Process for preparing bis(hydroxymethyl) methylphosphine oxide

A process is provided for preparing bis(hydroxymethyl)methylphosphine oxide, comprising (1) thermally rearranging tris(hydroxymethyl)phosphine at elevated temperatures in the presence of a catalytic amount of a strong, non-oxidizing acid, having an ionization constant greater than 10.sup.-3, or a source thereof, to obtain the desired bis(hydroxymethyl)methylphosphine oxide, and (2) recovering the latter compound from the reaction mixture.

BACKGROUND OF THE INVENTION 
This invention relates to a process for preparing 
bis(hydroxymethyl)methylphosphine oxide. 
Tris(hydroymethyl)phosphine (THP) is a hazardous material because of its 
lability which sometimes leads to expolsions on heating at high 
temperatures without a solvent. The heating of crude THP at high 
temperatures is particularly hazardous. 
The preparation of bis(hydroxymethyl)methylphosphine oxide by thermally 
rearranging tris(hydroxymethyl)phosphine in the presence of a solvent such 
as dimethyl sulfoxide, acetic acid, N-methyl pyrrolidone, 
N,N-dimethylforamide, and the like, at elevated temperatures, as disclosed 
by Lin in U.S. Pat. No. 3,732,316, moderates the aforementioned hazard. 
In the process of Lin (see Example 1) 40% of the final product composition 
is tris(hydroxymethyl)phosphine oxide (THPO) when solvent used is dimethyl 
sulfoxide. When acetic acid is used as the solvent the product is a 
mixture of various phosphorous compounds containing acetate groups. When 
N-methyl pyrrolidone is used as the solvent the product composition is 
78-79% of bis(hydroxymethyl)methylphosphine oxide and between 21-22% of 
the THPO. 
Thus, there is a need for an improved process which moderates the hazardous 
lability of tris(hydroxymethyl)phosphine and produces a product having a 
significantly higher percentage of bis(hydroxymethyl)methylphosphine 
oxide. 
Accordingly, it is an object of the present invention to provide an 
improved process for preparing bis(hydroxymethyl) -methylphosphine oxide 
by thermally rearranging tris(hydroxymethyl)phosphine. 
It is a further object of this invention to provide an improved process for 
preparing bis(hydroxymethyl)methylphosphine oxide by thermally rearranging 
tris(hydroxymethyl)phosphine in an inert solvent. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description, or may be realized by practice of the invention, the 
objects and advantages being realized and attained by means of the 
methods, processes, instrumentalities and combinations particularly 
pointed out in the appended claims. 
SUMMARY OF THE INVENTION 
It has been discovered that the foregoing objects may be achieved by an 
improved process which comprises heating tris(hydroxymethyl)phosphine and 
a catalytic amount of a strong, non-oxidizing acid, or source of said 
acid, at elevated temperatures to form the desired 
bis(hydroxymethyl)methylphosphine oxide and recovering the latter 
compound. 
More particularly, it has been discovered that the foregoing objects are 
preferably achieved by heating a solution of tris(hydroxymethyl)phosphine 
in an inert solvent containing a catalytic amount of a strong, 
non-oxidizing acid, or source of said acid at elevated temperatures and 
recovering the desired bis(hydroxymethyl)methylphosphine oxide therefrom. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are not 
restrictive of the invention. 
The bis(hydroxymethyl)methylphosphine oxide which is provided in improved 
quality and yield by the advent of this invention finds utility as a flame 
retardant for synthetic polymers, particularly polyurethanes and 
polyesters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention pertains to an improved process for preparing 
bis(hydroxymethyl)methylphosphine oxide by heating 
tris(hydroxymethyl)phosphine, preferably in an inert solvent in the 
presence of 0.75 to 8.0 percent mole of a catalyst which is a strong 
non-oxidizing acid having an ionization constant greater than 
1.times.10.sup.-3, or a source thereof, preferably 0.75 to 2.0 mole 
percent of methanesulfonic acid, at a temperature from about 100.degree. 
to about 190.degree. C., preferably at a temperature from about 
115.degree. to about 140.degree. C., for a period from about 15 to about 
80 hours, preferably from about 20 to about 40 hours, with the higher 
temperature in each range requiring the shorter heating period, and 
recovering the product therefrom. 
