Process for making alkyl or arylthiophosphonic acid

The disclosure relates to a process for making alkyl or arylthiophosphonic acid dichlorides by reacting alkyl or aryldichlorophosphanes with sulfur at a temperature above the melting point of sulfur and in the presence of a catalyst. More particularly, the reaction is effected at a temperature between the melting point of sulfur and the boiling point of the resulting alkyl or arylthiophosphonic acid dichloride and in the presence of a catalyst of the following general formula I or II ##STR1## in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4 stand for identical or different alkyl, aryl, alkaryl, or aralkyl groups having from 1 to 22 carbon atoms and A stands for the anionic group of an organic or inorganic acid.

The present invention relates to a process for making alkyl or 
arylthiophosphonic acid dichlorides by subjecting alkyl or 
aryldichlorophosphanes to catalytic reaction with sulfur. 
It has already been described (cf. Houben-Weyl, Methoden der organischen 
Chemie, volume XII/1, page 555) that alkyl or arylthiophosphonic acid 
dichlorides can be made by reacting suitable alkyl or 
aryldichlorophosphanes with sulfur under the catalytic action e.g. of 
AlCl.sub.3, FeCl.sub.3 or ZnCl.sub.3. The continuous manufacture of 
methanethiophosphonic acid dichloride has been described in Fench Pat. No. 
2 064 774, wherein methyldichlorophosphane is reacted with sulfur at about 
150.degree. C. with addition of 4 weight % AlCl.sub.3, based on CH.sub.3 
PCl.sub.2. After distillative separation of the product from the reaction 
mixture, the yield is about 92% of the theoretical. While the 
black-colored distillation residue can under circumstances be used again 
in further reactions, the fact remains that its catalytic activity 
decreases rapidly so that it is invariably necessary for further 
quantities e.g. of AlCl.sub.3 to be added to the reaction batch. As an 
inevitable result of this, increased quantities of distillation residue 
which, for reasons of environmental protection, can be disposed of with 
considerable expense only, are obtained. 
We have now unexpectedly found that substituted phosphonium or ammonium 
salts equally catalyze the reaction of alkyl or aryldichlorophosphanes 
with sulfur, the salts retaining their catalytic activity during the 
reaction. In other words, it is just necessary for the catalyst to be 
added once to the initial batch, at the onset of the reaction, the next 
batches having the distillation residue originating from the respective 
preceding batch added thereto. 
More specifically, the invention relates to a process for making alkyl or 
arylthiophosphonic acid dichlorides by reacting alkyl or 
aryldichlorophosphanes with at least stoichiometric proportions of sulfur 
at a temperature above the melting point of sulfur and in the presence of 
a catalyst, and separating the resulting alkyl or arylthiophosphonic acid 
dichloride from the reaction mixture after the reaction has been 
terminated, which comprises: effecting the reaction at a temperature 
between the melting point of sulfur and the boiling point of the resulting 
alkyl or arylthiophosphonic acid dichloride and in the presence of a 
catalyst of the following general formula I or II 
##STR2## 
in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4 stand for identical or 
different alkyl, aryl, alkaryl or aralkyl groups having from 1 to 22 
carbon atoms and A stands for the anionic group of an organic or inoganic 
acid, the catalyst being used in a proportion of 0.01 to 5 weight %, based 
on the quantity of the phosphane component; terminating the reaction and 
separating the alkyl or arylthiophosphonic acid dichloride from the 
reaction mixture; and using resulting catalyst-containing residue in 
further reactions of alkyl or aryldichlorophosphanes with sulfur. 
The alkyl or aryldichlorophosphanes as well as the thiophosphonic acid 
dichlorides obtainable by the process of this invention preferably contain 
1 to 8 carbon atoms. 
The feed materials comprise sulfur and preferably methyl, ethyl or 
phenyldichlorophosphanes, the reaction being preferably effected at a 
temperature of 130.degree. up to 170.degree. C. The reaction should 
preferably be accelerated using tetraalkylammonium or 
tetraalkylphosphonium chloride, bromide, sulfate, nitrate or acetate as 
the catalyst. 
