|Jmol-3D images||Image 1|
|Molar mass||179.20 g mol−1|
7.20 °C, 280 K, 45 °F
232.5 °C, 506 K, 451 °F (CRC)
|Flash point||104.4 °C (219.9 °F)|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Hexamethylphosphoramide, often abbreviated HMPA, is a phosphoramide (i.e. an amide of phosphoric acid) having the formula [(CH3)2N]3PO. This colorless liquid is a useful polar aprotic solvent and additive in organic synthesis.
Structure and reactivity
HMPA is the oxide of the highly basic tertiary phosphine hexamethylphosphorous triamide (HMPT), P(NMe2)3. Like other phosphine oxides (e.g., triphenylphosphine oxide), the molecule has a tetrahedral core and a P-O bond that is highly polarized, with significant negative charge residing on the oxygen atom.
HMPA is a speciality solvent for polymers, gases, and organometallic compounds. It improves the selectivity of lithiation reactions by breaking up the oligomers of lithium bases such as butyllithium. Because HMPA selectively solvates cations, it accelerates otherwise SN2 reactions by generating more "naked" anions. The basic oxygen centers in HMPA coordinate strongly to Li+.
Dimethyl sulfoxide can often be used in place of HMPA as a solvent. Both are strong hydrogen bond acceptors, and their oxygen atoms bind metal cations. Other alternatives to HMPA include the tetraalkylureas and the cyclic alkylureas like DMPU.
HMPA is only mildly toxic but has been shown to cause nasal cancers in rats. Still, many organic chemists regard HMPA as an exceptionally hazardous molecule due to its known carcinogenicity, and avoid its use when possible. HMPA can be degraded to less toxic compounds by the action of hydrochloric acid. For laboratory uses it can be substituted by the less carcinogenic solvent DMI.
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