US2426691A - Process for making neutral trisubstituted phosphates - Google Patents

Description

Patented Sept. 2, 1947 PROCESS FOR, MAKING NEUTRAL 'mr- SUBSTITUTED PHOSPHATES Russell L. Jenkins, Anniston, Ala., assignor to Monsanto Chemical Company, St. Louis, Mo., a.

corporation of Delaware No Drawing. Application October 30, 1944, Serial No. 561,162

The present invention relates to simple and mixed neutral phosphoric acid esters and to a novel process for producing same.

One object of the invention is to provide an improved process whereby phosphoric acid esters of hydroxy hydrocarbon compounds and derivatives thereof may be produced in a state of high purity and in an economical and efficient manner.

Another object is to provide a process for making trimethyl phosphate and other neutral watersoluble phosphoric acid esters wherein each step is carried out in the substantial absence of water, thereby avoiding the product recovery problems inherent in the conventional phosphorus oxychloride process and thus making possible the production and recovery of the ester products in substantially quantitative yields.

An additional object is to provide a process for making mixed neutral esters of phosphoric acid in a substantially pure form and in high yields, containing a minimum of undesirable by-products such as mixtures of simple and/or mixed neutral esters.

Other objects and advantages will be apparent to those skilled in the art as the description of the invention proceeds.

The process generally employed in the production of simple and mixed neutral esters of phosphoric acid consists essentially in reacting phosphorus oxychloride with hydroxy hydrocarbon compounds or mixtures thereof, quenching, and

'neutralizing the reaction mixture with sodium carbonate, and then recovering the esters from the neutralized product. It will be evident, however, from the considerations which immediately follow that this process is not suitable for making soluble esters of phosphoric acid which hydrolyze readly in the presence of water.

For example, according to conversion calculations a high yield of trimethyl phosphate is obtainable by the above procedure, but the separation of this compound from the reaction mixture presents a serious problem since it is so soluble in water. Attempts have been made to recover this compound from the reaction mixture by extraction with various solvents, but, so far as applicant is aware, they have been unsuccessful. Steam distillation has also been resorted to, but this results in excessive hydrolysis of the ester and requires the handling of such large volumes of liquid that this method of operation is impractical. Etl'orts have also been made to solve this problem by neutralizing the reaction mixture in 13 Claims. (Cl. 260-461) 2 the absence of water, but it was found that this reaction is so slow as to be of no practical value.

As applied to the production of mixed neutral esters, the above method is subject to the disadvantage that some hydroxy hydrocarbon compounds are more reactive than others and consequently the reaction product contains a mixture of simple esters and/or neutral mixed esters. This makes it necessary to separate the esters not only from the reaction mixture but also from each other with the result that the cost of operating the process is materially increased and the yield of the desired ester is substantially decreased.

As a modification of the foregoing process it has also been proposed to manufacture phosphoric acid esters by reacting phosphorus oxychloride with metal derivatives of hydroxy hydrocarbon compounds such as sodium methylate, sodium ethylate, etc., but this procedure'has the added disadvantage of producing three moles of a metallic chloride for each mole of phosphorus oxychloride, thus requiring the separation of excessive amounts of this impurity from the product.

Now I have developed an economical and commercially feasible process for producing phos phoric acid esters of the above type which has none of the objectionable features enumerated above. This process involves reacting phosphorus trichloride with a hydroxy hydrocarbon compound to form a disubstituted hydrogen phosphite, chlorinating the crude reaction mixture thus obtained to produce the corresponding disubstituted chlorophosphate and then reacting the latter with an esterifying compound to form a simple or mixed neutral ester depending upon the hydrocarbon compound and the esterifying compound employed. The reactions which take place in this process are represented by the following equations:

(l) ROH PClz (RO)2POH RC1 21101- (2) (ROhPOH Ch (RO)zPOCl HCl v RQ (3) (ROMPOCI (RO)yX -v RO 7P=O XCI where R is an aliphatic or an alicyclic radical, R is a hydrocarbon radical, X is a hydrogen atom or a metal selected from the group consisting of alkali metals, calcium, barium, strontium, magnesium and aluminum, and Y is a positive number having a value corresponding to the valence of X.

For a more detaileddescription of the above process for producing simple and mixed neutral,

esters of phosphoric acid, reference is made to the following examples which are illustrative of the preferred embodiments of the present invention.

