Formation of methylphosphonous acid by alkylation of yellow phosphorus

Article abstract of DOI:10.1080/10426500212237

Methylphosphonous acid is a key intermediate in the synthesis of phosphinic acid salts, a new class of halogen-free flame retardants. This paper describes a novel and simple synthesis of this synthon: By alkylation of elemental yellow phosphorus with meth

Full text of DOI:10.1080/10426500212237

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Phosphorus, Sulfur, and Silicon and the Related
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Formation of Methylphosphonous Acid by Alkylation of
Yellow Phosphorus
Norbert Weferling a , Martin Sicken a & Sebastian Hoerold a
a Clariant GmbH , Huerth, Germany
Published online: 27 Oct 2010.
To cite this article: Norbert Weferling , Martin Sicken & Sebastian Hoerold (2002) Formation of Methylphosphonous Acid
by Alkylation of Yellow Phosphorus, Phosphorus, Sulfur, and Silicon and the Related Elements, 177:6-7, 1757-1761, DOI:
10.1080/10426500212237
To link to this article: http://dx.doi.org/10.1080/10426500212237
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Phosphorus, Sulfur and Silicon, 2002, Vol. 177:1757–1761
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DOI: 10.1080/10426500290092901
FORMATION OF METHYLPHOSPHONOUS ACID
BY ALKYLATION OF YELLOW PHOSPHORUS
Norbert Weferling, Martin Sicken, and Sebastian Hoerold
Clariant GmbH, Huerth, Germany
(Received July 29, 2001; accepted December 25, 2001)
Methylphosphonous acid is a key intermediate in the synthesis of phos-
phinic acid salts, a new class of halogen-free flame retardants. This
paper describes a novel and simple synthesis of this synthon: By alkyla-
tion of elemental yellow phosphorus with methyl chloride under phase-
transfer conditions, methylphosphonous acid alkali salt is formed as
the main product. Most of the by-products were identified. The mecha-
nism of the reaction is discussed.
Keywords: Alkylation; phase-transfer catalysis; two-phase system;
yellow phosphorus
In 1979 Sandler1 reported for the first time the flame-retarding effect
of salts of the dialkylphosphinic acid for technical thermoplastics like
polyesters and polyamides. However, only a few years ago Kleiner2 dis-
covered that the aluminum salt of methyl-ethylphosphinic acid showed
excellent flame retardancy in these polymers. Trying to find an econom-
ically feasible route of synthesis for salts of methyl-ethylphosphinic
acid, a process was developed that started from methyldichlorophos-
phine, which is commercially available.3 Since it was necessary to find
an even less expensive process for this class of compounds, a route of
synthesis was developed that was based on methylphosphonous acid as
a key substance. The subsequent synthetic steps, like the addition of
ethylene and precipitation of aluminum salt, are also innovative, but
are not treated here.4
The authors thank Dr. A. Beard and Professor Dr. O. Stelzer for their support.
Address correspondence to Norbert Weferling, Clariant GmbH, Werk Knapsack,
D-50351 Huerth, Germany. E-mail: Norbert.weferling@clariant.com
1757

1758
N. Weferling et al.
ALKYLATION OF P4 UNDER PHASE-TRANSFER
CONDITIONS
In 1991 Trofimov et al.5,6 described the alkylation of red phosphorus un-
der phase-transfer (PT) conditions [KOH, water, dioxane, PT catalyst
(PTC)] with alkyl- and benzylhalogenides, which resulted in a yield of
65–72% of the corresponding trialkylphosphinoxides. However, when
we reacted yellow phosphorus with alkylhalogenides in a two-phase
system (aqueous alkali, organic solvent/PTC) we obtained the corre-
sponding alkylphosphonous acids and their sodium salts respectively
at yields of 65–75% based on the amount of phosphorus. The qualitative
and quantitative result of the reaction is mostly independent of the base
used (NaOH, KOH), on the akylation reagent (alkylchloride, -bromide,
dialkylsulfate), and the PTC (phosphonium-, ammonium-quat). Cata-
lysts with relatively long alkyl chains generally produce better yields
from alkylphosphonous acids than catalysts with short-chain quats.
Over a temperature range of 10 C to more than +30 C the reac-
tion progression is mostly independent of temperature. With increas-
ing temperature, higher concentrations of di- and trialkylated products
are found. The proportion of oxidated products and unidentified side-
products increases as well.
We now focus on the reaction of methyl chloride under PTC condi-
tions, because this reaction allows a simple and inexpensive route to
methyl phosphonous acid from which mentioned salts of methyl phos-
phonous acid can be obtained in turn.
This reaction requires intensive dispersion and mixing of the reac-
tion components, because there is a multiphase reaction system (solid,
liquid organic, aqueous liquid, gas). Otherwise, exchange processes at
phase boundaries would become limiting for the reaction rate. First
yellow phosphorus is added to the reactor, with the aprotic nonpolar
solvent containing the PTC. The yellow phosphorus is heated above
its melting point and subsequently finely dispersed by cooling to the
desired process temperature. This will also lead to a partial dissolu-
tion of the yellow phosphorus in the organic solvent. Next, an amount
of methyl chloride equivalent to the amount of phosphorus is added
under pressure. Further addition of an aqueous alkali hydroxide so-
lution starts an exothermic reaction. Even though methyl chloride is
consumed in the reaction, the overall pressure increases because of the
released hydrogen.
Table I shows the molar ratios of phosphorus-containing reaction
products in the aqueous phase (average values of a series of experi-
ments). Besides the added catalyst, the organic phase contains only mi-
nor amounts of phosphines [mainly CH3PH2, some (CH3)2PH, no PH3].

