Checked by B. C. McKusick and R. J. Shozda.
1. Procedure
In a
3-l. three-necked flask equipped with an
efficient mechanical stirrer of high torque (Note
1), a
reflux condenser with a
calcium chloride drying tube, a
thermometer, and a
dropping funnel are placed
133 g. (1.00 mole) of anhydrous aluminum chloride and
200 ml. of dry carbon disulfide. The mixture is cooled in an
ice bath and stirred while
182 g. (1.50 moles) of N,N-dimethylaniline (Note
2) is added through the dropping funnel during a period of 15 minutes. The dropping funnel is rinsed with
20 ml. of carbon disulfide which is then run into the flask. Any
aluminum chloride sticking to the walls of the flask is now scraped into the mixture, which is an easily stirred slurry of a white solid in a light-green liquid.
The reaction mixture is cooled to 5–10° in an
ice-salt bath (Note
3), and, with continued stirring, a solution of
31.7 g. (21.3 ml., 0.250 mole) of oxalyl chloride in
200 ml. of dry carbon disulfide is added through the dropping funnel in the course of 20 minutes. After the addition is complete, the thick black reaction mixture is allowed to warm to room temperature, refluxed for 1 hour, and then cooled to 0–5° in an ice bath. The mixture is stirred throughout these steps. One hundred grams of chipped ice is added with stirring, followed by 400 ml. of cold water. Steam is then passed into the flask until the
carbon disulfide and unreacted
dimethylaniline are removed, and the green-black
aluminum complex is decomposed to a mixture of a green solid and a blue solid; this requires 1–2 hours (Note
4). The mixture is cooled to 50°, and the solid, which is principally
4,4'-bis(dimethylamino)benzil, is collected on a
Büchner funnel. In order to remove the major part of the impurity, which is somewhat soluble in water, the solid is slurried in 200 ml. of water at 50°, and the slurry is filtered. This process is repeated twice, and the crude
benzil, now a green solid, is washed successively on the funnel with 200 ml. of water at 50° and with
100 ml. of cold methanol. After being dried in air, it weighs
44–55 g. and melts at
191–196°.
The crude
benzil is dissolved in
500 ml. of chloroform. To remove the impurity that remains (Note
5), the solution is shaken with three
400-ml. portions of 6% aqueous hydrogen peroxide solution containing
1.0 g. of sodium hydroxide in each portion, and finally with 500 ml. of water. The aqueous layers are combined, warmed to drive off dissolved
chloroform, and filtered to separate about
1.5 g. of a yellow-green solid, which is dissolved in the
chloroform solution.
The
chloroform layer is distilled to dryness and the residue is dissolved in
1.5 l. of acetone under reflux. The hot
acetone solution is filtered and then allowed to cool in a
refrigerator. Yellow
4,4'-bis(dimethylamino)benzil crystallizes from the
acetone solution. It is separated by filtration and washed with
100 ml. of cold methanol. After being dried in air, it weighs
28–31 g. (
38–42%); m.p.
200–202°; λ
maxCH3OH 371 mμ(ε 44,700) (Note
6).
The
acetone filtrate is concentrated to 700 ml. and cooled to 0–5°. An additional
4–8 g. (
6–11%) of slightly less pure
benzil, m.p.
198–201°, crystallizes from solution (Note
7).
2. Notes
1. A magnetic stirrer is unsatisfactory. The submitters used a
glass-blade stirrer at 300 r.p.m. The stirrer shaft must be rigidly attached to the motor because the reaction mixture becomes very thick during the addition of
oxalyl chloride.
2. Eastman Kodak white label compounds used without further purification are satisfactory starting materials.
3. Cooling below −10° should be avoided because the reaction stops at that temperature and large amounts of
oxalyl chloride accumulate in the flask. If this mixture is then allowed to come to room temperature, a vigorous reaction that may get out of control will take place. It is probable that, at reaction temperatures about 10°, the yield of
4,4'-bis(dimethylamino)benzil is less and some
Crystal Violet is formed as an impurity, for it has been reported that
aluminum chloride effects the conversion of
N,N-dimethylaniline and
oxalyl chloride to Crystal Violet in
92–95% yield when the reaction is allowed to proceed without cooling.
2
One liter of water is added, making the total volume about 2 l., and, after the solution has been cooled to room temperature, it is extracted with
1 l. of chloroform and then with
150 ml. of chloroform. The combined dark-blue extracts are washed with
550 ml. of 8.5% hydrochloric acid, then with 200 ml. of water and dried over anhydrous
sodium sulfate. The
chloroform solution is distilled until the volume is 250 ml., and it is then passed through an
8.5 × 25-cm. column (300 g.) of Alcoa F-20 alumina. The adsorbate is eluted with
1 l. of chloroform. The eluate is evaporated to a volume of 250 ml., washed with
500 ml. of 10% sodium hydroxide solution then with 100 ml. of water, and distilled to essential dryness.
The residual crude, yellow, semi-solid product is stirred and brought to a boil with
250 ml. of ethyl acetate and then allowed to cool to room temperature while stirring. Filtration affords
39–41 g. (
52–55%)
4,4'-bis(dimethylamino)benzil, m.p.
201–203°. Concentration of the mother liquor to 50 ml. gives, after cooling, an additional
3.5–4.5 g. of product, m.p.
175–180°. Recrystallization of
10 g. of the combined products from
120–150 ml. of benzene gives
9.1–9.3 g. of yellow crystals, m.p.
202–203°.
5. The checkers found that at least part of the colored impurity is Crystal Violet. Alkaline
hydrogen peroxide is reported to cleave Crystal Violet to
N,N-dimethylaniline and Michler's ketone.
3
6. The product is sometimes pale green because of traces of impurities, but it is nevertheless very pure, for repeated recrystallization does not change ε
max.
7. Addition of 1 l. of cold water to the
acetone filtrate from which the second crop of
benzil is separated causes about
5 g. of impure
benzil to precipitate. This may be added to the crude
benzil of a subsequent run prior to the treatment with
hydrogen peroxide and alkali.
3. Discussion
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
hydrochloric acid (7647-01-0)
Benzene (71-43-2)
ethyl acetate (141-78-6)
methanol (67-56-1)
sodium hydroxide (1310-73-2)
carbon monoxide (630-08-0)
chloroform (67-66-3)
sodium sulfate (7757-82-6)
aluminum (7429-90-5)
acetone (67-64-1)
aluminum chloride (3495-54-3)
Benzil (134-81-6)
Benzoin (119-53-9)
carbon disulfide (75-15-0)
hydrogen peroxide (7722-84-1)
dimethylaniline,
N,N-dimethylaniline (121-69-7)
N,N-diethylaniline (91-66-7)
oxalyl chloride (79-37-8)
4-dimethylaminobenzaldehyde (100-10-7)
N,N-dipropylaniline (2217-07-4)
4,4'-Bis(dimethylamino)benzil,
Benzil, 4,4'-bis(dimethylamino)- (17078-27-2)
4,4'-bis(diethylamino)benzil
4,4'-bis(di-n-propylamino)benzil,
4,4'-bis(dipropylamino)benzil
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