Organic Syntheses | Page 9

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. of cracked ice are added during
the course of five minutes. The dimethylaminobenzaldehyde generally
separates gradually in fifteen to twenty minutes, but in some cases does
not. If the precipitate does not form, the solution is placed in a
refrigerator for a few hours or overnight. The mixture is filtered with
suction and washed at least ten times with 300 cc. of water. The
precipitate is sucked as dry as possible for fifteen to thirty minutes.
The slightly moist aldehyde is distilled under diminished pressure from
an oil bath, by means of a 1-l. Claisen flask. A small amount of water
comes over first, then the thermometer rises rapidly to the boiling point
of the aldehyde (180'0/22 mm.). In changing receivers between the
water fraction and the aldehyde, care should be taken to keep the

side-arm of the distilling flask warm; otherwise, on starting the
distillation again, the aldehyde will solidify in the side-arm and cause
trouble. It is advisable not to collect the very last portion of the
distillate with the main portion, as the former is frequently quite red.
This is best added to crude material from another run. The main
distillate is dissolved in 100 cc. of alcohol in a 2-l. beaker, then 1000 cc.
of water are gradually added with vigorous mechanical stirring to
prevent lumping. The aldehyde separates, and is filtered with suction.
The product, when dry, weighs 125-130 g. (56-59 per cent of the
theoretical amount), and melts at 73'0.
The aldehyde prepared in this way is in the form of small granular
crystals, which vary in different runs from a flesh color to a lemon
yellow. For practically all purposes, this slightly colored product is
entirely satisfactory and is essentially pure, as can be judged by the
melting point. For reagent purposes it is desirable to remove the color
completely, particularly since the product obtained as just described has
a tendency to take on a reddish tinge on exposure to light. Further
purification can be accomplished by dissolving the aldehyde (it
dissolves slowly) in dilute hydrochloric acid (1 part of concentrated
acid, sp. gr. 1.19, to 6 parts of water), 125 g. of aldehyde requiring 700
cc. of the acid. The solution is placed in a jar and diluted with half its
volume of water, and dilute sodium hydroxide solution (15-20 per cent)
is added slowly with mechanical stirring. At the beginning, the
aldehyde comes down slightly colored. After about 10 to 30 g. are
precipitated, however, the product appears white; this point can be
readily seen. The first precipitate is filtered off and added to the next
run of crude material, or fractionally precipitated again from
hydrochloric acid. The rest of the aldehyde is now precipitated by
means of more sodium hydroxide solution, and comes down almost
white. At the very end of the neutralization, particularly if the original
product was quite yellow, the last 4 to 5 g. of aldehyde should be
precipitated separately, as they are inclined to be slightly colored. If too
much alkali is added towards the end of the neutralization, a brown
color appears, but the addition of a little hydrochloric acid will destroy
this color. The main portion of the precipitate is filtered and dried; it
weighs 95-100 g., m. p. 73'0. The succeeding runs yield 115-128 g. of
finished product, on account of the extra crude material obtained from

the distillation and reprecipitation of the previous run. 2. Notes
The aldehyde that is obtained without reprecipitation