Sodium hydride
Sodium hydride is the chemical compound with the empirical formula NaH. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline hydride, composed of and ions, in contrast to molecular hydrides such as borane, silane, germane, ammonia, and methane. It is an ionic material that is insoluble in all solvents, consistent with the fact that ions do not exist in solution.
Basic properties and structure
NaH is colorless, although samples generally appear grey. NaH is around 40% denser than Na.NaH, like LiH, KH, RbH, and CsH, adopts the NaCl crystal structure. In this motif, each ion is surrounded by six centers in an octahedral geometry. The ionic radii of and are comparable, as judged by the and distances.
"Inverse sodium hydride" (hydrogen sodide)
A very unusual situation occurs in a compound dubbed "inverse sodium hydride", which contains and ions. 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|adamanzane. This molecule irreversibly encapsulates the and shields it from interaction with the alkalide. 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.Preparation
Industrially, NaH is prepared by introducing molten sodium into mineral oil with hydrogen at atmospheric pressure and mixed vigorously at ~8000 rpm. The reaction is especially rapid at.The resultant suspension of NaH in mineral oil is often directly used, such as in the production of diborane.
Applications in organic synthesis
As a strong base
NaH is a base of wide scope and utility in organic chemistry. 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 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. Notably boron trifluoride reacts to give diborane and sodium fluoride: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.