Sodium hydride

Sodium hydride
Identifiers
CAS number	7646-69-7 Y
PubChem	24758
Properties
Molecular formula	NaH
Molar mass	23.99771 g/mol
Appearance	colorless to grey solid
Density	1.396 g/cm3
Melting point	800 °C decomp.
Solubility in water	reacts
Solubility	insoluble in ammonia, benzene, CCl4, CS2
Refractive index (nD)	1.470
Structure
Crystal structure	fcc (NaCl)
Hazards
MSDS	External MSDS
EU Index	Not listed
NFPA 704	4 3 2 W
Flash point	combustible
Related compounds
Other cations	Lithium hydride Potassium hydride
Related compounds	Sodium borohydride
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.

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 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.^[2]

Applications in organic synthesis

As a strong base

First and foremost, NaH is a base of wide scope and utility in organic chemistry.^[3] 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 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:^[1]

6 NaH + 2 BF₃ → B₂H₆ + 6 NaF

Si-Si and S-S bonds in disilanes and disulfides are also reduced.

Drying agent

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.

Hydrogen storage

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.^[4]

Practical considerations

Sodium hydride is sold by many chemical suppliers such as Dow (previously Rohm and Haas),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, a solvent that resists deprotonation but solvates many organosodium compounds.

Safety

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.

References