Note: Descriptions are shown in the official language in which they were submitted.
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Thi~ invention relates to an improved process for pre-
paring mercaptobenzothiazole.
U.S. Patent 3,031,073 describes the manufacture of
mercaptobenzothiazole wherein aniline, sulfur and carbon di-
sulfide are reacted together. In accordance with this patent,
the residue left after separation of mercaptobenzothiazole
from the crude reaction product, normally by a water immiscible
organic solvent, after removal of the solvent, was recycled
to an autoclave with fresh reactants. While this is alleged
to represent an improvement in the yield of desired mercapto-
benzothiazole, purification problems are introduced and the
recycle product mercaptobenzothiazole so obtained is difficult
to purify because of the continued recycling of the tars and
other discolored and undesired reaction products. If the by-
products are discarded, there is a loss of valuable materials.
Improvement in this process is desired, particularly the
obtainment of maximum amounts of substantially color-free mer-
captobenzothiazole from these by-product residues.
According to the invention there is provided in the
process for preparing mercaptobenzothiazole by the reaction
of aniline, sulfur and carbon disulfide, the improvement
comprising separating the mercaptobenzothiazole formed from
the other reaction by-products of said reaction, fractionating
- said by-products to recover benzothiazole and heating said
1, benzothiazole with sulfur, whereby additional yields of
i~ mercaptobenzothiazole are obtained.
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Typically the residue obtained after the 2-mercapto-
benzothiazole is separated from the reaction product of aniline,
culfur and carbon disulfide is fractionated to provide a
fraction containing at least 90~ benzothiazole, which benzo-
thiazole iA then heated with sulfur, and carbon disulfide if
required to react with any aniline impurity in the benzothia-
zole, to provide a high purity substantially color-free 2-
mercaptobenzothiazole.
In the preparation of the 2-mercaptobenzothiazole
by the reaction of aniline, sulfur and carbon disulfide,
usually in about equimolar amounts, after the initial reaction,
the crude mercaptobenzothiazole iQ normally solubilized in
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a caustic solution, which solution is then extracted with a
solvent as toluene. The toluene removes the tar and other
impurities from the mercaptobenzothiazole. In accordance
with this invention this solvent solution is then distilled
to remove the toluene and the fractionated to obtain benzo-
thiazole. This benzothiazole fraction is then reacted with
sulfur, and carbon disulfide if required, to obtain an
improved yield of high purity mercaptobenzothiazole having
improved color.
When the total by-product is recycled or further -
reacted, conversion of this residue to some additional
mercaptobenzothiazole is obtained; however, the resulting
product is difficult to purify because of color and other
impurities. In contrast, the distilled benzothiazole product,
which normally contains a small amount of aniline, is readily -
converted into 2-mercaptobenzothiazole in improved overall
; yields with the elimination of color and purification problems
observed when recycling the total tarry by-product residue.
The impurities from the mercaptobenzothiazole
reaction product are readily separated from the mercaptobenzo-
thiazole with solvents. Preferably, aromatic solvents are
used and those containing one aromatic ring including benzene,
xylene, toluene, chlorobenzene, aniline and the like have
been found to be useful. Other cyclic solvents such as
cyclohexane and methyl cyclohexane may be used as well as
halogenated hydrocarbons and less desirably, higher aliphatics
such as heptane and octane.
In the separation of the benzothiazole from the
other mercaptobenzothiazole by-products, distillation under
; 30 vacuum is the most convenient method. The solvent is readily
. . .
removed at lower temperatures. Aniline is removed at about
15 milliliters, normally at about 77C., and benzothiazole
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at about 120 to 121C. Higher boiling impurities such as
resinous materials, methyl mercaptobenzothiazole, sulfur and
the like remain in the still pot because of much higher
boiling points. The benzothiazole distillate used normally
has a purity of about 90YO and more preferably, about 95 to
9g%. Small amounts of aniline, as 0.1 up to 10~ or even 15%,
and typically up to about 5% can be tolerated as well as 2-
anilino-benzothiazole. Less desirable, toluidines, methyl
benzothiazole and the like are preferably kept low, below
about 1%, and normally do not appear when the by-products are
carefully distilled. A typical analysis of the by-product from
the aniline, sulfur, carbon disulfide reaction wherein 11.4% by-
product tars were isolated from the mercaptobenzothiazole by
toluene was aniline, 0.34%, benzothiazole, 5.64%, sulfur, 1.32~,
2-anilinobenzothiazole, 1.12%; other, 2.98æ. The other
impurities include undesirable resinous black materials, and
possibly small amounts of toluidines, methylbenzothiazole,
methyl mercaptobenzothiazole and other benzothiazole derivatives.
