BACKGROUND OF THE INVENTION

This invention relates to organophosphorus compounds, especially organophosphates and organothiophosphates. More particularly, it relates to the electrochemical synthesis of organophosphates and organothiophosphates in compartmented or uncompartmented cells at ambient temperature and pressure.

Neutral organic esters and thioesters of phosphoric acid have been synthesized in the past by the reaction of phosphorus trichloride and phosphorus oxychloride with alcohols, phenols, mercaptans, or thiophenols. More recently organic phosphates have been prepared by the reaction of phosphorus pentachloride with ethers, as disclosed in U.S. Pat. No. 2,407,279. Employing phosphorus halides to make organic phosphates or thiophosphates generates hydrogen halides as by-products, which are corrosive, difficult to handle, and troublesome to dispose of in an environmentally sound manner. The reaction of phosphorus pentoxide with organic hydroxyl or sulfhydryl compounds requires high temperatures, often high pressure, and the maintenance of supplies of reactive chemicals in inventory.

Warshawsky, Tomilov, and Smirnov published in the All-Union Journal of General Chemistry 7, 598 (1962-USSR) a description of an electrochemical process for the synthesis of trialkyl phosphates by passing current between graphite electrodes in a cell containing alcoholic hydrogen chloride and a suspension of red phosphorus. Trimethyl, triethyl, tributyl and triamyl phosphates were made from the respective alcohols by this procedure.

Fraser in U.S. Pat. No. 2,133,290 of 1938 disclosed a process for making iron oxide and sodium phosphate in an electrolytic cell with a ferrophosphorus anode, a graphitic or lead cathode, and a concentrated aqueous caustic electrolyte.

In U.S. Pat. No. 2,173,103 of 1939 Fraser disclosed a process for making iron phosphate in an electrolytic cell with a ferrophosphorus anode, a graphitic or lead cathode, and a concentrated aqueous solution of an alkali metal salt plus, optionally, alkali metal hydroxide as the electrolyte.

In U.S. Pat. No. 3,730,864 of 1973 Tarjanyi disclosed a process for decreasing the phenolic content of dilute aqueous solutions electrochemically by passing current through a cell packed 40-80% by volume with a bed of particles in the electrolyte. One of the numerous types of particles which Tarjanyi discloses for the electrolytic bed is ferrophosphorus, which is not named as a possible electrode, however.

A principal object of the invention is to produce organophosphorus compounds directly from ferrophosphorus. Another object of the invention is to utilize ferrophosphorus, a by-product of the winning of elemental phosphorus from phosphate rock. Still another object of the invention is to produce trialkyl phosphates and thiophosphates without generating hydrogen chloride as a by-product. A further object of the invention is to produce organophosphorus compounds at ambient temperatures and pressures. Other objects of the invention will be evident to those skilled in the art from study of the description and examples below.

SUMMARY OF THE INVENTION

Surprisingly, it has been found that electrolysis of alcohols, phenols, mercaptans, and thiophenols made conductive by the presence of an inorganic salt in a cell equipped with an anode of ferrophosphorus leads to the production of organophosphorus compounds in the electrolyte. Electrolysis of alcohols primarily produces trialkyl phosphates. The product from phenols in the electrolyte is triaryl phosphates. Sulfhydryl compounds yield thioesters. Polyhydric compounds such as ethylene glycol may also be employed.

DESCRIPTION OF THE INVENTION

Elemental phosphorus is produced commercially from phosphate rock in an electrically heated furnace by reduction with coke in the presence of sand. In the course of this reduction iron impurities from all sources, including the furnace, react with phosphorus to produce ferrophosphorus, which is tapped off from the furnace below the silicate slag. For each kilogram of elemental phosphorus product, approximately 0.3 kg ferrophosphorus is made. The main use of ferrophosphorus is in steel making where about 0.1 percent phosphorus in the steel is considered advantageous. Other applications for ferrophosphorus by-product would be desirable.