A preferred feature of the present process provides for the phosphane 
component to be introduced portionwise into a mixture of sulfur and 
catalyst and, after the phosphane addition is complete, for the reaction 
mixture to be subjected to a post-reaction over a period of 5 to 60 
minutes. The catalyst should preferably be used in a proportion of 0.05 to 
2 weight %, based on the quantity of the phosphane component. The use of 
more catalyst could not be found to result in adverse effects; it should 
however be avoided for reasons of economy and as being redundant. Under 
the action of the catalyst, the reaction of this invention proceeds 
rapidly and completely. After the reaction has been terminated, the 
thiophosphonic acid dichloride is distillatively removed from the crude 
reaction mixture. The catalyst is retained in the distillation residue 
which is admixed with sulfur and used in the next batch. 
The phosphonium and ammonium salts, respectively, suggested to be used as 
catalyst in accordance with this invention compare favorably with the 
metal chlorides used heretofore inasmuch as hydrolytic action does not 
result in the phosphonium or ammonium salts becoming decomposed as it is 
the case with metal chlorides. Particularly stable are the phosphonium 
salts, which remain chemically inaltered during the reaction of the feed 
materials and also during distillation of the thiophosphonic acid 
dichloride. As a result, it is possible for the distillation residue to be 
recycled and used again in the next following reaction. In a test series 
made on 19 batches, where the distillation residue was recycled in each 
case, the catalytic activity could not be found to have decreased, so that 
the catalyst added once can reasonably be assumed to be useful in even a 
considerably larger number of reaction cycles. In other words, the present 
process with the use of relatively costly catalyst therein can be carried 
out under commercially more attractive conditions than the prior art 
methods, after a few reaction cycles. In addition to this, less 
distillation residue is obtained which means less expensive work-up or 
disposal. 
The alkyl and arylthiophosphonic acid dichlorides made by the process of 
this invention are obtained in higher yields than heretofore, and they are 
extremely pure.

EXAMPLE 1 
184 g (5.75 mols) sulfur and 1.6 g tetrabutylphosphonium bromide were 
introduced into a multi-necked flask provided with a stirrer, dropping 
funnel, reflux condenser and thermometer, and 577 g (4.93 mols) 
methyldichlorophosphane was added dropwise with agitation at 140.degree. 
to 150.degree. C. After a post-reaction period of 30 minutes at 
150.degree. C., methanethiophosphonic acid dichloride was distilled off 
from the reaction mixture at atmospheric pressure, after separation of 
first runnings. 
The distillation residue was admixed with further sulfur and the first 
runnings of the first batch, and methyldichlorophosphane was added 
dropwise. In this manner, four further batches were processed and 
altogether 2670 g CH.sub.3 PCl.sub.2 was reacted with sulfur to give 3356 
g CH.sub.3 PSCl.sub.2. The yield was 98.7% of the theoretical, the product 
having a purity of moe than 98% (n.sub.D.sup.23 =1.5481). The distillation 
residue obtained at a rate of 57.5 g, corresponding to 1.7% based on the 
quantity of product, contained the catalyst of inaltered activity, which 
could be used in further batches. 
EXAMPLE 2 
192 g (6 mols) sulfur and 1.5 g tetramethylphosphonium chloride were 
introduced into an apparatus as described in Example 1. Next, 585 g (5 
mols) CH.sub.3 PCl.sub.2 was added dropwise to the mixture at about 
150.degree. C. and the reaction initiated. After a post-reaction period of 
15 minutes, the crude product was distilled and 3 to 5 weight %, based on 
the total quantity, first runnings were obtained. Methanethiophosphonic 
acid dichloride with a purity of 99.5% was obtained in a yield of 636 g. 
The first runnings and distillation residue were used in another batch. 
In this manner, 18 batches were processed without any intermediary addition 
of fresh catalyst. After altogether 19 batches, the test series was 
interrupted; the catalyst activity could not be found to have been 
reduced. 
Altogether 13 871 g methanethiophosphonic acid dichloride with a purity of 
99.5% was obtained in a yield of 98.0% of the theoretical. 
