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].