Sodium hydride
Strong base and reducing agent in organic synthesis.
Sodium hydride (NaH) is an alkali metal hydride that serves as a strong, flammable base in organic synthesis. It is a saline hydride, made up of Na⁺ and H⁻ ions, unlike molecular hydrides such as borane or methane. Because H⁻ ions do not exist in solution, NaH is insoluble in all solvents except molten sodium metal.
Pure NaH is colorless, though samples usually appear grey. It is about 40% denser than sodium metal. Like other alkali metal hydrides, it crystallizes in the sodium chloride structure: each Na⁺ ion sits in an octahedral arrangement with six H⁻ centers. The ionic radius of H⁻ in NaH (146 pm) is close to that of F⁻ (133 pm), as seen in the similar Na–H and Na–F distances.
A rare compound called "inverse sodium hydride" contains H⁺ and Na⁻ ions. This alkalide differs from ordinary NaH by having a much higher energy content, due to the net shift of two electrons from hydrogen to sodium. A derivative forms in the presence of the base [36]adamanzane, which irreversibly encapsulates the H⁺ and keeps it away from the Na⁻. Theoretical work suggests that an unprotected protonated tertiary amine complexed with sodium alkalide might be metastable under certain solvent conditions, but the reaction barrier would be small and finding a suitable solvent would be challenging.
Industrially, NaH is made by mixing molten sodium into mineral oil with hydrogen at atmospheric pressure, stirred vigorously at about 8000 rpm. The reaction is especially fast at 250–300 °C. The resulting suspension of NaH in mineral oil is often used directly, for example in making diborane.
In organic synthesis, NaH is a versatile superbase. It deprotonates weak Brønsted acids—including alcohols, phenols, pyrazoles, and thiols—to give sodium derivatives. It also deprotonates carbon acids like 1,3-dicarbonyls (e.g., malonic esters), which can then be alkylated. NaH promotes condensation reactions such as the Dieckmann, Stobbe, Darzens, and Claisen condensations. It also deprotonates sulfonium salts and DMSO, and is used to make sulfur ylides for converting ketones into epoxides via the Johnson–Corey–Chaykovsky reaction.
As a reducing agent, NaH reduces certain main-group compounds, though this is rare in organic chemistry. For example, it reacts with boron trifluoride to give diborane and sodium fluoride. It also reduces Si–Si and S–S bonds in disilanes and disulfides. A composite reag
- formula
- NaH
- type
- Alkali metal hydride
- density
- ~40% denser than Na (0.968 g/cm3)
- crystal_structure
- NaCl structure
- color
- Colorless (samples generally appear grey)
- primary_use
- Strong base in organic synthesis
Lore & Background
Sodium hydride is a saline hydride, contrasting with molecular hydrides such as borane and methane. It adopts the NaCl crystal structure, with each Na+ ion surrounded by six H− centers in an octahedral geometry. The ionic radius of H− in NaH is 146 pm, comparable to that of F− at 133 pm. Industrially, NaH is prepared by introducing molten sodium into mineral oil with hydrogen at atmospheric pressure, mixed vigorously at around 8000 rpm, with the reaction especially rapid at 250–300 °C. The resultant suspension in mineral oil is often used directly, such as in the production of diborane.
A very unusual compound dubbed 'inverse sodium hydride' contains H+ and Na− ions, differing from ordinary sodium hydride by a much higher energy content due to the net displacement of two electrons from hydrogen to sodium. A derivative arises in the presence of the base [36]adamanzane, which irreversibly encapsulates the H+ and shields it from interaction with the alkalide Na−. Theoretical work has suggested that 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.
In organic synthesis, NaH is a superbase capable of deprotonating a range of weak Brønsted acids, including O-H, N-H, and S-H bonds, as well as carbon acids such as 1,3-dicarbonyls. It is widely used to promote condensation reactions via the Dieckmann, Stobbe, Darzens, and Claisen condensations. NaH also reduces certain main group compounds, such as boron trifluoride to diborane, and Si–Si and S–S bonds. It has been proposed for hydrogen storage, though not commercially significant, with one experimental implementation using plastic pellets containing NaH crushed in the presence of water to release hydrogen.
Reader's Guide
Sodium hydride holds significance as a versatile and powerful base in organic chemistry, enabling deprotonation of even weak acids and facilitating key condensation reactions. Its ionic nature and insolubility in most solvents make it distinct from molecular hydrides, and its preparation as a dispersion in mineral oil improves safety. The compound's reactivity with water and air requires careful handling, and its use in solvents like DMSO, DMF, or DMAc has been associated with fires and explosions. The existence of 'inverse sodium hydride' highlights unusual electronic configurations, though such species remain largely theoretical or highly specialized. While not commercially significant for hydrogen storage, NaH has been explored for that purpose, with challenges in regenerating NaH from NaOH. Overall, NaH remains a staple in synthetic laboratories for its strong basicity and reducing capabilities, despite safety concerns.
Did You Know?
- NaH is around 40% denser than sodium metal.
- The ionic radius of H− in NaH is 146 pm, comparable to that of F− at 133 pm.
- Industrially, NaH is prepared by mixing molten sodium with hydrogen in mineral oil at ~8000 rpm.
- A compound called 'inverse sodium hydride' contains H+ and Na− ions and has a much higher energy content than ordinary NaH.
More in Sodium Compounds 1-24
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