Methylphosphonic acid

Methylphosphonic acid is an organophosphorus compound with the chemical formula CH₃P(O)(OH)₂. The phosphorus center is tetrahedral and is bonded to a methyl group, two OH groups and an oxygen. Methylphosphonic acid is a white, non-volatile solid that is poorly soluble in organic solvent but soluble in water and common alcohols.^[2]

Methylphosphonic acid

Identifiers
CAS Number	993-13-5 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:45129
ChemSpider	13220
ECHA InfoCard	100.012.370
EC Number	213-607-2
KEGG	C20396
MeSH	C032627
PubChem CID	13818
UNII	329W4YM10Z Y
CompTox Dashboard (EPA)	DTXSID5047748
InChI InChI=1S/CH5O3P/c1-5(2,3)4/h1H3,(H2,2,3,4) Key: YACKEPLHDIMKIO-UHFFFAOYSA-N InChI=1/CH5O3P/c1-5(2,3)4/h1H3,(H2,2,3,4) Key: YACKEPLHDIMKIO-UHFFFAOYAB
SMILES CP(=O)(O)O
Properties
Chemical formula	CH5O3P
Molar mass	96.02
Appearance	White Solid
Melting point	105 to 107 °C (221 to 225 °F; 378 to 380 K)[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation

Methylphosphonic acid can be prepared from triethylphosphite by first using a Michaelis-Arbuzov reaction to generate the phosphorus(V) centre:^[3]

CH₃Cl + P(OC₂H₅)₃ → CH₃PO(OC₂H₅)₂ + C₂H₅Cl

The resulting dialkylphosphonate is then treated with chlorotrimethylsilane before hydrolysis of the siloxyphosphonate to generate the desired product.^[3]

CH₃PO(OC₂H₅)₂ + 2 Me₃SiCl → CH₃PO(OSiMe₃)₂ + 2 C₂H₅Cl

CH₃PO(OSiMe₃)₂ + 2H₂O → CH₃PO(OH)₂ + 2 HOSiMe₃

The reaction pathway proceeds via the siloxyphosphonate intermediate due to the difficulty in directly hydrolysing dialkylphosphonates. Katritzky and co-workers published a one-pot synthesis of methylphosphonic acid in 1989.^[3]