Exsmrtr: I

500 c. c. of anhydrous benzene and 372 c. c. (9.2 moles) of anhydrous methyl alcohol were introduced into a three neck flask which was equipped with a stirrer, a fritted glass gas disperser, a dropping funnel and a downward condenser. The solution thus obtained was cooled in an ice-salt bath to C. and then. the system was subjected to a pressure of about 700 millimeters of mercury. While the system was under this pressure, 262 c. c. (3 moles) of phosphorus trichloride was added slowly with vigorous stirring in order to maintain the temperature of the reaction below 5 C., the time of addition being approximately 1 hour.

As soon as the reaction had been completed, the system was subjected to a pressure of from 50 to 200 millimeters of mercury, whereupon anhydrous chlorine was gradually bubbled into the mixture for one hour and a quarter at such a rate that the reaction temperature was maintained below 5 C. The end point of the chlorination was indicated by a change in the color of the solution to a permanent green and also by a rapid drop in temperature. At this point the chlorination was stopped, 200 c. c. of benzene was added to the mixture and, while the system was subjected to a pressure of from 50 to 200 millimeters of mercury to remove the residual hydrogen chloride, the temperature was allowed to rise slowly over a period of two hours. At the end of the first hour heat was applied to distill the benzene, but the temperature was not permitted to exceed C. At the end of this period most of the benzenehad been distilled and the byproduct HCl removed, leaving crude dimethyl chlorophosphate as a residue.

The crude chlorophosphate product thereby obtained was added with stirring to a sodium methylate solution containing phenolphthalein, which had been previously cooled on an ice bath. During this operation the reaction temperature was maintained at from 10 to 15 C. The addition of chlorophosphate was continued until the indicator changed color showing that the mixture was on the acid side. The sodium chloride which precipitated as a result of this reaction was filtered off, washed with dry benzene and the filtrate and washings distilled at a temperature of 55 to 60 C. and a pressure of 3 to 5 millimeters of mercury. The distillate obtained was refractionated to purify the product at a temperature of 67-68 C. and under a pressure or 6 to 7 mm. of mercury. 388 grams of trimethyl phosphate was obtained, representing a yield of 80% of theory, basis PCla.

EXAMPLE II In a three neck flask of suitable size fitted with stirrer, fritted glass gas bubbler, dropping funnel and downward condenser, 1818 c. c. of anhydrous benzene and 1272 c. c. (31.4 moles) of methyl alcohol were mixed and cooled in an ice-salt bath to 0 C. The system was thereupon subjected to a pressure of about 700 millimeters of mercury and 909 c. c. (10.7 moles) of phosphorus trichloride was added for a period of three hours, with vigorous stirring. through the dropping funnel at the fastest rate that would allow the temperature to be maintained below 5 C. At the end of the reaction, a vacuum corresponding to an absolute pressure of 50 to 200 millimeters of mercury was applied and then chlorine, which had been dried by means of sulfuric acid, was bubbled into the mixture for a period of four and one quarter hours, the rate of the chlorine addition being so controlled as to maintain the reaction temperature below 5 C. The end point of the chlorination was indicated by the solution tuming a permanent greenish color together with a rapid drop in temperature as the reaction ceased. At this point the chlorination was discontinued.

The temperature of the chlorinated product was then maintained between 0 and 5 C. for about 3 hours under an absolute pressure of 50 to 200 millimeters of mercury. This was followed by. a 3 hour evacuation at a still lower pressure by means of a Kinney large capacity pump, during which period the temperature of the reaction was slowly raised to a maximum of 30 C. This latter operation effected a distillation of most of the benzene which carried with it all or substantially all of the residual hydrogen chloride and left behind a residue of crude dimethyl chloro phosphate.

This crude product was added with vigorous stirring to a previously prepared cold sodium methylate solution containing a small quantity of phenolphthalein indicator. The temperature of the reaction was maintained at 15 to 20 C. and the addition of the crude chloroph'osphate was continued until the indicator changed color showing that the mixture was on the acid side. The precipitated sodium chloride was filtered off, washed with dry benzene and the filtrate and washings distilled and refractionated as in Example I. A distillate was collected consisting of 2.75 lbs. of trimethyl phosphate, a y eld of 83% of theory, basis PO13.

The trimethyl phosphate obtained in Examples I and IIpossessed the following physical propert es:

Specific gravity 1213-1214 at 25/15.5 C. Boiling point 189 C. at 760 mm. Vapor pressure 0.52 mm. at 22 C.

- Pour points Liquid at -70 C.

Melting point Liquid at 70 C. Solubility in water Infinitely In view of the fact that trimethyl phosphate i extremely soluble in water and is also readily hydrolyzed thereby, the reaction is preferably carried out in the substantial absence of water, otherwise the product yield will be materially reduced. This precaution is also important in the production of other water-soluble phosphoric acid esters.