While the process of this invention can be effected by heating the THP neat 
in the presence of a strong non-oxidizing acid alone or a source thereof, 
it is preferable to carry out the reaction in the further presence of an 
alcoholic solvent; methyl cellosolve is a preferred solvent. 
Illustrative of the strong non-oxidizing acids, which may be used in the 
process of this invention are hydrochloric acid, hydrobromic acid, 
hydroiodic acid, phosphoric acid (K.sub.1 =7.5.times.10.sup.-3), 
trichloroacetic acid (K=1.3.times.10.sup.-1), oxalic acid (K.sub.1 
=6.5.times.10.sup.-1), sulfonic acids, such as p-toluene sulfonic acid, 
phenylsulfonic acid (K=2.8.times. 10.sup.-3), .alpha.-napthalene sulfonic 
acid (K=6.8" 10.sup.-1), methanesulfonic acid, and the like. Strong 
oxidizing acids such as sulfuric, nitric, and chloric acid are not useful. 
Sources of such acids include tetrakis (hydroxymethyl)phosphonium chloride, 
tetrakis (hydroxymethyl) phosphonium phosphate, and tetrakis 
(hydroxymethyl) phosphonium oxalate. 
It is critical that the crude tris(hydroxymethyl) -phosphine starting 
material be essentially neutral, that is a 50 percent aqueous solution of 
this material should have a pH no higher than 6.9-7.0. If the pH of the 
starting material is slightly above 7.0, the pH should be adjusted to pH 
6.9-7.0 by addition of an acid thereto. In the case of crude THP the 
amount of acid required to adjust the pH to 6.9-7.0 may vary depending on 
amount of impurities present. 
The following examples are illustrative of the process of this invention, 
and will enable persons skilled in the art to better understand and 
practice the invention. 
EXAMPLE 1 
To 22.0 g. of crude tris(hydroxymethyl)phosphine (90% real, 0.158 mole; 5% 
water, 4% THPO, 0.5% formaldehyde) in a suitable reaction vessel is added 
50 mls of deaerated toluene and the mixture is azeotroped under nitrogen 
to remove water and then concentrated under vacuum to remove all but 2 
mls. of the toluene. To the reaction mixture are added 18 mls. of 
nitrogen-deaerated methyl cellosolve and 0.11 g. (0.0012 mole; 0.75 mole 
percent based on the THP) of methanesulfonic acid. The reaction mixture is 
then refluxed (134.degree.-135.degree. C.) under nitrogen for 23 hours and 
cooled to room temperature. A sample is removed and labeled A. The 
reaction mixture is then heated at 134.degree.-135.degree. C. for an 
additional 18 hours and cooled to room temperature. Another sample is 
removed and labeled B. 
The analytical results obtained by hydrogen, nuclear magnetic resonance 
analysis of these samples are reported in Table I as mole percent of the 
original real THP. 
TABLE I 
______________________________________ 
Mole % 
Sample 
##STR1## THPO Phosphonium Salts 
______________________________________ 
A 91 5 4 
B 92 5 3 
______________________________________ 
example 2 
the following example illustrates the process of this invention carried out 
in methyl cellosolve with 2 mole percent of acid. 
To 14.0 g. of crude tris(hydroxymethyl)phosphine (90% real, 0.100 mole) in 
11 mls. of nitrogen-deaereated methyl cellosolve in a suitable reaction 
vessel is added 0.16 g. (0.0017 mole; 1.7 mole percent based on THP) of 
methanesulfonic acid and the reaction mixture is heated at 
128.degree.-130.degree. C. for a period of 23 hours under a nitrogen 
atmosphere. The reaction mixture is then cooled to room temperature. 
Analysis of the reaction mixture shows that 79% of the original THP is 
rearranged to bis(hydroxymethyl)methylphosphine oxide. 
EXAMPLE 3 
This example shows that the mole percent of THP rearranged to 
dihydroxymethyl phosphine oxide is significantly lowered if an acid 
catalyst is not included. 