Ferrophosphorus compounds with formulae, FeP.sub.2, FeP, Fe.sub.2 P, and Fe.sub.3 P are known and have been characterized. All the ferrophosphides are good conductors of electricity, and therefore can serve as electroconductive sources of phosphorus. Ferrophosphides with a phosphorus content of from about 22 to about 30 weight percent phosphorus are preferred as anodes in the present invention.

Two commercial samples of ferrophosphorus employed as anodes in the examples described below, designated MP and SB, had the following elemental analysis corresponding to Fe.sub.2 P:

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                 Wgt. %      Wgt. %
    Element      in Sample MP
                             in Sample SB
    ______________________________________
    Phosphorous  26.2        26.9
    Iron         66.3        61.0
    Chromium     1.0         3.92
    Vanadium     0.2         6.05
    Silicon      0.32        0.28
    Manganese    2.2         0.19
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As a source of direct current for the electrolysis a battery or rectifier may be employed. The voltage impressed should be sufficient to electrolyse the hydroxyl or sulfhydryl compound and can range from about 5 volts to about 50 volts, with about 15 volts to about 25 volts preferred. The area of the electrodes, the salt content of the electrolyte, the cell, and the impressed voltage should be so chosen that the current density varies from about 0.001 amp/cm.sup.2, with about 0.05 amp/cm.sup.2 to about 0.2 amp/cm.sup.2 preferred.

In order to make the hydroxyl or sulfhydryl compound ionically conductive, an indifferent ionizable salt should be added to the electrolyte. For this purpose the nonreactive salts of an alkali metal or ammonium cation are preferred. Halogen, nitrate, bicarbonate, bisulfate, or sulfate anions are preferred. Tetramethylammonium chloride is especially preferred as an ion-conductivity agent in the electrolyte. (TMAC1).

The examples below illustrate the present invention. Those skilled in the art of organic electrochemistry may conceive of other illustrative alternative modes for practicing this invention which would still be within the scope of this disclosure.

EXAMPLE 1

This example illustrates the preparation of triethyl phosphate.

A cylindrical, jacketed electrolysis cell was constructed of glass 8 cm in diameter and 20 cm high. The jacket had an inlet and outlet for the circulating water used to control the temperature. Approximately 15 cm from the bottom was a fitting for a reflux condenser. A hemispherical, glass top was mounted on the electrolysis cell by means of a ground-glass joint fitted with a number 50, rubber O-ring, through which copper wires connecting to the electrodes were run. Also the top bore an inlet for nitrogen gas. Metal clips held the electrodes suspended from the copper wire leads. A magnetic stirring bar was placed in the bottom of the cell which normally held 350 ml of electrolyte. A direct current rectifier, (Model B-1000, R. O. Hull, Cleveland, Ohio) was the source of power.

The electrolyte was 14.1 g. tetramethylammonium chloride in 310 ml anhydrous ethanol. After purging with nitrogen the electrolysis was carried out at 28.degree. C. for 73 hours with 10 v. D.C. applied, a current varying between 0.1 to 0.38, a, a graphite rod cathode 10 cm long and 0.6 cm in diameter, and a ferrophosphorus (MP) anode, weighing 21.7 g. During the electrolysis the anode lost 9.7 g. weight.

After the electrolysis, a sample of the amber reaction liquor subjected to mass spectrographic analysis showed the presence of both triethyl phosphate and diethylmethyl phosphate. Upon vacuum distillation 29.4 percent of triethyl phosphate was isolated based on the loss of weight of the anode.

EXAMPLE 2

This example illustrates the preparation of tripropyl phosphate directly from ferrophosphorus.

The same equipment and procedure as in Example 1 was used. The electrolyte was 10.9 g. tetramethylammonium chloride in 310 ml n-propanol. After purging with nitrogen the electrolysis was run at 70.degree. C. for 117 hours with 18.5 v. applied voltage, a current of 0.21 a, and a ferrophosphorus (MP) anode, which lost 11.3 g. during the reaction.

