Hexamethylphosphoramide

Hexamethylphosphoramide (abbreviated HMPA) is a colorless organic liquid with the formula [(CH₃)₂N]₃PO.

HMPA is the oxide of the highly basic tertiary phosphine P(NMe₂)₃. Like other phosphine oxides, e.g. Triphenylphosphine oxide, the molecule has tetrahedral core and the P-O bond is highly polarized, with significant negative charge residing on oxygen.

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Applications

HMPA is used a polymer solvent, as a selective solvent for gases, stabilizer in polystyrene against thermal degradation, as a laboratory solvent for organometallic and organic reactions. It has been tested as a fire retardant and as an insect chemosterilant, but there is no known current use of HMPA for these applications.

It is useful for improving the selectivity of certain organic reactions. For instance in some deprotonation reactions HMPA breaks up the oligomers of lithium bases such as butyl lithium. Also because HMPA solvates cations so well, while not solvating anions, it has been used as a solvent for some difficult S_N2 reactions. The basic oxygen atom in HMPA coordinates strongly to lithium cation.^[1] A molybdenum peroxide complex of HMPA is used in synthetic chemistry as an oxidant.

Alternative reagents

Dimethyl sulfoxide can often be used in place of HMPA as a solvent. Both are strong hydrogen bond acceptors, and their oxygen atoms are able to bind to metal cations. Other alternatives to HMPA include the tetraalkylureas.^[2]

Toxicity

HMPA is only mildly toxic but has been shown to cause nasal cancers in rats.^[1] It is frequently nicknamed in chemistry labs as "liquid cancer", although this characterization is somewhat extreme.

HMPA can be degraded to less toxic compounds by the action of hydrochloric acid. In phosphorus chemistry it is the case in general that the treatment of a compound containing a nitrogen phosphorus single bond with hydrogen chloride will form a protonated amine and a P-chloro compound. For example if P-dimethylamino-P, P-diphenyl phosphine is treated with hydrogen chloride then P-chloro-P, P-diphenyl phosphine and dimethylamine HCl salt will be obtained.

References

1. ^ ^{a b} Dykstra, R. R. "Hexamethylphosphoric Triamide" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley & Sons, Ltd. DOI: 10.1002/047084289X.rh020
2. ^ Beck, A. K.; Seebach, D. "N,N'-Dimethylpropyleneurea" in Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley & Sons, New York. DOI: 10.1002/047084289X.rd366.

• Merck Index, 12th Edition, 4761.