To 8.7 g. of methyl cellosolve which has been deaerated with nitrogen is 
added 11.4 g. of crude tris(hydroxymethyl)phosphine (79% real, 0.072 mole) 
which also contains 16% THPO and about 5% water. The mixture is stirred to 
dissolve the THP and 0.15 g. of dimethylaniline is added thereto to 
neutralize* any acid which may be present. The mixture is then heated at 
reflux (131.degree.-134.degree. C.) for 23 hours under nitrogen. The 
reaction mixture is cooled to room temperature and a sample is removed for 
analysis and labeled A. The reaction mixture is then refluxed at 
131.degree.-134.degree. C. for an additional 18 hours and cooled to room 
temperature. A second sample is then removed for analysis and labeled B. 
FNT *Dimethylaniline is added until a 50% solution of the reaction mixture in 
water has a pH of 7. 
The analytical results obtained by 31 P nuclear resonance analysis are 
reported in Table II as mole percent based on the orignial THP present. 
TABLE II 
______________________________________ 
Mole % 
Sample 
##STR2## THPO THP 
______________________________________ 
A 64 18 18 
B 75 21 4 
______________________________________ 
example 4 
this example illustrates a rearrangement of THP (neat) without an acid 
catalyst. 
A sample of 9.6 g. (0.077 mole) of tris(hydroxymethyl)phosphine is heated 
under a nitrogen atmosphere for 81/4 hours at 116.degree.-122.degree. C. 
while stirring slowly. At the end of this period the reaction mixture is 
cooled to room temperature and a sample is analyzed by hydrogen nuclear 
magnetic resonance. Analysis shows that only 9 mole percent of the 
original THP has rearranged to 
##STR3## 
EXAMPLE 5 
This example illustrates the process whereby THP (neat) is heated for a 
relatively short period with hydrochloric acid. 
A mixture of 10.7 g. (0.086 mole) of tris(hydroxymethyl)phosphine and 0.17 
g. (0.0017 mole, 1.97 mole percent) of concentrated hydrochloric acid is 
heated under a nitrogen atmosphere at 120.degree.-125.degree. C. while 
stirring slowly for 61/2 hours. Hydrogen nuclear magnetic analysis of the 
reaction mixture after this period shows that 35 mole percent of the 
original THP has been converted to bis(hydroxymethyl)methylphosphine 
oxide. 
EXAMPLE 6 
This example illustrates the process carried out with THP(neat) and 
hydrochloric acid. 
A mixture of 8.0 g. (0.064 mole) of tris(hydroxymethyl)phosphine and 0.20 
g. (0.002 mole, 3.1 mole percent) of concentrated hydrochloric acid is 
heated under a nitrogen atmosphere while stirring at 
119.degree.-120.degree. C. for 15 hours. The temperature is then raised to 
126.degree.-127.degree. C. for 6 hours at which point the heating is 
discontinued and the mixture is cooled to 30.degree. C. A small sample (A) 
is removed and 0.12 g. (0.001 mole, 1.6 mole percent) of concentrated 
hydrochloric acid is added to the reaction mixture. Heating is resumed 
under nitrogen at 126.degree.-127.degree. C. for an additional 16 hours 
after which the reaction mixture is cooled to room temperature and a 
second sample (B) is removed. Both samples are then analyzed by hydrogen 
nuclear magnetic resonance; the results obtained are reported in Table III 
as mole percent of the THP present originally. 
TABLE III 
______________________________________ 
Mole % 
Sample 
##STR4## Phosphonium Salts 
THP 
______________________________________ 
A 65 7 28 
B 89 8 none detected 
______________________________________ 
EXAMPLE 7 
The following example illustrates the process carried out in THP (neat) 
with tetrakis(hydroxymethyl)phosphonium chloride as the catalyst. 
A mixture of 9.9 g. (0.079 mole) of tris(hydroxymethyl)phosphine and 1.05 
g. (0.006 mole; 7.6 mole percent based on THP) of 
tetrakis(hydroxymethyl)phosphonium chloride is heated under a nitrogen 
atmosphere at 111.degree.-115.degree. C. while stirring slowly for 241/2 
hours. The reaction mixture is then cooled to room temperature and 
analyzed by "hydrogen nuclear magnetic resonance." The analysis shows that 
77 mole percent of the THP has rearranged to 
bis(hydroxymethyl)methylphosphine oxide.