Sodium hydride

Sodium hydride is the chemical compound with the formula NaH. It is primarily used as a strong base in organic synthesis. NaH is representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na⁺ and H⁻ ions, in contrast to the more molecular hydrides such as borane, methane, ammonia and water. It is an ionic material that is insoluble in organic solvents (although apparently soluble in molten Na), consistent with the fact that H⁻ remains an unknown anion in solution. Because of the insolubility of NaH, all reactions involving NaH occur at the surface of the solid.

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Basic properties and structure

NaH is produced by the direct reaction of hydrogen and liquid sodium.^[1] Pure NaH is colorless, although samples generally appear gray. NaH is ca. 40% more dense than Na (0.968 g/cm³).

NaH, like LiH, KH, RbH, and CsH, adopts the NaCl crystal structure. In this motif, each Na⁺ ion is surrounded by six H⁻ centers in an octahedral geometry. The ionic radii of H⁻ (146 pm in NaH) and F⁻ (133 pm) are comparable, as judged by the Na−H and Na−F distances.^[2]

Applications in organic synthesis

First and foremost, NaH is a base of wide scope and utility in organic chemistry.^[3] It is capable of deprotonating a wide range of even weak Brønsted acids to give the corresponding sodium derivatives. Typical "easy" substrates contain O-H, N-H, S-H bonds, including alcohols, phenols, pyrazoles, and thiols.

NaH most notably is employed to deprotonate carbon acids such as 1,3-dicarbonyls and analogues such as malonic esters. The resulting sodium derivatives can be alkylated. NaH is widely used to promote condensation reactions of carbonyl compounds via the Dieckmann condensation, Stobbe condensation, Darzens condensation, and Claisen condensation. Other carbon acids susceptible to deprotonation by NaH include sulfonium salts and DMSO.

In an illustrative reaction, NaH is used to make sulfur ylides, which in turn are used to convert ketones into epoxides. Also, it is used in one synthesis of the dithioimidodiphosphinates which are an interesting class of acac like ligands in inorganic chemistry.

NaH reduces Si-Si and S-S bonds in disilanes and disulfides. Otherwise, NaH rarely exhibits the reducing qualities of Na itself.

Practical considerations

Sodium hydride is sold by many chemical suppliers such as Sigma-Aldrich and ACROS, usually as a mixture of 60% sodium hydride (w/w) in mineral oil. Such a dispersion is safer to handle and weigh than pure NaH. The pure white solid is prepared by rinsing the oil with pentane or THF, care being taken that the washings will contain traces of NaH that can ignite in air. Reactions involving NaH require an inert atmosphere, such as nitrogen gas. Typically NaH is used as a suspension in THF; THF resists deprotonation but solvates many organo sodium compounds.

Safety

NaH can ignite in air, especially upon contact with water. The reaction gives off hydrogen, possibly explosively. Hydrolysis converts NaH into sodium hydroxide (NaOH), a caustic base.

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press
3. ^ Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.