Phosphoric Acids and Phosphates - Polyphosphoric Acids

Polyphosphoric Acids

When two orthophosphoric acid molecules are condensed into one molecule, pyrophosphoric acid (H₄P₂O₇) is obtained as follows:

2 H₃PO₄ → H₄P₂O₇ + H₂O

The chemical structure of pyrophosphoric acid is also shown in the illustration. There is also a separate article on Pyrophosphoric acid. Three orthophosphoric acid molecules can condense in a row to obtain tripolyphosphoric acid (H₅P₃O₁₀), which is also shown in the illustration. This condensation process can continue with additional orthophosphoric acid units to obtain tetrapolyphosphoric acid (H₆P₄O₁₃, pictured) and so on. Note that each extra phosphoric unit adds 1 extra H (hydrogen) atom, 1 extra P (phosphorus) atom, and 3 extra O (oxygen) atoms. The "backbone" chain of these types of molecules consists of alternating P and O atoms covalently bonded together. Polyphosphoric acid molecules can have dozens of such phosphoric units bonded in a row. A general formula for such poly-acid compounds is HO(PO₂OH)_xH, where x = number of phosphoric units in the molecule. The four oxygen atoms bonded to each phosphorus atom are in a tetrahedral configuration with the phosphorus in the center of the tetrahedron and the oxygens in each of the four corners.
Used in organic synthesis for cyclizations and acylations.