IN a 3-l. round-bottom flask fitted with a mechanical stirrer 150 g.
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.
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
gradually takes on a pinkish tinge on exposure to light.
After the reprecipitation, however, this characteristic disappears.
Thorough washing of the crude aldehyde is particularly desirable,
as it removes a reddish impurity which tends to distil over and
color the product lemon yellow or sometimes even brownish yellow.
When such a brownish product is obtained, it is quite necessary to make
a second precipitation, as well as to observe the directions mentioned
in the purification of the crude aldehyde, namely, to precipitate
the first few grams and the last few grams of the aldehyde separately.
The precaution of rejecting the first and last portions
of the precipitate is unnecessary in the reprecipitation.
In the reprecipitation of a deeply colored product, the portion
of aldehyde at the end may be even purplish in color and particular
care must be taken to keep this separate.
Vigorous mechanical stirring must be employed during the precipitation
of the crude aldehyde, as otherwise large lumps are formed which
make washing difficult.
A previous investigator has mentioned that the crude product must
be dried before distilling. This, however, is unnecessary.
If the aldehyde is dried before distilling, it is possible to use
a 500-cc. distilling flask instead of a 1-l one.
In purifying the aldehyde by dissolving in acid and reprecipitating,
it is essential not to use stronger acid than that specified
(1:6), as stronger acid causes a deepening of the color of the solution.
If the concentrated acid, which is to be diluted and used in this
procedure, does not have a sp. gr. of 1.19, it will be necessary
to add the equivalent amount of weaker acid in order to dissolve
the _p_-dimethylaminobenzaldehyde. In purifying the aldehyde,
sodium carbonate may be used in place of sodium hydroxide
for precipitation, but it causes much foaming.
When the apparatus for distilling, etc., is all set up,
a run such as described above requires about five to six hours
for completion. 3. Other Methods of Preparation
_p_-Dimethylaminobenzaldehyde has been made by the condensation
of chloral with dimethylaniline, and subsequent hydrolysis;[1]
by the hydrolysis of tetramethyldiaminobenzhydrol with acetic
acid;[2] by the condensation of dimethylaniline, formaldehyde and
_m_-sulfo-_p_-tolyl hydroxylamine followed by hydrolysis;[3] by the
electrolytic reduction of a mixture of sodium nitrobenzene sulfonate,
dimethylaniline and formaldehyde, and subsequent hydrolysis;[4]
by the reduction of a mixture of dimethylaniline, formaldehyde and
sodium nitrobenzene sulfonate with iron and hydrochloric acid,
followed by hydrolysis;[5] by the condensation of alloxan with
dimethylaniline followed by hydrolysis;[6] by the condensation
of dimethylaniline, formaldehyde and sodium _p_-toluidine sulfonate
in the presence of hydrochloric acid and potassium dichromate
followed by hydrolysis.[7] The most satisfactory method, however,
is the condensation of dimethylaniline, formaldehyde and nitroso
dimethy]aniline, followed by hydrolysis,[8] a method which was first
described by E. Noelting and later perfected in detail by L. Baumann.
[1] Ber. 18, 1519 (1885); 19, 366 (1886); D. R. P. 61, 551; Frdl.
3, 109 (1892).
[2] Ber. 27, 3317 (1894).
[3] D. R. P. 103,578; Frdl. 5, 101 (1899).
[4] D. R. P. 105,103; Frdl. 5, 107 (1899).
[5] D. R. P. 105,105; Frdl. 5, 109 (1899).
[6] D. R. P. 108,086; Frdl. 5, 117 (1899).
[7] D. R. P. 118,567; Frdl. 6, 133 (1901).
[8] Ber. 37, 858 (1904); J. Biol. Chem. 41, 146 (1920).