Basic properties and structure
NaH is produced by the direct reaction of hydrogen and liquid sodium. Pure NaH is colorless, although samples generally appear grey. NaH is ca. 40% denser 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.
"Inverse sodium hydride"
A very unusual situation occurs in a compound dubbed "inverse sodium hydride", which contains Na− and H+ ions. Na− is an alkalide,
and this compound differs from ordinary sodium hydride in having a much
higher energy content due to the net displacement of two electrons from
hydrogen to sodium. A derivative of this "inverse sodium hydride"
arises in the presence of the base adamanzane. This molecule irreversibly encapsulates the H+ and shields it from interaction with the alkalide Na−.
Theoretical work has suggested that even an unprotected protonated
tertiary amine complexed with the sodium alkalide might be metastable
under certain solvent conditions, though the barrier to reaction would
be small and finding a suitable solvent might be difficult.
Applications in organic synthesis
As a strong base
NaH is a base of wide scope and utility in organic chemistry. It is capable of deprotonating a 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. NaH is used to make sulfur ylides, which in turn are used to convert ketones into epoxides, as in the Johnson–Corey–Chaykovsky reaction.
As a reducing agent
NaH reduces certain main group compounds, but analogous reactivity is unknown in organic chemistry. Notably boron trifluoride reacts to give diborane and sodium fluoride:
- 6 NaH + 2 BF3 → B2H6 + 6 NaF
Si-Si and S-S bonds in disilanes and disulfides are also reduced.
Because of its rapid and irreversible reaction with water, NaH can be
used to dry some organic solvents. Other drying agents are far more
widely used, such as calcium hydride.
The use of sodium hydride has been proposed for hydrogen storage for use in fuel cell vehicles, the hydride being encased in plastic pellets which are crushed in the presence of water to release the hydrogen.
Sodium hydride is sold by many chemical suppliers 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
compound is often used in this form but the pure grey solid can be
prepared by rinsing the oil with pentane or THF, care being taken
because the washings will contain traces of NaH that can ignite in air.
Reactions involving NaH require an inert atmosphere, such as nitrogen or argon gas. Typically NaH is used as a suspension in THF, a solvent that resists deprotonation but solvates many organosodium compounds.
NaH can ignite in air, especially upon contact with water to release hydrogen, which is also flammable. Hydrolysis converts NaH into sodium hydroxide (NaOH), a caustic base. In practice, most sodium hydride is dispensed as a dispersion in oil, which can be safely handled in air.