After 19 batches, 165 g or 1.2%, based on the product quantity, 
distillation residue was obtained. 
EXAMPLE 3 
256 g (8 mols) sulfur and 1.5 g tetrabutylammonium chloride was introduced 
into an apparatus as described in Example 1 and 910 g (7.78 mols) 
methyldichlorophosphane was added dropwise with agitation at about 
140.degree. to 150.degree. C. After a post-reaction period of 15 minutes 
at 150.degree. C., first runnings were separated and 1025 g 
methanethiophosphonic acid dichloride was distilled off from the reaction 
mixture. The dichloride had a purity of more than 98%. The distillation 
residue was suitable for use in further batches. 
EXAMPLE 4 
74 g (2.31 mols) sulfur and 1.6 g tetrabutylphosphonium bromide were 
introduced into an apparatus as described in Example 1 and 272 g (2.08 
mols) ethyldichlorphosphane was added dropwise with agitation at about 
160.degree. C. The reaction mixture was allowed to undergo post-reaction 
over a period of 30 minutes at 160.degree. C. Under a reduced pressure of 
about 95 millibars, the reaction mixture was first freed distillatively 
from first runnings. Next, ethanethiophosphonic acid dichloride was 
distilled off at a head temperature of 92.degree. to 94.degree. C. 280 g 
dichloride with a purity of 98.1% and a refractive index n.sub.D.sup.22 
=1.5406 was obtained. 
By recycling the first runnings and distillation residue, it was possible 
considerably to increase the yield. 
EXAMPLE 5 
40 g (1.25 mols) sulfur and 2 g tetrabutylphosphonium bromide were 
introduced into an apparatus as described in Example 1 and 210 g (1.17 l 
mols) C.sub.6 H.sub.5 PCl.sub.2 was added dropwise to the mixture with 
agitation at about 160.degree. C. After a post-reaction of 30 minutes at 
160.degree. C., the mixture was distilled under vacuum of 4 millibars. 212 
g benzenethiophosphonic acid dichloride with a purity of 99.6% and a 
refractive index n.sub.D.sup.22 =1.6230 was obtained. 
By recycling the first runnings and distillation residue, it was possible 
considerably to increase the yield. 
COMATIVE EXAMPLE 1 
In an apparatus as described in Example 1, altogether 11 561 g (98.8 mols) 
methyldichlorophosphane was reacted batchwise with 3 162 g (98.8 mols) 
sulfur with the addition of altogether 139.5 g aluminium chloride as 
catalyst, the reaction being effected at 140.degree. to 150.degree. C. 14 
154 g methanethiophosphonic acid dichloride with a purity of more than 98% 
and n.sub.D.sup.22 =1.5488 was distilled off from the reaction mixture. 
The yield was 96.1% of the theoretical and 550 g (3.9%, based on the 
product quantity) distillation residue was obtained. 
A comparison of the test data of Example 2 and comparative Example 1 show 
that the present process compares favorably with prior art methods. 
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Comparative 
Example 2 Example 1 
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Product quantity 
13 871 g 14 154 
g 
Catalyst quantity 
1.5 g (CH.sub.3).sub.4 PCl 
139.5 g AlCl.sub.3 
Distillation res- 
165 g 550 g 
idue 
Yield 98% of theo- 96.1% of theo- 
retical retical 
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COMATIVE EXAMPLE 2 
1259 g (39.3 mols) sulfur and 4599 g (39.3 mols) methyldichlorophosphane 
were introduced into an apparatus as described in Example 1 and reacted 
therein at 140.degree. to 150.degree. C. with addition of 28 g AlCl.sub.3 
as a catalyst. Methanethiophosphonic acid dichloride was distilled off 
from the reaction mixture and the distillation residue was then admixed 
with 1074 g sulfur without addition of fresh AlCl.sub.3. After the 
addition of 1200 ml CH.sub.3 PCl.sub.3, spontaneous strong reflux of 
unreacted CH.sub.3 PCl.sub.2 set in, which reacted very reluctantly only. 
This phenomenon was attributable to the decreasing activity of the 
catalyst. 20 g fresh AlCl.sub.3 was added and the reaction was terminated.