Sodium hydride is the
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with the
empirical formula
In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is the ...
Na H. This
alkali metal hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...
is primarily used as a strong yet combustible
base in
organic synthesis. NaH is a saline (salt-like)
hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...
, composed of Na
+ and H
− ions, in contrast to molecular hydrides such as
borane
Trihydridoboron, also known as borane or borine, is an unstable and highly reactive molecule with the chemical formula . The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated ...
,
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
,
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
, and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
. It is an ionic material that is insoluble in organic solvents (although soluble in molten Na), consistent with the fact that H
− ions do not exist 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.
[Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. .] Pure NaH is colorless, although samples generally appear grey. NaH is ca. 40% denser than Na (0.968 g/cm
3).
NaH, like
LiH
Lithium hydride is an inorganic compound with the formula Lithium, LiHydride, H. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a Hydride#Ionic hydrides, salt-like (ionic) hydride, it has a ...
,
KH,
RbH, and
CsH, adopts the
NaCl
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
. In this motif, each Na
+ ion is surrounded by six H
− centers in an
octahedral
In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...
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.
"Inverse sodium hydride"
A very unusual situation occurs in a compound dubbed "inverse sodium hydride", which contains H
+ and Na
− ions. Na
− is an
alkalide An alkalide is a chemical compound in which alkali metal atoms are anions (negative ions) with a charge or oxidation state of −1. Until the first discovery of alkalides in the 1970s, alkali metals were known to appear in salts only as cations ...
, 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
6damanzane">
6damanzane. This molecule irreversibly encapsulates the H
+ and shields it from interaction with the alkalide Na
−. 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.
Applications in organic synthesis
As a strong base
NaH is a base of wide scope and utility in organic chemistry. As a
superbase
A superbase is a compound that has a particularly high affinity for protons. Superbases are of theoretical interest and potentially valuable in organic synthesis. Superbases have been described and used since the 1850s.''Superbases for Organic ...
, 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,
phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...
s,
pyrazole
Pyrazole is an organic compound with the formula C3H3N2H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen atoms, which are in ortho-substitution. Pyrazole is a weak base, with p''K''b 11.5 ( ...
s, and
thiol
In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...
s.
NaH notably deprotonates carbon acids (i.e., C-H bonds) such as 1,3-
dicarbonyl
In organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain ...
s 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
The Dieckmann condensation is the intramolecular chemical reaction of diesters with base to give β-keto esters. It is named after the German chemist Walter Dieckmann (1869–1925). The equivalent intermolecular reaction is the Claisen condensat ...
,
Stobbe condensation
The Claisen condensation is a carbon–carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base, resulting in a β-keto ester or a β-diketone. It is named after Ra ...
,
Darzens condensation, and
Claisen condensation
The Claisen condensation is a carbon–carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base, resulting in a β-keto ester or a β-diketone. It is named after Ra ...
. Other carbon acids susceptible to deprotonation by NaH include sulfonium salts and
DMSO
Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds ...
. NaH is used to make
sulfur ylides, which in turn are used to convert
ketones into
epoxide
In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale ...
s, as in the
Johnson–Corey–Chaykovsky reaction
The Johnson–Corey–Chaykovsky reaction (sometimes referred to as the Corey–Chaykovsky reaction or CCR) is a chemical reaction used in organic chemistry for the synthesis of epoxides, aziridines, and cyclopropanes. It was discovered in 1961 b ...
.
As a reducing agent
NaH reduces certain main group compounds, but analogous reactivity is very rare in organic chemistry (''see below''). Notably
boron trifluoride reacts to give
diborane
Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
and
sodium fluoride
Sodium fluoride (NaF) is an inorganic compound with the formula . It is used in trace amounts in the fluoridation of drinking water, in toothpaste, in metallurgy, and as a flux. It is a colorless or white solid that is readily soluble in water. I ...
:
[
:6 NaH + 2 BF3 → B2H6 + 6 NaF
Si–Si and S–S bonds in ]disilane
Disilane is a chemical compound with chemical formula Si2H6 that was identified in 1902 by Henri Moissan and Samuel Smiles (1877–1953). Moissan and Smiles reported disilane as being among the products formed by the action of dilute acids on meta ...
s and disulfide
In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
s 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).
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.
Practical considerations
Sodium hydride is sold as a mixture of 60% sodium hydride (w/w) in mineral oil
Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils.
The name 'mineral oil' by itself is imprecise, ...
. Such a dispersion is safer to handle and weigh than pure NaH. The compound is often used in this form but the pure grey solid can be prepared by rinsing the commercial product with pentane or THF, with care being taken because the waste solvent will contain traces of NaH and can ignite in air. Reactions involving NaH require air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less ...
s. Typically NaH is used as a suspension in THF
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
, a solvent that resists attack by strong bases but can solvate many reactive sodium compounds.
Safety
NaH can ignite spontaneously in air. It also reacts vigorously with water to release hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
, which is also flammable, and 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. Although sodium hydride is widely used in DMSO, DMF or DMA, there have been many cases of fires and/or explosions from such mixtures.
References
Cited sources
*
{{Hydrides by group
Metal hydrides
Reagents for organic chemistry
Sodium compounds
Superbases
Rock salt crystal structure
Semiconductor materials