HYDROXYMETHYL PHOSPHORUS DERIVATIVES FROM TETRAKIS(HYDR0XYMETHYL) PHOSPHONIUM CHLORIDE. REACTION WITH LEAD CARBONATE AND OXIDE

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1 HYDROXYMETHYL PHOSPHORUS DERIVATIVES FROM TETRAKIS(HYDR0XYMETHYL) PHOSPHONIUM CHLORIDE. REACTION WITH LEAD CARBONATE AND OXIDE N. FILIPESCU, L. M. KINDLEY, H. E. PODALL, AND F. A. SERAFIN Melpar, Incorporated, Research Division, Falls Church, Virginia, U.S.A. Received November 13, 1962 ABSTRACT A novel and convenient method has been developed for the preparation of tris(hydroxymethyl) phosphine oxide (THPO) from tetrakis(hydroxymethy1) phosphonium chloride (THPC). The method consists essentially of oxidative deformylation of THPC with lead carbonate in n-propyl alcohol. The reaction is facilitated by azeotropic distillation of di-npropylformal and n-propyl alcohol. The reaction of THPC with lead carbonate as well as with lead oxide has been studied in detail in aqueous solution and in n-propyl alcohol. The reaction proceeds via the initial formation of tetrakis(hydroxymethy1) phosphonium hydroxide (THPOH) followed by deformylation to tris(hydroxymethy1) phosphine (THP). THP then undergoes oxidation by air as well as by lead carbonate or oxide to THPO. If air is bubbled through the mixture at elevated temperatures, atmospheric oxidation predominates with little or no oxidation by the lead compound. Depending upon the reaction conditions, a phosphonium hydroxide, a phosphine, or a phosphine oxide are products of the reaction of tetrakis(hydoxymethy1) phosphonium chloride with lead carbonate or oxide. For several years this laboratory has been engaged in the synthesis of phosphoruscontaining polymers in order to provide flame resistant and thermally stable end products. One of the starting materials for the introduction of phosphorus into the polymeric molecules was tetrakis(hydroxymethy1) phosphonium chloride, referred to as THPC. Although several polymers derived from THPC have been reported (5, 6), we have found that in many instances phosphorus was not incorporated into the polymeric chains which are primarily formed by polycondensation reaetions of formaldehyde resulting from the degradation of THPC. These polymers, insufficiently characterized, possessed poor hydrolytic stability and fracture strength. By use of tris(hydroxymethy1) phosphine oxide (THPO) and its monobenzoylated derivative we were able to obtain polymers containing phosphorus in the polymeric chain which possessed markedly improved hydrolytic stability, fracture strength, flame resistance, etc. (3). A key problem which arose in the initial stages of work with THPO-derived polymers was the conversion of commercially available THPC to THPO. Methods previously described for this conversion required a number of tedious operations to isolate the product. In attempting to devise a better method for the synthesis of THPO from THPC some interesting chemistry was uncovered involving not only the preparation of THPO but also the formation of tetrakis(hydroxymethy1) phosphonium hydroxide (THPOH) and tris(hydroxymethy1) phosphine (THP). This paper is concerned with a new synthesis of THPO from THPC and with the properties and reactions of some of the intermediates. THPC was reported (1,2) to react with aqueous sodium hydroxide at room temperature or upon gentle heating below SO0 C to yield THPO with the evolution of one molecule of hydrogen. On boiling, a second molecule of hydrogen is given off with the formation of the sodium salt of bis(hydroxymethy1) phosphinic acid. The same results are obtained if one uses an alkali metal carbonate instead of the hydroxide, but the reaction rate is I Canadian Journal of Chemistry. Volume 41 (1963) 82 1