The various steps of the present process will now beconsidered in detail.

Step I In the first step or my process phosphorus trichloride is reacted with an aliphatic alcohol in the presence of benzene, carbon tetrachloride or other suitable inert solvent to form the corresponding dialkyl hydrogen phosphite. The optimum temperature for this reaction varies with the number of carbon atoms contained in the alkyl chain of the alcohol. For the dimethyl compound the reaction temperature should fall substantially in the range of from -15 C. to 5 C. but higher or lower temperatures may also be employed if desired and when the reaction is executed at a temperature below 0 C., the addition of an inert organic solvent may be omitted.

With regard to the quantities of reagents employed in the initial step of my process, an excess of the theoretical amount of alcohol required to form the desired phosphate should be used. Generally from 1 to 10% excess is satisfactory, but more or less alcohol may be employed so long as the theoretical requirements are met.

Step II In the production Of dialkyl chlorophosphates in accordance with the second step of my process, the dialkyl hydrogen phosphite is chlorinated in the presence of an inert organic solvent of the above type to produce a crude mixture containing the corresponding dialkyl chlorophosphate. In this reaction the chlorination temperature varies with the dialkyl hydrogen phosphite being treated and also with the amount and type of solvent used. In chlorinating dimethyl hydrogen phosphite the reaction should take place at a temperature substantially within the range of -5 to 8 C. and preferably at a. temperature below 5 C.

The use of reduced pressure in the chlorination step is desirable as it aids in controlling the temperature of the reaction and at the same time facilitates the removal of undesirable vapors, but

this method of operation is not essential as very high yields of dialkyl chlorphosphates are obtainable at atmospheric pressure if efliicent cooling means are provided.

Step III The removal of hydrogen chloride in the third step of my process is desirably carried out by adding benzene, carbon tetrachloride or other suitable inert solvent which decreases the solubility of the HCl in the mixture, and then sweeping out the acid by solvent vapors produced by distillation, preferably by vacuum distillation. The sweeping operation is initiated at a relatively low temperature which i gradually increased until all or g has been removed, care being taken to avoid raising the temperature to a point where substantial decomposition of the chlorophosphate takes place.

For example, when removing hydrogen chloride from mixtures containing dimethyl chlorophosphate, the sweeping operation preferably begins at a temperature of about 0 C. and ends at a temperature of 30 C. However, satisfactory results have been obtained at temperatures within the range of --15 C. and 50 C.

It should be understood that the starting temperature of the sweeping operation will vary with the amount of hydrogen chloride present in the crude chlorophosphate mixture and the chlorophosphate mixture being sweetened. If substantial amounts of hydrogen chloride are present, then astarting temperature of -15 C. or a lower temperature should be employed, but if only relatively small amounts of hydrogen chloride are present, then the sweetening step may be initiated at somewhat high temperatures.

Step IV In preparing trimethyl phosphate from dimethyl chlorophosphate in accordance with the fourth step of my process, the reaction is carried out at a temperature substantially in the range of from 10 to 20 C. and preferably in the rangeof flom to C. since in the latter range the sodiumv chloride precipitates in a form in which it may be easily filtered. It is also desirable, although not absolutely essential, to add the dimethyl chlorophosphate to the soduim methylate rather than the converse of this operation as I thereby obtain a material increase in the yield of the product.

Instead of employing sodium methylate, the

substantially all of the acid I reaction may be carried out using methyl alcohol, alone or in combination with ammonia or a nitrogenous base. When this is done, the reaction should likewise be eflected at a tempera-'- ture within the above range, but satisfactory results are also obtainable at somewhat higher or lower temperatures.

As an alternative to the foregoing procedures, the dimethyl chlorophosphate may be reacted with substantially one half of a mole of calcium barium, strontium or magnesium methylate or about one third of a mole of aluminum methylate.

When producing phosphoric acid esters from dialkyl or di-alicyclic chlorophosphates and other hydroxy hydrocarbon compounds or the metal derivatives thereof, the reaction conditions will vary somewhat with the compounds employed and in view of the very large number of esters obtainable by the present process, no attempt has been made to set forth the conditions required for the production of each ester.

Step V In the fifth and final step of my process for producing trimethyl phosphate, the sodium chloride is filtered oil and the filter cake is washed with methanol or benzene. Th'e filtrate and washings are combined and vacuum distilled to remove the solvents and the residue is distilled at to C. under a vacuum corresponding to 3 to 5 millimeters of mercury. The distillate thus obtained is refractionated at a pressure of 6 to '7 millimeters of mercury and at a temperature of from 67' to 68 C. to yield asubstantiaily pure product.

