"Inverse sodium hydride" (hydrogen sodide)

A very unusual situation occurs in a compound dubbed "inverse sodium hydride", which contains H⁺ and Na⁻ 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 [3⁶]adamanzane. This molecule irreversibly encapsulates the H⁺ and shields it from interaction with the alkalide Na⁻.^[9] 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.^[10]

As a strong base

NaH is a base of wide scope and utility in organic chemistry.^[12] As a superbase, 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 notably deprotonates carbon acids (i.e., C-H bonds) such as 1,3-dicarbonyls 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 very rare in organic chemistry (see below).^[13] Notably boron trifluoride reacts to give diborane and sodium fluoride:^[14]

6 NaH + 2 BF₃ → B₂H₆ + 6 NaF

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

A series of reduction reactions, including the hydrodecyanation of tertiary nitriles, reduction of imines to amines, and amides to aldehydes, can be effected by a composite reagent composed of sodium hydride and an alkali metal iodide (NaH⋅MI, M = Li, Na).^[15]

Hydrogen storage

Although not commercially significant sodium hydride has been proposed for hydrogen storage for use in fuel cell vehicles. In one experimental implementation, plastic pellets containing NaH are crushed in the presence of water to release the hydrogen. One challenge with this technology is the regeneration of NaH from the NaOH formed by hydrolysis.^[16]