2 822 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963 slower. An insoluble carbonate, such as calcium or barium carbonate, produces only THPO and one molecule of hydrogen even after prolonged boiling. Each of these procedures yields THPO in an aqueous solution mixed with soluble inorganic chloride (sodium, barium, or calcium). Separation of THPO from the inorganic chloride is difficult because of the former's extreme solubility in water and low solubility in organic solvents. Isolation by distillation is not possible because THPO decomposes at temperatures far below its boiling point. The most successful isolation of THPO to date involves long, tedious extractions of the dry mixture of THPO and inorganic salts with relatively low molecular weight alcohols, such as isopropyl alcohol. In our laboratory, attention was directed toward an approach which had appeal as a simple, economical reaction for converting THPC to THPO and at the same time avoiding the isolation problems encountered heretofore. The proposed method was: air 2[(HOCHn)4P]fC1- + PbC t B(HOCH,),PO + PbCZn f 2CHz0 + H20 + COe. Experimental investigation, however, proved that the reaction of THPC with lead carbonate was considerably more complex than originally believed, and that several different organophosphorus compounds can be formed depending on the reaction conditions. The conditions under which the reaction of THPC with lead carbonate was investigated may be summarized as follows: (I) in water at 55-65' C, (2) in water at temperatures from 65 to 90' C, (3) in n-propyl alcohol at 60-65' C, (4) in n-propyl alcohol at approximately 96' C. Under each of these conditions the reaction time was varied over wide ranges (1 hour to several days). In the reactions conducted in water, it was observed that metallic lead precipitated if the reaction temperature exceeded about 65' C. For this reason, a series of experiments was carried out below 65' C and another series above this temperature. In the "low" temperature experiments THPC and lead carbonate were heated in water at 55-65' C, and the solids were removed by filtration. After removal of water from the filtrate there remained a syrupy, liquid product. Removal of final traces of volatile materials, particularly water, was attempted by long periods of stirring under vacuum, azeotropic distillation with benzene, and repeated extractions with acetone. Infrared spectra of this product (still a syrupy liquid) indicated very little P -+ 0 absorption and thus very little formation of THPO. Furthermore, several qualitative reactions of the product were quite different from those of THPO. For example, the product reacted violently with oxidizing agents such as Hz02 and HN03 (indicating the presence of a strong reducing compound), it exhibited a highly exothermic reaction with a,p-unsaturated carbonyl compounds, and cured unsaturated polyester resins to elastomeric materials. Furthermore it possessed a strong odor suggestive of a phosphine. Based on the above results and evidence obtained in later experiments it appears that the main reaction occurring between THPC and lead carbonate in water at 55-65' is: PI (THPOH) The THPOH (tetrakis(hydroxymethy1) phosphonium hydroxide) is believed to exist in equilibrium with tris(hydroxymethy1) phosphine (THP), i.e., [(HOCH2),PlfOH- G (H0CHn)aP + CH2O + HzO. (31 (THP)

3 FILIPESCU ET AL.: HYDROXYMETHYL PHOSPHORUS DERIVATIVES 823 It was found that not all of the ionic chloride was completely removed as PbC12. The main reason for this appears to be due to the fact that hydroxymethyl-phosphorus compounds, such as THPC, THPOH, etc., were found to form soluble complexes with PbCla as well as with AgCl. We believe that these complexes are formed by coordination of the metal ion with the hydroxyl functions of the phosphorus compound, e.g. C1 H H + /CH2-0: PbCle + THPOH --t > pb$~:> P \CH,-O: OH-. C1 H H Not all of the complexes were water or alcohol soluble. The solids removed from the reaction by filtration even after repeated washing with hot water contained significant amounts of phosphorus compounds strongly adsorbed or complexed with the lead carbonate or lead chloride. These solids were similar in appearance to litharge-glycerine cements, suggestive of some polymerization. In the second series of experiments, THPC and lead carbonate were heated in water at 65-90' C for various lengths of time. In addition to the reactions which occur at lower temperature, a redox reaction was found to occur with the formation of metallic lead. The reaction appears to be as follows: The evidence for this reaction is based upon the following facts: (1) isolation and identification of THPO, (2) no evidence of hydrogen formation, (3) PbO gave similar results, and (4) similar results were obtained in n-proh instead of water at elevated temperatures. It was also found that when air was passed through the mixture while it was being heated, less metallic lead was produced, and the conversion of THP to THPO appeared to occur more rapidly. The predominant reaction under these conditions appears therefore to be simply the direct atmospheric oxidation of THP to THPO. The product recovered after filtration of the solids from the reaction mixture and exhaustive removal of water and other volatile materials was a partially crystallized syrupy mixture. Attempts to separate this mixture into pure individual compounds by a variety of techniques were not successful. It was, however, possible to separate two major fractions-one containing a large percentage of THPO and another consisting mostly of THP plus some THPOH. By thoroughly drying the reaction mixture and cooling it for a long period of time, it was possible to obtain a distinct crystalline solid which could be separated from the syrupy liquid. This solid reacted with benzoyl chloride in a Schotten-Bauman reaction to give the known pure tribenzoate (5) of THPO in good yield. The presence of THP in the syrupy liquid phase was indicated by formation of the known yellow benzoquinone-thp complex (4), its "characteristic" exothermic reactions with oxidants such as Hz02 and HN08, and finally its reactions with unsaturated carbonyl compounds such as diethyl maleate, acrolein, acrylamide, etc. (The latter reactions will be the subject of another communication.) Vacuum distillation (5 mm) of the product resulting from the reaction of THPC and lead carbonate at 65-90' C yielded a viscous colorless liquid distilling at ' C. The ph of this distillate was 4 and elementary analysis indicated its composition to be as follows: THP, 51.8; CH20, 31.5; water, 9.4; HCl, 7.6%. The distillate gave reactions with HzOz, HNOp, and unsaturated carbonyl compounds previously described as those of THP.

