Submitted by C. S. Marvel and R. L. Jenkins.
Checked by J. B. Conant and F. C. Whidden.
1. Procedure
In a
5-l. round-buttoned flask, fitted with a stopper holding a
condenser set for downward distillation and a
thermometer which will extend well into the liquid, are placed
4 kg. (3711 cc., 47–53 moles) of technical formaldehyde (35–40 per cent; sp. gr. 1.078 at 20°) (see
Note 1 on p. 378) and
2 kg. (37 moles) of technical ammonium chloride. The mixture is heated on the
steam bath until no more distillate comes over and then over a flame until the temperature of the solution reaches 104°. The temperature is held at this point until no more distillate comes over (four to six hours). The distillate, which consists of
methylal,
methyl formate, and water, may be treated as described in (Note
1).
The contents of the reaction flask are cooled to room temperature (Note
2), and the
ammonium chloride which separates is filtered off and centrifuged (Note
3). The mother liquor is concentrated on the steam bath under reduced pressure to 2500 cc., and again cooled to room temperature, whereupon a second crop of
ammonium chloride is obtained. The total recovery of
ammonium chloride up to this point amounts to
780–815 g.
The mother liquor is again concentrated under reduced pressure until crystals begin to form on the surface of the solution (1400–1500 cc.). It is then cooled to room temperature, and a first crop of
methylamine hydrochloride, containing some
ammonium chloride, is obtained by filtering the cold solution and centrifuging the crystals (Note
3). At this point
625–660 g. of crude product is obtained. The mother liquor is now concentrated under reduced pressure to about 1000 cc. and cooled; a second crop of
methylamine hydrochloride (
170–190 g.) is then filtered off and centrifuged. This crop of crystals is washed with
250 cc. of cold chloroform, filtered, and centrifuged, in order to remove most of the
dimethylamine hydrochloride which is present; after the washing with
chloroform the product weighs
140–150 g. The original mother liquor is then evaporated under reduced pressure, as far as possible, by heating on a steam bath, and the thick sirupy solution (about 350 cc.) which remains is poured into a
beaker and allowed to cool, with occasional stirring, in order to prevent the formation of a solid cake (Note
2). The thick paste so obtained is centrifuged, and the crystals (170–190 g.) are washed with
250 cc. of cold chloroform; the solution is filtered and the crystals are centrifuged, thus yielding
55–65 g. of product. There is no advantage in further concentrating the mother liquor.
The total yield of crude centrifuged
methylamine hydrochloride is
830–850 g. This product contains, as impurities, water,
ammonium chloride, and some
dimethylamine hydrochloride. In order to obtain a pure product, the crude
methylamine hydrochloride is recrystallized from
absolute alcohol (Note
4). The crude salt is placed in a
5-l. round-bottomed flask fitted with a
reflux condenser protected at the top with a
calcium chloride tube;
2 l. of absolute alcohol is added and the mixture heated to boiling. After about one-half hour the undissolved material is allowed to settle and the clear solution is poured off. When the alcoholic solution is cooled,
methylamine hydrochloride crystallizes out. It is filtered off and centrifuged, and the alcohol used for another extraction. The process is repeated until the alcohol dissolves no more of the product (about five extractions are necessary). In the flask
100–150 g. of ammonium chloride remains, making the total recovery of
ammonium chloride 850–950 g. The yield of recrystallized
methylamine hydrochloride is
600–750 g. (
45–51 per cent of the theoretical amount, based on the
ammonium chloride used up in the process) (Note
5) and (Note
6).
2. Notes
1.
Methylal and
sodium formate may be obtained from the first distillate of the main reaction mixture. The crude distillate is placed in a flask fitted with a
reflux condenser, and to it is added a solution of
200 g. of sodium hydroxide in 300 cc. of water. The
methyl formate is hydrolyzed to
sodium formate. The
methylal layer is separated, dried over
calcium chloride, and distilled. In this way
240–260 g. of
methylal, boiling at
37–42°, is obtained. By evaporating the watery portion to dryness, a residue of about
280 g. of crude sodium formate is obtained.
2. The
methylamine hydrochloride solutions should be cooled rapidly in order to bring the salt down in small crystals which may be easily purified.
3. Centrifuging the precipitates is the only satisfactory method of drying, as all the products tend to take up water when allowed to stand in the air.
4. Since
ammonium chloride is not absolutely insoluble in
100 per cent ethyl alcohol (100 g. dissolve 0.6 g. at 15°), the
methylamine hydrochloride purified in the manner described contains appreciable traces of it. A purer product can be prepared by recrystallizing from
n-butyl alcohol, in which the solubility of
ammonium chloride even at the boiling temperature is negligibly small.
Methylamine hydrochloride is somewhat less soluble in this solvent than in
ethyl alcohol, but as a rule three extractions carried out at 90–100° with 4–6 parts of fresh
butyl alcohol for each extraction result in a substantially complete separation. Since the last traces of the solvent are not readily removed by exposure to air, a solution of the recrystallized material in a small quantity of water should be distilled until free of alcohol, and allowed to crystallize.
6.
Methylamine can be purified through its
benzal derivative, which is distilled and then hydrolyzed by concentrated hydrochloric acid.
3 This method has been checked and found satisfactory by one of the editors.
3. Discussion
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