off and dried.
The free base is liberated by adding to the phenylhydrazine
hydrochloride 1 l. of a 25 per cent solution of sodium hydroxide.
The phenylhydrazine separates and is taken up with benzene
(two 300-cc. portions). The combined extractions are well dried
with 200 g. of solid sodium hydroxide, poured off, and distilled.
Most of the benzene may be distilled under ordinary pressure,
and the remainder, and any low-boiling impurities,
under diminished pressure. The pure phenylhydrazine boils
at 137-138'0/18 mm., and is obtained as a pale-yellow liquid.
It can be crystallized on cooling in an ice bath; the crystals
melt at 230. The crude phenylhydrazine from two lots of aniline
(744 g.) is best distilled at one time and gives 695-725 g.
of pure product (80-84 per cent of the theoretical amount).