In any event, it is desired to carefully distill the tars so
as to obtain primarily benzothiazole of at least 90YO purity.
The presence of small amounts of aniline impurity are not
undesirable.
On a molar basis of one mol of benzothiazole,
,
about an equal or a slight molar excess of sulfur provides
excellent yields of the desirable 2-mercaptobenzothiazole.
- While lesser amounts of sulfur as 0.5 mol may be used, lower
; yield of desired product are obtained and with larger amounts
of sulfur, as greater than about 1.5 to 2 mols of sulfur,
problems may be introduced in removing the excess sulfur,
~`~ 30 often requiring the use of a sulfur solvent. When aniline
is pre~ent as an impurity, carbon disulfide is included as a
xeactant. Normally about equimolar amounts will be used.
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From 0.5 to 2.0 mols/mol of aniline may be used, but the
lower amounts provide less desired product and the excess
must be removed.
The reactants are mixed together in any standard
reactor or autoclave, preferably a pressure autoclave and
heated to a temperature greater than about 200C. to about
300C. Better yields are generally obtained in the range of
about 220C. to about 280C. for periods of about 2 to 6
hours under autogeneous pressure, although higher pessures
may be used and longer times at lower temperatures.
EXAMPLE I
To demonstrate the advantages of this invention, a
series of reactions were run to prepare mercaptobenzothiazole
by reacting a molar ratio of 1.5 mols each of aniline,
~ 15 sulfur and carbon disulfide at 260C. for 2 hours. The
; resulting reaction products were dissolved in a lN solution
of sodium hydroxide and the tars and other by-products
extracted with toluene. A toluene extract of tar from these
mercaptobenzothiazole reactions was combined and distilled
at 13-18 mm mercury and 21-32C. to remove toluene, and 10-
17 mm mercury and 110 to 120C. to collect benzothiazole
containing about 2~ aniline. 1.46 mols (210 grams) of
benzothiazole obtained from the by-product tar along with
the 0.04 mol of aniline was heated with 1.5 mols (41.8
grams) of sulfur and 0.04 mol (3 grams) of carbon disulfide
for 2 hours at 260C. The reaction product was dissolved in
: 850 ml of 2N sodium hydroxide. The resulting solution was
free of color. This solution was extracted with a total
volume of 1 liter of toluene. The separated aqueous solution,
which was substantially colorless, was acidified to a pH of
3 with 5N sulfuric acid and the precipitated mercaptobenzo-
thiazole was filtered and washed with water. A yield of
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mercaptobenzothiazole of 89.5% (212 grams) having a melting
point of 178-181C. was obtained. After drying this product
had only a pale yellow discoloration.
EXAMPLE II
To demonstrate the disadvantages of using recovered
by-products as such from the mercaptobenzothiazole reaction,
112 grams of by-product was collected as described in Example
I and the toluene distilled off. This material, 0.76 mol
equivalent of benzothiazole, 2-anilinobenzothiazole and
aniline with the required amount of sulfur and carbon di-
sulfide, 0.64 mol of sulfur and 0.11 mol of carbon disulfide
and the mixture heated in a rocking autoclave at 260C. for
2 hours. The reactor was vented and the reaction mixture
removed from the vessel. The reaction product was dissolved
in 1 liter of lN sodium hydroxide. The resulting black
mixture containing black suspended matter was treated with
14 grams of activated charcoal and filtered. The resulting
filtrate was still black. The filtrate was then extracted
~; three times with 200 ml portions of toluene and the aqueous
~, 20 phase treated again with 20 grams of activated charcoal.
After filtration, the filtrate was still colored. The
filtrate was acidified with 5N sulfuric acid to a pH of 3
and the precipitate washed with water and dried. A 70~
yield of mercaptobenzothiazole (89 grams) having a melting
- 25 point in the range of 173 to 180C. was obtained. This
- product had an undesirable yellowish brown color and could
not be used as a compounding ingredient in white rubber
- stocks.
2-Mercaptobenzothiazole (MBT) is a valuable com-
mercial rubber accelerator, normally used to accelerate the
sulfur vulcanization of olefinically unsaturated rubbers
such as butadiene acrylonitrile and styrene rubbers, poly-
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butadiene rubber, polyisoprene, neoprene and the like, as is
well known to those skilled in the art.
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