After the electrolysis the reaction liquor was stripped of solvent by a rotary vacuum distillation, leaving a tarry residue which was no longer examined. The liquid residue was subjected to a vacuum distillation, leaving a tarry residue which was no longer examined. The liquid residue was subjected to another vacuum distillation. The distillate showed a strong P.dbd.O band in infrared spectroscopy, a .sup.31 P nuclear magnetic resonance spectrum similar to that of tripropyl phosphate, and a gas chromatographic spectrum identifying the product as tripropyl phosphate. Disregarding the tarry fraction, the liquid fraction showed 16 percent yield based on weight loss of the anode and 18 percent yield based on current efficiency.

EXAMPLE 3

This example illustrates the preparation of triisopropyl phosphate.

Using the same equipment, electrodes, and procedure as in Examples 1 and 2 an electrolysis was carried out at 40.degree. C. with 11 g. of tetramethylammonium chloride in 310 ml of isopropyl alcohol for 99 hours with 20 v. applied and a current of 0.15 a. The ferrophosphorus anode lost 6.8 g. weight during the electrolysis.

After filtration, stripping the solvent, and vacuum distillation, the product was subjected to infrared spectroscopy, gas chromatography and .sup.31 P nmr. The predominant product was triisopropyl phosphate with an indication of some phosphonate.

EXAMPLE 4

This example illustrates the use of a bromide as electrolyte.

Following the procedure of the previous examples in the same equipment, two electrolyses were carried out using 7.1 g. sodium bromide in 310 ml anhydrous ethanol as the electrolyte. In each case in addition to triethyl phosphate a significant proportion of triethyl phosphate was produced. Also in both cases the length of the electrolysis was longer than the previous examples. The parameters of these two runs are given below:

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    Time    Volt-  Current  Temp.  %       %
    (hrs)   age    (a)      .degree.C.
                                   Phosphate
                                           Phosphite
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    4A   237    11     0.12   32     75      25
    4B   194    18.5   0.27   45     85      15
    ______________________________________

    ______________________________________
                         Cur-
                  Volt-  rent Temp. Time
    Run  Anode    age    (a)  .degree.C.
                                    (hrs)
                                         Remarks
    ______________________________________
    5A   slug     10     0.21 am-   17   Mass spec. and .sup.31 P
                              bient      nmr show organo-
                                         phosphate
    5B   slug     11     0.16 am-   37   IR shows organo-
                              bient      phosphate
    5C   slug     10     0.26 35.degree.
                                    80   triethyl phosphate
                                         (TEP) and diethyl-
                                         methyl phosphate by
                                         mass spectroscopy
    5D   ore(MP)  10     0.29 37.degree.
                                    204  30% TEP at 44%
                                         current efficiency
    5F   ore(MP)  18     0.33 40.degree.
                                    83   53% TEP at 55%
                                         current efficiency
    5P   ore(SB)  9-25   0.04-
                              40.degree.
                                    80   TEP containing a
                         0.3             little triethyl
                                         vanadate
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    ______________________________________
                                     Cur-
                                Volt-
                                     rent Time Temp.
    Run  Electrolyte  Anode     age  (a)  (hrs)
                                               .degree.C.
    ______________________________________
    6J   phenol, TMACl
                      ore(MP)   20   0.02-
                                          3    52.degree.
                                     0.17
    6L   ethylene glycol,
                      slug      16   0.01 102  am-
         TMACl                                 bient
    6N   isopropyl-   ore(PMP)  24   0.02 195  45.degree.
         phenol, TMACl
    6Q   neopentyl alc.
                      ore(MP)   24   0.01-
                                          470  65.degree.
         tetraethyl-                 0.04
         ammonium p-
         toluene
         sulfonate
    6S   ethanol, TMACl
                      ore(SB)   13   0.04-
                                          184  am-
                                     0.3       bient
    6T   ethanol, sodium
                      ore(TVA)  10   0.04-
                                          5    30.degree.
         cyclopentadene-             0.2
         ide in THF
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