Patent GB-763311-A (GB)
Title: Improvements in or relating to to a process and apparatus for the manufacture of triaryl esters of ortho-phosphoric acid
IPC: C07F9/12
Filed: 1954-09-30
Abstract: Tri-aryl esters of orthophosphoric acid are prepared by heating a liquid mixture comprising a di-aryl phosphate and a hydroxyaryl compound to a reaction temperature promoting the formation of a tri-aryl ester and water, removing water from the reaction mixture as the reaction proceeds to maintain the water content below the equilibrium concentration in the reaction mixture, continuing said water removal to bring the proportion of water in the vapours emerging from the reaction mixture down to a value lower than the proportion of water soluble in the liquid hydroxyaryl compound, and recovering the tri-aryl phosphates from the reaction mixture.  The water content of the reaction mixture is preferably kept at below 0.1 per cent by weight, using for example calcium chloride or silica gel or by blowing dry air through it, but the preferred method is by internally contacting the mixture with vapours of the hydroxyaryl compound, the water being entrained in the vapours.  The hydroxyaryl compound vapours may be generated internally in the reaction mixture, or may be generated externally and fed into the reaction mixture.  In both cases the water content of the hydroxyaryl compound must be substantially lower than the solubility of water in the hydroxyaryl compound at room temperature.  The final product of the reaction is essentially a mixture of tri- and di-aryl phosphates and hydroxyaryl compound and may be separated by vacuum distillation, the di-aryl phosphate remaining in the still. The di-aryl phosphate starting material may be prepared by reacting the hydroxyaryl compound with phosphoric acid. Alternatively other phosphoric acids such as meta-, pyro- or poly-phosphoric acids or phosphorus pentoxide may be used and instead of the hydroxyaryl compounds their ethers or esters such as mono-aryl phosphates.  Specified hydroxyaryl compounds are phenol, phenols substituted by halogen atoms or nitra-, amino- or sulpho-groups, and phenols substituted by from one to five methyl, ethyl, n-or iso-propyl, n- or sec.- or tert.-butyl or amyl groups. Mixtures of hydroxyaryl compounds may be used.  In a batchwise process, a hydroxyaryl compound and phosphoric acid are added in appropriate amounts to a still containing the residue from a previous batch (i.e. di-aryl phosphate) and the mixture is heated to boiling, the vapours of the hydroxyaryl compound passing up a column into a condenser and thence to a separator wherein the condensate is separated into hydroxyaryl compound and water, the water being discarded.  The hydroxyaryl compound is returned to the still by passing it down the column in countercurrent to the rising hydroxyaryl compound vapours which remove water from the hydroxyaryl compound so that it is substantially anhydrous when it reaches the still.  When the reaction is terminated the reaction mixture is distilled to recover tri-aryl phosphate; hydroxyaryl compound and diaryl phosphate being recycled.  In a continuous process, a mixture of hydroxyaryl compound and phosphoric acid is passed down a heated column through a series of compartments, in countercurrent to a stream of hydroxyaryl compound vapour.  The mono-, di- and tri-esters are successively formed in the column, the hydroxyaryl compound entering at the bottom of the column serving to remove water in the tertiary stage of the esterification and then passing on to remove water from the primary and secondary stages of the esterification wherein the vapour pressure of water is much higher than in the tertiary stage.  The hydroxyaryl compound vapours emerging at the top of the column are condensed, separated from water, dried as in the batch process by passing down a column in countercurrent to rising hydroxyaryl compound vapours and the substantially anhydrous compound recycled.  The tri-aryl phosphate is recovered by distillation from the mixture emerging from the bottom of the column, the other components being recycled.  The temperature of the reaction varies according to the phenol used, being about 240-280 DEG  C. for hydroxyaryl compounds such as cresols.  The reaction may also be conducted at sub-atmospheric pressures.  In examples: (1) a mixture of m- and p-cresol isomers is mixed with phosphorus pentoxide, the mixture heated, further cresol added, water being removed from the reaction mixture as in the batch process described above and the mixture vacuum distilled to give cresols as a first fraction, tricresyl phosphate as a second fraction and a residue which is reacted further with the first fraction; (2) as in (1) using phosphoric acid; (3) as in (1) but using phenol; (4) as in (2) but using a mixture of high-boiling phenols containing alkyl substituents totalling about 4 carbon atoms mainly in the 3 : 5-positions and reacting under a partial vacuum.  U.S.A. Specification 1,869,312 is referred to.