Tetrakis(hydroxymethyl)phosphonium chloride (THPC) is an organophosphorus compound with the chemical formula [P(CH<sub>2</sub>OH)<sub>4</sub>]Cl. It is a white water-soluble salt with applications as a precursor to fire-retardant materials and as a microbiocide in commercial and industrial water systems.
THPC can be synthesized with high yield by treating phosphine with formaldehyde in the presence of hydrochloric acid.
The cation P(CH<sub>2</sub>OH)<sub>4</sub><sup>+</sup> features four-coordinate phosphorus, as is typical for phosphonium salts.
THPC converts to tris(hydroxymethyl)phosphine upon treatment with aqueous sodium hydroxide:
THPC has industrial importance in the production of crease-resistant and flame-retardant finishes on cotton textiles and other cellulosic fabrics. A flame-retardant finish can be prepared from THPC by the Proban Process, in which THPC is treated with urea. The urea condenses with the hydroxymethyl groups on THPC. The phosphonium structure is converted to phosphine oxide as the result of this reaction.
This reaction proceeds rapidly, forming insoluble high molecular weight polymers. The resulting product is applied to the fabrics in a "pad-dry process". This treated material is then treated with ammonia and ammonia hydroxide to produce fibers that are flame-retardant.
THPC can condense with many other types of monomers in addition to urea. These monomers include amines, phenols, and polybasic acids and anhydrides.