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Org. Synth. 1941, 21, 81
DOI: 10.15227/orgsyn.021.0081
METHYL ISOTHIOCYANATE
[Isothiocyanic acid, methyl ester)]
Submitted by Maurice L. Moore and Frank S. Crossley.
Checked by Nathan L. Drake and Ralph Mozingo.
1. Procedure
In a 1-l. round-bottomed three-necked flask, surrounded by an ice bath and fitted with a mechanical stirrer, a reflux condenser, thermometer, and a 250-ml. dropping funnel, are placed 137 g. (110 ml., 1.8 moles) of carbon disulfide and a cold solution of 72 g. (1.8 moles) of sodium hydroxide in 160 ml. of water. To this mixture, cooled to 10–15°, is added, with stirring, 180 ml. (56 g., 1.8 moles of methylamine) of 35% aqueous methylamine solution (Note 1) and (Note 2) over a period of 30 minutes. Stirring is continued, and the mixture is warmed gently on a steam bath for 1–2 hours to ensure complete reaction (Note 3). The bright red solution is cooled to 35–40°, and to it is added over a period of 1 hour, with stirring, 196 g. (175 ml., 1.8 moles) of ethyl chlorocarbonate (Note 4). The stirring is continued for 30 minutes after all the ethyl chlorocarbonate has been added, at which time the temperature should have fallen to 30–40°. The methyl isothiocyanate, which separates on top, is removed from the reaction mixture and weighs 170–190 g.
The product is dried over 10 g. of sodium sulfate and distilled under atmospheric pressure through a short Vigreux column; the fraction which boils at 115–121° is collected. The yield is 85–100 g. (65–76%) (Note 5) and (Note 6). The product may be further purified by refractionation. The portion which boils at 117–119° is collected.
2. Notes
1. The monomethylamine used in this preparation was a "Commercial Special 35% Solution" obtained from Rohm and Haas Company.
2. Methylamine hydrochloride can be used in place of the commercial aqueous methylamine solution by a slight modification of the above procedure. The carbon disulfide and a solution of 122 g. (1.8 moles) of methylamine hydrochloride in 200 ml. of water are mixed together in the flask, and a cold solution of 144 g. (3.6 moles) of sodium hydroxide in 320 ml. of water is added, with stirring, over a period of 30 minutes. Two equivalents of sodium hydroxide must be used in this case. The remainder of the procedure is the same as with the free base.
3. The temperature gradually rises from 25°, which is that noted at the end of the addition of the methylamine solution, to 75–85°.
4. The temperature may rise rather rapidly during this addition; it is advisable to maintain the rate of addition constant so that the reaction does not become too vigorous.
5. Larger runs, up to 5.4 moles of carbon disulfide, have been made with only a slight reduction in yield.
6. This reaction is general for the preparation of alkyl isothiocyanates in good yields; thus, according to the submitters, ethyl isothiocyanate is obtained in yields of 60–70% from ethylamine hydrochloride.
3. Discussion
Methyl isothiocyanate has been prepared from methyl thiocyanate by rearrangement with heat1 and from N,N'-dimethylthiuramdisulfide by the action of iodine2 or by heating with water or methanol.3 The most useful method of preparation has been the reaction of methylamine with carbon disulfide to form methyldithiocarbamic acid which is decomposed by steam distillation of the lead salt,4 or by reaction with ethyl chlorocarbonate.5
This preparation is referenced from:

References and Notes
  1. Hofmann, Ber., 13, 1349 (1880).
  2. v. Braun, Ber., 35, 817 (1902).
  3. Freund and Asbrand, Ann., 285, 166 (1895).
  4. Delépine, Bull. soc. chim. France, (4) 3, 641 (1908); Delépine, Compt. rend., 144, 1125 (1907); Worrall, J. Am. Chem. Soc., 50, 1456 (1928).
  5. Slotta and Dressler, Ber., 63, 888 (1930).

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

lead salt

methanol (67-56-1)

sodium hydroxide (1310-73-2)

sodium sulfate (7757-82-6)

iodine (7553-56-2)

carbon disulfide (75-15-0)

Methylamine hydrochloride (593-51-1)

methylamine,
monomethylamine (74-89-5)

ethyl chlorocarbonate (541-41-3)

ethyl isothiocyanate (542-85-8)

Methyl isothiocyanate,
Isothiocyanic acid, methyl ester (556-61-6)

ethylamine hydrochloride (557-66-4)

methyl thiocyanate (556-64-9)

methyldithiocarbamic acid

N,N'-dimethylthiuramdisulfide