of technical dimethylaniline are dissolved in 750 cc.
of diluted hydrochloric acid (1 part concentrated acid to 1
part water). This solution is now cooled to 0'0 and a solution
(previously cooled to 0'0) of 90 g. of technical sodium nitrite
in 150 cc. of water is added through a separatory funnel.
During the addition of the nitrite solution, mechanical stirring
should be employed and the flask cooled well with ice and salt.
The addition is made at such a rate (thirty to forty minutes
for the entire addition) that the temperature does not rise above
5'0. The precipitate of nitroso dimethylaniline hydrochloride
is filtered off with suction, then washed with about 300 cc.
of diluted hydrochloric acid (1:1).

In a 2-l. beaker, 180 g. of technical dimethylaniline, 125 cc.
of formaldehyde (technical 40 per cent), and 300 cc.
of concentrated hydrochloric acid are mixed and heated for ten
minutes on a steam bath. The mixture is now placed in a hood
and the nitroso dimethylaniline added all at once, or as rapidly
as possible. The beaker is then covered with a watch glass.
A vigorous reaction soon occurs and is complete in about five minutes.
The resulting solution is transferred to a 5-l. flask and diluted
to 4 l.; stirring is started, and a 25 per cent solution of sodium
hydroxide is added until the red color disappears (about 650 cc.
are required). The yellow benzylidene compound separates,
is filtered with suction and washed with water. The moist precipitate
is transferred to a 4-l. glass jar, covered with 1000 cc.
of 50 per cent acetic acid and 250 cc. of formaldehyde,
and stirred until twenty minutes after the benzylidene compound has
gone into solution. While the mixture is being stirred vigorously
to prevent lumping of the precipitate, 400 cc. of water and 200 g.