If methyl alcohol is employed in the fourth step of my process, then the reaction product is vacuum distilled in the presence or absence of benzene or other high boiling solvent to remove the byproducts, meth'yl chloride and hydrogen chloride, after which the residue is distilled and refractionated to recover trimethyl phosphate in the manner described in Examples I, and II.

However, when dialkyl or di-alicyclic chlorophosphates are reacted with hydroxy hydrocarbon compounds to form water insoluble esters, the hydrogen chloride contained in the neutral ester mixture is preferably not removed by distillation; it is extracted by means of a solution of sodium hydroxide or sodium carbonate and after separating the aqueous layer, the esters are recovered from the non-aqueous layer-by distillation.

If methyl alcohol and ammonia or a nitrogenous base of the type indicated above are employed, then in this case it is also unnecessary to remove the hydrogen chloride from the neutral ester mixture by distillation as it is absorbed by these nitrogen compounds to produce the corresponding salts which are then separated from the 'product by filtration. The filtrate thus obtained is distilled and retractionated to recover trimethyl phosphate in a substantially pure form. The same procedure is employed when other hydroxy hydrocarbon compounds are used in combination with ammonia or nitrogenous bases in the production of other phosphoric acid esters.

When calcium methylate, magnesium methylate, barium methylate, strontium methylate and aluminum methylate are reacted with dimethyl chlorophosphates or other dialkyl (or di-alicyclic) chlorophosphates, the procedure for recovering the esters is identical with the sodium methylate process except that instead of sodium chloride, calcuim, barium, strontium, mag- 1. Simple and mixed neutral phosphoric acid esters of aliphatic alcohols containing up to 20 or more carbon atoms.

2. Simple and mixed neutral phosphoric acid esters of cycloaliphatic alcohols.

3. Mixed dialkyl-alicyclic esters of phosphoric acid. a

4. Mixed di-alicyclic-alkyl esters' of phosphoric acid.

5. Mixed dialkyl-aryl esters of phosphoric acid.

6. Mixed di-alicyclic-aryl esters of phosphoric acid.

The compounds produced in accordance with this invention have the following general formula:

wherein R is an aJwl radical containing up to 20 or more carbon atoms or an alicyclic radical 2. The process defined in claim 1 wherein the methylate employed is sodium methylate.

3. The process defined in claim 1 wherein the methylate employed is an alkaline earth methylate.

4. The process defined in claim 1 wherein the methylate employed is aluminum methylate.

5. The process for manufacturing trimethyl phosphate, which comprises reacting about 3 molecular proportions of methyl alcohol with about 1 molecular proportion of phosphorus trichloride to form a crude product principally containing dimethyl hydrogen phosphite, chlorinating said product at a temperature of from about -5 C. to about 8 C. to produce dimethyl chlorophosphate and thereupon reacting said chlorophosphate with sodium methylate in the substantial absence of water to form trimethyl phosphate, said methyl alcohol and said phosphorus trichloride being the sole reactants in said initial step of the process.

6. The process defined in claim 5 wherein the reaction betweenmethyl alcohol and phosphorus trichloride is carried out at a temperature of from 15 C. to 5' C.

7. The process defined in claim 5 wherein the reaction between methyl alcohol and phosphorus trichloride is carried out at a temperature below 0 C.

8. The process defined in claim 5 wherein the dimethyl hydrogen phosphite is chlorinated at a and R is an aryl radical. a cycloaliphatic radi-.

cal or an alkyl radical containing up to 20 or more carbon atoms.

These compounds are useful as addition agents for lubricating oils, lacquers, varnishes, etc., and also as plasticizers for cellulose esters, cellulose ethers and synthetic resins.

Reference is hereby made to application, S. N. 520,100, filedin the name of Edgar E. Hardy and Gennady M. Kosolapofl, for a more detailed description of the method for producing the dialkyl cholorophosphates used in the production of neutral phosphoric acid esters in accordance with this invention.

Where the expression hydroxy hydrocarbon compounds is used in the specification and claims,-it is to be understood that both the substituted and unsubstituted hydroxy hydrocarbon compounds and the metal derivatives thereof are contemplated.

As many widely different embodiments of this invention-may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the following claims.

