Continuous preparation of phosphoric acid alkyl ester dichlorides

In the preparation of a phosphoric acid alkyl ester dichloride by reacting phosphorus oxychloride with an aliphatic alcohol according to the equation ##STR1## in which R represents C.sub.1 -C.sub.5 -alkyl, the improvement which comprises carrying out the reaction continuously at a temperature of about 10.degree. to 50.degree. C. using an excess of phosphorus oxychloride of about 5 to 40%. The excess phosphorus oxychloride is reacted with the alcohol by continuous mixing, the reaction mixture is cooled, substantially all the hydrogen chloride formed is then separated off in a preliminary degassing stage, the reaction mixture is worked up in a distillation column, the unreacted phosphorus oxychloride and any diluent is separated off over the top and condensed, the condensate is recycled to the reaction and the corresponding alkyl ester dichloride is removed continuously as the bottom product.

The invention relates to an unobvious process for the preparation of 
certain phosphoric acid alkyl ester dichlorides. 
It is known that phosphoric acid alkyl ester dichlorides are obtained when 
phosphorus oxychloride is allowed to react with the corresponding alcohol 
in the presence of a solvent, for example carbon tetrachloride or 
petroleum ether (Compt. rend. 246, 1879 (1958); J. Am. Chem. Soc. 51, 953 
(1929)). However, the yields of 60-70% are low, and they are decreased 
still further if the reaction temperatures are increased. 
According to Houben-Weyl XII, 2, page 212, the following criteria are 
important for a good yield; low reaction temperatures (about .+-.0.degree. 
C.), an excess of phosphorus oxychloride and as rapid as possible a 
removal of the hydrogen chloride formed. Only under these preconditions 
can the formation of phosphoric acid dialkyl ester chlorides, phosphoric 
acid trialkyl esters and alkyl chlorides be suppressed. 
According to U.S. Pat. No. 2,487,859, carbon dioxide is bubbled through the 
reaction mixture to remove, at a rapid rate, the hydrogen chloride formed. 
In Chemicky prumysl, 11, 461 (1961), in addition to discontinuous 
reactions, semi-continuous reactions according to the equation 
EQU POCl.sub.3 +CH.sub.3 OH.fwdarw.CH.sub.3 O--P(O)Cl.sub.2 +HCl 
are also described. The reaction components are first heated to 
temperatures which are 10.degree.-15.degree. C. above the boiling point of 
the alcohol, and then mixed in a jet. In spite of the relatively good 
yields, this process is unsuitable for carrying out on an industrial scale 
because of the high energy consumption. 
A continuous process has now been found for the preparation of phosphoric 
acid alkyl ester dichlorides, in which an excess of phosphorus oxychloride 
is reacted with the corresponding alcohol, a diluent being added if 
necessary. 
The invention accordingly provides a process for the preparation of a 
phosphoric acid alkyl ester dichloride by reacting phosphorus oxychloride 
with an aliphatic alcohol according to the equation 
##STR2## 
in which R represents C.sub.1 -C.sub.5 -alkyl, 
which process is characterized in that the reaction is carried out 
continuously at a temperature of about 10.degree. to 50.degree. using an 
excess of phosphorus oxychloride of about 5 to 400%. 
An excess of phosphorus oxychloride is employed in the process according to 
the invention in order to largely suppress the formation of by-products. 
Although an excess of 5 to 400% can be used, an excess of between about 10 
to 50% is preferred. 
The process according to the invention is preferably carried out in the 
temperature range from about 20.degree. C. to 40.degree. C. 
The process according to the invention can be carried out under reduced 
pressure, normal pressure or increased pressure, preferably under normal 
pressure. 
The process according to the invention is preferably carried out 
continuously, in a reaction apparatus in which the residence time in the 
reaction mixture of the hydrogen chloride formed is only short. 
Diluents which are to be used if necessary are, in general, solvents with 
approximately the same boiling point as phosphorus oxychloride; toluene is 
preferred.

The invention will be further described with reference to the accompanying 
drawing which is a flow sheet of a system for carrying out the instant 
continuous process. 
Referring now more particularly to the drawing, phosphorus oxychloride (A), 
the alcohol (B) and if necessary a diluent (A') are mixed continuously in 
the mixing chamber (1). The reaction mixture is cooled in the cooler (2), 
and the hydrogen chloride formed is then separated off in a preliminary 
degassing stage (3) and passed on at (C) to an HCl absorption stage. At 
(C) there is also a connection to a vacuum pump. The reaction mixture, 
which is essentially free from HCl, is worked up in a distillation column 
(4); phosphorus oxychloride and the diluent used thereby pass over the top 
and are condensed in the condenser (5). The condensate is recycled into 
the reaction. The bottom product (D) from the distillation column (4) 
consists of the corresponding alkyl ester dichloride to the extent of 
about 95%. The yields are between 90 and 95%. 
It is surprising that the phosphoric acid alkyl ester dichlorides are 
formed in such a high purity and yield in the process according to the 
invention, since it had to be expected, from the state of the art, that 
the yield would be drastically reduced at the relatively high reaction 
temperatures used. 
The process according to the invention thus represents an enrichment of the 
art. 
EXAMPLES 
Phosphoric acid ethyl ester dichloride 
148 kg/hour of phosphorus oxychloride, 148 kg/hour of toluene and 40 
kg/hour of ethanol were mixed in the mixing chamber (1) of the apparatus 
illustrated in the drawing. The reaction mixture was kept at +20.degree. 
C. by means of the cooler (2). The hydrogen chloride formed was removed in 
vacuo in the preliminary degassing stage (3) (T=60.degree. C., P=100 
mbars). In the column (4), 15 kg/hour of phosphorus oxychloride and 148 
kg/hour of toluene were distilled off over the top, also in vacuo, and 
were recycled into the reaction. 141 kg/hour of a product having the 
following composition ran out at the bottom of the column: 95% of 
phosphoric acid ethyl ester dichloride, 3% of phosphoric acid diethyl 
ester chloride and 2% of unknown substances. 
Phosphoric acid methyl ester dichloride 
Phosphoric acid methyl ester dichloride could be prepared in the same 
manner, with an excess of phosphorus oxychloride of about 30%. In this 
case, the yields were 90%. 
It will be appreciated that the instant specification and examples are set 
forth by way of illustration and not limitation, and that various 
modifications and changes may be made without departing from the spirit 
and scope of the present invention.