4 824 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963 The residue from the distillation failed to react with any of the above-mentioned reagents. It had a ph of 6 and elementary analysis which corresponded to the following composition: THPO, 53.0; THPC, 18.0; CH20, 25.6; water, 3.4%. Further verification of the composition of the distilled and residue fractions was obtained by measuring the amount of hydrogen evolved upon boiling a sample of each fraction with an excess of aqueous sodium hydroxide. The reactions of sodium hydroxide with the compounds of interest are as follows: (HOCHZ)~PO + NaOH -, (HOCHZ)ZPO(ON~) + CHzO + Hz 161 (HOCH2)3P + NaOH + Hz0 -+ (HOCH2)2PO(ONa) + CH20 + 2Hz [7] [(HOCHz)~P]+Cl- + 2NaOH -+ (HOCHZ)ZPO(ON~) + 2CHz0 + NaCl + 2H2. PI The experimental results were as follows (based on 100 g of sample). Distillate: of Hz; calc. for indicated composition (51.8% THP), Residue: of HP; calc. for 53.0y0 THPO and 18.0yo THPC, In the third series of experiments, water was present only in traces in the reaction of THPC with lead carbonate. The THPC and lead carbonate was suspended in n-propyl alcohol and the mixture was heated at 60-65' C for 6 to 8 hours. After filtration and removal of alcohol, a syrupy liquid was obtained. This product contained practically no ionic chloride and reacted exothermally with oxidizing agents and with a,p-unsaturated carbony1 compounds. Comparison of this product with samples prepared by an alternate route* strongly indicated that it was essentially THPOH containing some THP. The reaction scheme may be described as follows: n-proh 2[(HOCH2)rP]+C1- + PbCO3 + Hz0-4 2[(HOCH2)aP]+OH- + PbClz + COz [9I THPOH E THP + CHzO + HzO. [lo] If an excess of lead carbonate is used in the above reaction and the intermediate product is not isolated, further reaction occurs when the reaction temperature is raised to 96' C. The reactions in this step appear to be as follows: [(HOCHz)rP]+OH-S (H0CHz)aP + CHzO + Hz0 CHzO $. n-proh 4-(n-Pr0)zCHz + Hz0 THP + PbC08 4 THPO + Pb + COz. During reaction, metallic lead precipitated and the dipropyl formal - propyl alcohol azeotrope was removed at 96' C. Removal of formaldehyde as an azeotrope evidently facilitates the formation of THP, which is immediately oxidized to THPO. The final product (THPO) was recovered as a white solid after filtration of metallic lead and removal of volatile components by vacuum distillation at about 50' C. Its identity was confirmed by formation of the tribenzoate derivative and by comparison of its infrared spectrum with an authentic sample of THPO. In a typical reaction 1 mole of THPC and 1.8 mole of PbC03 in 1500 ml n-propyl alcohol were stirred with heating. During the reaction (8-10 hours) the distillate was continuously removed and fresh n-propyl alcohol added from time to time. The reaction mixture was cooled in an ice bath and filtered. The filtrate was dried in vacuo to obtain crystalline THPO. The yields were in excess of 90%. *THPOH was prepared by reaction of THPC with methanolic sodzum hydroxide at room temperature, filtration of NaC1, and vacuum removal of solvent. THP was prepared by passing phosphine through an alcoholic solution of THPOH in the presence of a platinum oxide catalyst. The THP was recovered as a white waxy solzd after solvent removal in vacuo and identi$ed via its benzoquinone and mercuric chloride complexes.

5 FILIPESCU ET AL.: HYDROXYMETHYL PHOSPHORUS DERIVATIVES 825 A possible explanation for the difficulties encountered in separating THPO from THP and/or THPOH may be due to the high association and possible complex formation between these compounds, e.g., 0 t THP + THPO -+ [THPH]+(HOCH~)ZP-CH20- [I41 THPOH + THPO -+ [(HOCH~)~~CHZOH...O-P(CHeOH)3]0H-. [I51 In summary it has been found that: (1) THPO can be conveniently prepared by reaction of THPC with PbC03 in n-proh at 96" while azeotropically distilling di-n-propyl formal and n-propyl alcohol; (2) the conversion of THPC to THPO in the presence of PbCOs proceeds via THPOH and THP, respectively; (3) PbCO3 and PbO each oxidize THP to THPO; (4) PbClz and AgCl form two types of complexes with THPO and related compounds, viz. soluble and insoluble products. REFERENCES 1. A. HOFFMAN. J. Am. Chem. Soc. 43, 1684 (1921). 2. A. HOFFMAN. J. Am. Chem. Soc. 52, 2995 (1930). 3. L. M. KINDLEY, H. E. PODALL, and N. FILIPESCU. SPE Trans. 2, 122 (1962). 4. G. M. KOSOLAPOFF. Organophosphorus compounds. J. Wiley and Sons, Inc., New York W. A. REEVES and J. D. GUTHRIE. U.S.Dep. Agr., Agr. Chem. Cir. Rev. 364 (1954). 6. R. A. WILSON and J. D. GUTHRIE. Incl. Eng. Chem. 48,64 (1956).

Properties of Compounds

Chapter 6. Properties of Compounds Comparing properties of elements and compounds Compounds are formed when elements combine together in fixed proportions. The compound formed will often have properties