Org. Synth. 1942, 22, 16
DOI: 10.15227/orgsyn.022.0016
2-AMINO-6-METHYLBENZOTHIAZOLE
[Benzothiazole, 2-amino-6-methyl-]
Submitted by C. F. H. Allen and James VanAllan.
Checked by W. E. Bachmann and N. W. MacNaughton.
1. Procedure
A solution of 107 g. (1 mole) of p-toluidine (Note 1) in 700 ml. of chlorobenzene is prepared in a 3-l. three-necked, round-bottom flask fitted with a stirrer, reflux condenser, thermometer, and dropping funnel. Over a period of 5 minutes, 54 g. (29.3 ml., 0.55 mole) of concentrated sulfuric acid is added dropwise. To the finely divided suspension of p-toluidine sulfate is added 90 g. (1.1 moles) of sodium thiocyanate, and the mixture is heated for 3 hours at 100° (inside temperature) in an oil bath (Note 2). The solution, which now contains the thiourea, is cooled to 30°, and 180 g. (108 ml., 1.34 moles) of sulfuryl chloride is added over a period of 15 minutes, with care that the temperature does not exceed 50°. The mixture is kept at 50° for 2 hours (no further evolution of hydrogen chloride), after which the chlorobenzene is removed by filtration (Note 3).
The solid residue is then dissolved in 1 l. of hot water, and the remainder of the solvent is removed by a current of steam (Note 4). The solution is filtered from a little solid and is then made alkaline to litmus by the addition of 200 ml. of concentrated ammonium hydroxide (sp. gr. 0.90). The precipitated aminomethylbenzothiazole is filtered and washed with 200 ml. of water. The solid, which melts over the range 123–128°, is dissolved in 300 ml. of hot ethanol (Note 5), 10 g. of Norit is added, and the hot suspension is filtered. The filtrate is diluted with 500 ml. of hot water, and the mixture is vigorously stirred and quickly chilled. After 30 minutes, the pale yellow granular product is filtered and washed with 150 ml. of 30% ethanol. After drying to constant weight, the product weighs 100–105 g. and melts at 135–136°, with preliminary shrinking at 130–131° (Note 6). On the addition of 200 ml. of water to the filtrate, a further 5–8 g. of product is recovered, making the total yield 105–110 g. (64–67%).
2. Notes
1.
All the chemicals used were from the Eastman Kodak Company. The checkers used the practical grade of
p-toluidine, which was distilled just before use.
2.
The
p-tolylthiourea may be isolated at this point, if desired, by filtering and washing the solid residue with
ether. The dried solid (
172 g.) is a mixture of the
urea and
sodium sulfate. It is extracted with 250 ml. of warm (50–60°) water; the residual
urea, m.p.
188–189°, is completely soluble in
ethanol. The yield is
139 g. (
84%).
3.
The recovery is about 600 ml.; it may be used again without purification.
4.
Treatment of the solution with
Norit at this point does not give a product of any better quality.
5.
If less
ethanol is used, crystallization takes place during filtration.
6.
The literature gives melting points ranging from 128° to 142°, the higher value being for
aminomethylbenzothiazole prepared via the hydrochloride (m.p.
250–253°). The submitters report that the product obtained above is unchanged in melting point after regeneration from the hydrochloride. By this treatment, however, the color is removed.
3. Discussion
2-Amino-6-methylbenzothiazole has been prepared by the action of
cupric thiocyanate,
1,2,3 or of
chloramine and
ammonium thiocyanate,
4 on
p-toluidine; by the action of
chlorine on
di-p-tolylthiourea;
5 or of
bromine on
acetyldi-p-tolylthiourea;
6 by treatment of
p-tolylthiourea with halogens
7 or acid halides.
8,9,10,11,12 2-Aminobenzothiazole has been prepared in excellent yield by the action of
hydroxylamine upon
benzothiazole,
13 and has also been obtained from
2-mercaptobenzothiazole by the action of
ammonium bisulfite at 150°.
14 It has been prepared by the electrolysis of
p-toluidine in the presence of
ammonium thiocyanate.
15 Two German patents report a general method for the preparation of
2-aminobenzothiazoles16 and
2-aminonaphthothiazoles17 consisting of the treatment of an aromatic amine with
potassium thiocyanate and
acetic acid and subsequently with
bromine in
acetic acid.
Nearly quantitative yields are reported for a process using a modified
cupric thiocyanate method.
18
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
2-aminobenzothiazoles
2-aminonaphthothiazoles
ethanol (64-17-5)
sulfuric acid (7664-93-9)
hydrogen chloride (7647-01-0)
acetic acid (64-19-7)
ether (60-29-7)
ammonium thiocyanate (1762-95-4)
bromine (7726-95-6)
sodium sulfate (7757-82-6)
Norit (7782-42-5)
chlorobenzene (108-90-7)
sulfuryl chloride (7791-25-5)
chlorine (7782-50-5)
urea (57-13-6)
ammonium hydroxide (1336-21-6)
potassium thiocyanate (333-20-0)
hydroxylamine (7803-49-8)
thiourea (62-56-6)
sodium thiocyanate (540-72-7)
2-Amino-6-methylbenzothiazole,
Benzothiazole, 2-amino-6-methyl- (2536-91-6)
aminomethylbenzothiazole
cupric thiocyanate (18223-42-2)
chloramine (10599-90-3)
2-Aminobenzothiazole (136-95-8)
benzothiazole (95-16-9)
2-mercaptobenzothiazole (149-30-4)
ammonium bisulfite (10192-30-0)
p-toluidine (106-49-0)
p-toluidine sulfate
p-tolylthiourea (622-52-6)
di-p-tolylthiourea
acetyldi-p-tolylthiourea
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