Process for preparing dialkylphosphinic acids and their salts

Details Process for preparing dialkylphosphinic acids and their salts Process for preparing dialkylphosphinic acids and their salts

2000-05-24

2002-03-19

Vollano, Jean F. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Phosphorus acids or salts thereof

C558S089000, C558S104000

Reexamination Certificate

active

06359171

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for the preparation of dialkylphosphinic acids and salts thereof, and to the use of the dialkylphosphinic acids and salts thereof prepared by this process.
Phosphinic acids and salts thereof can be prepared by various methods and have been described in large number (Houben-Weyl IV, Vol. XII/1, pp. 220 to 240).
The preparation of these compounds is technically complex and is carried out, for example, by oxidation of the corresponding phosphines or via the esters of alkylphosphonous acids, which are themselves prepared from the corresponding phosphonous acid dihalides by reaction with alcohols. The phosphines and phosphonous acid dihalides, for example methyldichlorophosphine, on which the process is based have hitherto been prepared by complex syntheses (Houben-Weyl, Volume 12/1, p. 306). In addition, various by-products are formed which, like some of the abovementioned starting materials too, are toxic, self-igniting and/or corrosive, i.e. are highly undesired.
The organic phosphinic acids and salts thereof based on the abovementioned starting materials, and processes for their preparation and use have in some cases been described.
Thus, for example, the aluminum salts of organic phosphoric acids are known as flame retardants. They can be prepared by various processes.
EP-A-0 299 922 describes a process for the preparation of aluminum salts of phosphoric and phosphonic acid esters by reacting aluminum compounds with phosphoric acid, phosphonic acid or an ester thereof.
In the process described in EP-A-0 245 207, aluminum compounds are reacted with alkylphosphonic acid diesters to give the corresponding aluminum salt.
According to EP-A-0 327 496, the reaction of aluminum hydroxide with alkylphosphonic acid diesters at about 180° C. in the absence of water likewise results in aluminum salts of the phosphonic acid monoesters.
EP-A-0 699 708 describes flame-retardant polyester molding compositions in which the polyesters have been rendered flame-resistant by the addition of calcium salts or aluminum salts of phosphinic or diphosphinic acids. The abovementioned salts are obtained by reacting the corresponding dialkylphosphinic acids with calcium hydroxide or aluminum hydroxide.
DE-A-24 47 727 describes flame-retardant polyamide molding compositions which comprise a (metal) salt of a phosphinic acid or of a diphosphinic acid.
SUMMARY OF THE INVENTION
However, the abovementioned processes have the disadvantage that the suitable organophosphorus compounds first of all have to be prepared in an inconvenient manner. This is particularly true of the dialkylphosphinic acids, whose aluminum salts are highly suitable for use as flame retardants in certain polymers.
The object of the invention was therefore to provide a process for the preparation of dialkylphosphinic acids and salts thereof in which both the dialkylphosphinic acids themselves and the likewise desired end products, namely dialkylphosphinic acid salts of certain metals, can be prepared in a particularly simple and economical manner.
There is therefore a need for a process for the preparation of dialkylphosphinic acids and salts thereof which can be carried out in a simple manner and in which uniform products are obtained in high yield. A process of this type should also be significantly superior to the processes known hitherto in economic and environmental terms.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
This object is achieved by a process of the type mentioned at the outset which comprises
a) reacting elemental yellow phosphorus with alkylating-agents in the presence of a base to give a mixture which comprises, as principal constituents, the (metal) salts of alkyl-phosphonous, phosphorous and hypophosphorous acids,
b) esterifying the principal constituents of the mixture from a) to give an ester mixture,
c) isolating the ester of the alkylphosphonous acid from the ester mixture,
d) preparing the corresponding ester of the dialkyl-phosphinic acid from the ester of the alkylphosphonous acid by free-radical-initiated reaction with olefins,
e) converting the ester of the dialkylphosphinic acid into the free acid or into the dialkylphosphinic acid salts of metals from groups IA, IIA, IIIA, IVA, VA, IIB, IVB, VIIB or VIIIB of the Periodic Table or of cerium.
The process according to the invention has the considerable advantages over the processes known hitherto that it has a positive balance in the product distribution and at the same time avoids the phosphine and phosphonous acid dihalide starting materials, which are regarded as undesired, and in addition produces no halogenated organic by-products.
The alkylating agents are preferably alkyl halides, dialkyl sulfates, trialkyl phosphates, dialkyl carbonates and/or formic acid ortho-esters.
The alkylating agents are particularly preferably methyl chloride, methyl bromide and/or dimethyl sulfate.
The bases are preferably hydroxides, carbonates, bicarbonates, amides, alkoxides and/or amine bases, such as, for example, amines and ammonia.
The reaction in step a) is preferably carried out in a two-phase system comprising aqueous alkali or alkaline-earth metal hydroxide or mixtures thereof and an organic solvent.
The organic solvents employed in step a) are preferably straight-chain or branched alkanes, alkyl-substituted aromatic solvents, water-immiscible or only partially water-miscible alcohols or ethers, alone or in combination with one another.
The organic solvent employed is particularly preferably toluene, alone or in combination with alcohols.
The reaction can, if desired, also be carried out in a non-aqueous system, for example by using solid sodium hydroxide or amines.
The reaction is preferably carried out in the presence of a phase-transfer catalyst.
The phase-transfer catalyst is preferably a tetraalkyl-phosphonium halide, triphenylalkylphosphonium halide or tetraorganylammonium halide.
The temperature during the reaction is preferably from −20 to +80° C.
The temperature is particularly preferably from −10 to +30° C.
The reaction is preferably carried out under a pressure of from 0 to 10 bar.
Step a) of the process according to the invention is preferably carried out by suspending or dissolving the yellow phosphorus in a solvent or solvent mixture and then reacting it with an alkyl halide and a compound of the formula MOH or M′(OH)
2
or mixtures thereof, where M is an alkali metal and M′ is an alkaline-earth metal.
The yellow phosphorus and the alkyl halide are preferably reacted with one another in a molar ratio of from 1:1 to 1:3, where the molar ratio of yellow phosphorus to the compound of the formula MOH or M′(OH)
2
is from 1:1 to 1:5.
The principal constituents of the mixture from a) are preferably esterified in step b) using a linear or branched alcohol of the general formula R—OH, where R is a linear or branched alkyl radical having 1 to 10 carbon atoms.
In another preferred embodiment of the process according to the invention, the principal constituents of the mixture from a) are converted into a mixture of alkylphosphonous, phosphorous and hypophosphorous acids using mineral acids, with the (metal) salts of the mineral acids simultaneously being precipitated, and the mixture of these acids subsequently being esterified.
The water formed during the esterification is preferably removed by azeotropic distillation.
In other words, the esterification of the phosphonous acid to the corresponding monoester can be achieved by reaction with relatively high-boiling alcohols with removal of the resultant water by azeotropic distillation.
The precipitation of the metal salts, usually the alkali or alkaline-earth metal mineral salts, is preferably carried out here by replacement of the solvent water by the alcohol to be used in reaction step b).
The alkali or alkaline-earth metal mineral salt which has already precipitated is preferably filtered off before the esterification.
The alcohol is preferably n- or i-butanol, n-hexanol, ethylhexanol and/or amyl alcohol.
The mine

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