Methylphosphonous Acid from Yellow Phosphorus
1759
TABLE I Products of the PT-Catalyzed Alkylation of Yellow Phosphorus
(Aqueous Phase)
Product
Formular
Yield (%)
72
Oxidation state
+1
Methylphosphonous acid
(sodium salt)
8
+3
Phosphorous acid
(disodium salt)
7.5
5
+1
Hypophosphorous acid
(sodium salt)
1
Dimethylphosphine oxide
3.5
0.5
+3
+1
Methylphosphonic acid
(disodium salt)
Dimethylphosphinic acid
(sodium salt)
Trimethylphosphine oxide
1
1
Unknown compounds and
2.5
diphosphorus compound 8
The formation of most of the products can be explained by known
mechanisms in a straightforward manner. With the help of the PTC,
hydroxide ions are transported to the yellow phosphorus, which is dis-
solved in the organic phase. These hydroxide ions undertake a nucle-
ophilic attack, resulting in the formation of phosphide ions, which ren-
ders the deeply red color of the reaction mixture. The phosphide ions
can now react either with methyl chloride to form a P C bond or with
water to form a P H bond and another hydroxide ion (see Figure 1).
On the one hand, oxidation products like methylphosphonate and
sodium phosphite can be formed via this successive degradation of the
P4 tetrahedrons. On the other hand, they can be formed from the sodium
salt of methylphosphonous acid, respectively the hypophosphite, by

1760
N. Weferling et al.
FIGURE 1 First steps of P4 cleavage reaction in the presence of methyl
chloride.
base-catalyzed oxidation of P H compounds7 accompanied by release
of hydrogen. The formation of trimethylphosphine oxide 7 might be the
successor of the intermediate (CH3)2P O , which reacts with methyl
chloride by loss of chloride to phosphinic acid methyl ester. This in turn
rearranges to the more stable tertiary phosphine oxide. The analogous
reaction with water would lead to dimethylphosphine oxide, which is
also observed.
Despite major analytical efforts, for the time being only one prod-
uct with a P P bond could be detected during the reaction: CH3PH-
P(:O)(OH)CH3 8, as its sodium salt. The cleavage of the P P bond in 8
by hydroxide ions is assumed to proceed as depicted in Figure 2.
1
1
The phosphine part of 8 (
:
109.7, JPP = 129.6 Hz, JPH =
P
209.3 Hz) reacts to the stable end products methylphosphine and
FIGURE 2 Suggested process for the cleavage of 8.

Methylphosphonous Acid from Yellow Phosphorus
1761
methylphosphonic acid via the fairly instable primary phosphine ox-
ide CH3P(:O)H2. The sodium salt of methylphosphonous acid 1 is a
1
result of the reaction of the oxidized part ( P: +51.4, JPP = 129.6 Hz)
with water. Oxidation by release of hydrogen will produce 5, whereas a
methylation after rearrangement will yield the dimethylphosphinate 6.
REFERENCES
[1] US 4, 180, 495, Pennwalt Corp. (1979).
[2] EP 0, 699, 708, Ticona GmbH (1998).
[3] N. Weferling and S. Hoerold, Phosphorus, Sulfur and Silicon, 144–146, 21 (1999).
[4] DE 196, 14, 424, Clariant GmbH (1996).
[5] DE 198, 51, 768, Clariant GmbH (1998).
[6] N. K. Gusarova, B. A Trofimov et al., Zh. Obsch. Khim, 63, 1753 (1993).
[7] B. A. Trofimov, N. K. Gusarova et al., Phosphorus, Sulfur and Silicon, 55, 271 (1991).
[8] S. M. Bloom, S. A. Buckler et al., Chem. Commun., 870 (1970).