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Methylphosphonic acid

Methylphosphonic acid is an organophosphorus compound with the chemical formula CH<sub>3</sub>P(O)(OH)<sub>2</sub>. 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.

Preparation

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

CH<sub>3</sub>Cl + P(OC<sub>2</sub>H<sub>5</sub>)<sub>3</sub> → CH<sub>3</sub>PO(OC<sub>2</sub>H<sub>5</sub>)<sub>2</sub> + C<sub>2</sub>H<sub>5</sub>Cl

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

CH<sub>3</sub>PO(OC<sub>2</sub>H<sub>5</sub>)<sub>2</sub> + 2 Me<sub>3</sub>SiCl → CH<sub>3</sub>PO(OSiMe<sub>3</sub>)<sub>2</sub> + 2 C<sub>2</sub>H<sub>5</sub>Cl
CH<sub>3</sub>PO(OSiMe<sub>3</sub>)<sub>2</sub> + 2H<sub>2</sub>O → CH<sub>3</sub>PO(OH)<sub>2</sub> + 2 HOSiMe<sub>3</sub>

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.

References