Sodium hydride

Sodium Hydride is an inorganic compound with the empirical formula NaH. This alkali metal hydride is primarily used as a strong yet combustible 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 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.

Properties

Sodium hydride is a white solid, although commercial samples appear grey. It adopts a fluorite structure, similar to CaF2, which means that the Na+ centers occupy the octahedral holes, while the hydride anions (H−) occupy the tetrahedral holes. Each Na+ center 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.

Reactions

A reaction of sodium hydride with water produces sodium hydroxide (NaOH), hydrogen gas (H2), and heat: NaH + H2O → NaOH + H2↑ + heat

Sodium hydride is used as a base in organic synthesis, where it is preferred to sodium hydroxide because the latter typically contains water, which slows reactions that are sensitive to water. Sodium hydride is used to deprotonate carbonyl compounds, giving rise to enolates.

Safety

Sodium hydride is a powerful base, which can ignite in air, especially upon contact with water to release hydrogen, which is also flammable. Sodium hydride is usually handled in mineral oil for this reason.

See also

• Sodium borohydride
• Sodium amide
• Sodium aluminium hydride

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