What I claim is:

1. The process for manufacturing trimethyl phosphate, which comprises reacting at a temperature of about 15 C. to about 5 C. substantially 3 molecular proportions of methyl alcohol with about 1 molecular proportion of phosphorus trichloride to form a crude product principally containing dimethyl hydrogen phosphite, chlorinating said product to produce dimethyl chlorophosphate and thereupon reacting said chlorophosphate with a compound selected from the group consisting of methyl alcohol and methylates in the substantial absence of Water to form trimethyl phosphate, said methyl alcohol and said phosphorus trichloride being the sole reactants in said initial step of the process. 1

temperature below 5 C.

9. The process defined in claim 5 wherein the dimethyl hydrogen phosphite is chlorinated under reduced pressure and at 'a temperature below 10. The process defined in claim 5 wherein the reaction between dimethyl chlorophosphate and sodium methylate is carried out at a temperature of from 10 C. to 20 C.

11. The process for manufacturing trimethyl phosphate, which comprises reacting about 3 molecular proportions of methyl alcohol with about 1 molecular proportion of phosphorus trichloride in the presence of an inert organic solvent and at a temperature of from 15 C. to 5 C. to form a crude mixture principally containing dimethyl hydrogen phosphite, chlorinating said crude mixture at a temperature of from -5 C. to 8 C. while under reduced pressure to produce a crude chlorinated product principally containing the corresponding dimethyl chlorophosphate and hydrogen chloride, removing said hydrogen chloride by distilling said crude prod uct under reduced pressure and at a temperature of from -15 C. to 50 C. introducing the resulting crude dimethyl chlorophosphate residue into a sodium methylate solution and thereby causing these compounds to react together in the substantial absence of water to produce a reaction product containing trimethyl phosphate and sodium chloride and then recovering said trimethyl phosphate from said reaction product by distillation, said methyl alcohol and said phosphorus trichloride being the sole reactants in said initial step of the process.

12. The process for manufacturing trimethyl phosphate, which comprises producing acrude mixture principally containing dimethyl hydrogen phosphite by reacting substantially 3 molecular proportions of methyl alcohol with substan- 9 lute pressure on said system to a value of from 50 to 200 millimeters of mercury, then chlorinating said crude mixture at a temperature below 5 C. to form a crude product principally containing dimethyl chlorophosphate and hydrogen chloride, removing said hydrogen chloride by ini- I tially distilling said crude product at a temperature of from C. to C. for about 3 hours while under an absolute pressure of from 50 to 200 millimeters of mercury and then continuing said distillation for an additional 3 hours at a lower absolute pressure while allowing the temperature of the crude product to gradually rise to a maximum of 30 C. introducing the resulting crude dimethyl chlorophosphate residue into a sodium methylate solution and thereby causing these compounds to react together in the substantial absence of water to produce a crude mixture containing trimethyl phosphate and then separating said trimethyl phosphate from said crude mixture by distillation, said methyl alcohol and said phosphorus trichloride being the sole reactants in said initial step of the process.

13. The process for manufacturing trimethyl phosphate, which comprises producing a crude mixture principally containing dimethyl hydrogen phosphite by reacting in the presence of benzene about 3 molecular proportions of methyl a1- cohol with about 1 molecular proportion of phosphorus trichloride at a temperature below 5 C. and while the system is under an absolute pressure 01' about 700 millimeters of mercury, reducing the absolute pressure on said system to a value within the range of from 50 to 200 millimeters of mercury, then chlorinating said crude mixture at a temperature below 5 C. to form a product principally containing dimethyl chlorophosphate and hydrogen chloride, distilling said product at an absolute pressure of from 50 to 200 millimeters of mercury to remove said hydro- ,0

gen chloride while permitting the temperature of said product to rise gradually to a point not exceeding 30 C., introducing the resulting crude dimethyl chlorophosphate residue into a sodium methylate solution and thereby causing these compounds to react together in the substantial absence of water to produce a crude product containing trimethyl phosphate and then recovering said trimethyl phosphate by distilling said crude' product at a temperature of from about C. to C. while under an absolute pressure of from about 3 to 5 milimeters of mercury, said methyl alcohol and said phosphorus trichloride being the sole reactants in said initial step of the process.

RUSSELL L. JENKINS.

' REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,023,758 Raschig Apr. 16, 1912 1,844,408 Nicolai Feb. 9, 1932 1,869,768 Nicolai Aug. 2, 1932 2,005,619 Graves June 18, 1935 2,176,416 Britton Oct. 17, 1939 2,409,039 Hardy et al. Oct, 8, 1946 FOREIGN PATENTS Number Country Date 566,514 Germany Dec. 17, 1932 OTHER REFERENCES Gerrard, Jour. Chem. Soc. (London), 1940, pp. 1464-1469.

Beilstein, "Handbuch der Org. Chem.," vol. I, 4th ed., p. 332.