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Org. Synth. 1953, 33, 27
DOI: 10.15227/orgsyn.033.0027
p-DIMETHYLAMINOBENZALDEHYDE
[Benzaldehyde, p-dimethylamino-]
Submitted by E. Campaigne and W. L. Archer1.
Checked by N. J. Leonard and R. W. Fulmer.
1. Procedure
In a 2-l. three-necked round-bottomed flask, equipped with a sealed stirrer, dropping funnel, and a reflux condenser topped by a calcium chloride tube, is placed 440 g. (6 moles) of dimethylformamide (Note 1). While the flask is carefully cooled in an ice bath, 253 g. (1.65 moles) of phosphorus oxychloride is added dropwise with stirring. An exothermic reaction occurs with the formation of the phosphorus oxychloride-dimethylformamide complex. When all the phosphorus oxychloride has been added, and the heat of the reaction has subsided, 200 g. (1.65 moles) of dimethylaniline (Note 2) is added dropwise with stirring. When the addition of the dimethylaniline is complete, a yellow-green precipitate begins to form. The reaction mixture is heated on a steam bath, and stirring is continued for 2 hours. The yellow-green precipitate redissolves when heating is begun. The mixture is then cooled and poured over 1.5 kg. of crushed ice in a 5-l. beaker. Any precipitate that remains in the flask may be washed into the ice mixture with cold water. The solution is neutralized to pH 6–8 (Universal Test Paper) by the dropwise addition of approximately 1.5 l. of saturated aqueous sodium acetate with vigorous stirring (Note 3). p-Dimethylaminobenzaldehyde begins to precipitate soon after the addition of the sodium acetate is begun. The neutral mixture (total volume about 4.5 l.) is stored in the refrigerator overnight (Note 4). The greenish-tinted crystalline precipitate is filtered by suction, with the aid of a rubber dam, and washed several times with water on the filter. The green color is readily removed during the washing. The very light-yellow to nearly colorless product, after air-drying, weighs 198–208 g. (80–84%) and melts at 73–74°. It is essentially pure and useful for most purposes as obtained (Note 5).
2. Notes
1. The dimethylformamide is available as technical grade DMF from the Grasselli Chemicals Department of E. I. du Pont de Nemours and Company. Dimethylformamide can be prepared by the method of Mitchell and Reid2 from dimethylamine and formic acid.
2. Dimethylaniline free from monomethylaniline (Eastman Kodak Company) is used.
3. It is possible to neutralize the acid solution partially with sodium hydroxide before the sodium acetate is added, but it is more difficult to avoid localized heating by this method. It is important to keep the reaction mixture below 20° during the neutralization, by the addition of ice if necessary, since any excessive increase in temperature of the aqueous solution leads to the formation of greenish blue dyestuffs, which are very difficult to remove from the product.
4. The mixture may turn orange-colored when allowed to stand overnight.
5. If a purer product is desired, the aldehyde may be purified by the method described by Adams and Coleman.3
3. Discussion
p-Dimethylaminobenzaldehyde has been prepared from dimethylaniline, formaldehyde, and p-nitrosodimethylaniline in 56–59% yield,3 by the formylation of dimethylaniline with N-methylformanilide in approximately 50% yield,4 by the formylation of dimethylaniline with dimethylformamide,5 and by the condensation of methyl formate with p-dimethylaminophenylmagnesium chloride in tetrahydrofuran.6
This preparation is referenced from:

References and Notes
  1. Indiana University, Bloomington, Indiana.
  2. Mitchell and Reid, J. Am. Chem. Soc., 53, 1879 (1931).
  3. Org. Syntheses Coll. Vol. 1, 214 (1941).
  4. Vilsmeier and Haack, Ber., 60, 119 (1927).
  5. Brit. pat. 607,920 [C. A., 43, 2232 (1949)].
  6. Ramsden (to Metal & Thermit Corp.), Brit. pat. 806,710 [C. A., 54, 2264 (1960)].

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

phosphorus oxychloride-dimethylformamide complex

sodium acetate (127-09-3)

sodium hydroxide (1310-73-2)

formaldehyde (630-08-0)

formic acid (64-18-6)

Phosphorus Oxychloride (21295-50-1)

dimethylaniline (121-69-7)

dimethylamine (124-40-3)

methyl formate (107-31-3)

monomethylaniline (100-61-8)

Tetrahydrofuran (109-99-9)

N-methylformanilide (93-61-8)

dimethylformamide (68-12-2)

p-Dimethylaminobenzaldehyde,
Benzaldehyde, p-dimethylamino- (100-10-7)

p-nitrosodimethylaniline (138-89-6)

p-dimethylaminophenylmagnesium chloride