 | |
 Sodium cation,Na+ Hydrogen anion,H− | |
| Names | |
|---|---|
| IUPAC name Sodium hydride | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.028.716 |
| EC Number |
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| UNII | |
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| Properties | |
| NaH | |
| Molar mass | 23.998 g/mol[1] |
| Appearance | white or grey solid |
| Density | 1.39 g/cm3[1] |
| Melting point | 638 °C (1,180 °F; 911 K)(decomposes)[1] |
| Reacts with water[1] | |
| Solubility | insoluble in all solvents |
| Band gap | 3.51 eV(predicted)[2] |
Refractive index (nD) | 1.470[3] |
| Structure | |
| fcc (NaCl),cF8 | |
| Fm3m, No. 225 | |
a = 498 pm | |
Formula units (Z) | 4 |
| Octahedral (Na+) Octahedral (H−) | |
| Thermochemistry[5][4] | |
| 36.4 J/mol K | |
Std molar entropy(S⦵298) | 40.0 J·mol−1·K−1[4] |
Std enthalpy of formation(ΔfH⦵298) | −56.3 kJ·mol−1 |
Gibbs free energy(ΔfG⦵) | −33.5 kJ/mol |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | highly corrosive, reacts violently with water or humid air. |
| GHS labelling:[6] | |
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| Danger | |
| H260 | |
| NFPA 704 (fire diamond) | |
| Flash point | combustible |
| Safety data sheet (SDS) | External MSDS |
| Related compounds | |
Otheranions | Sodium borohydride Sodium hydroxide |
Othercations | Lithium hydride Potassium hydride Rubidium hydride Caesium hydride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Sodium hydride is thechemical compound with theempirical formulaNaH. Thisalkali metal hydride is primarily used as a strong yet combustiblebase inorganic synthesis. NaH is a saline (salt-like)hydride, composed of Na+ and H− ions, in contrast to molecular hydrides such asborane,silane,germane,ammonia, andmethane. It is an ionic material that is insoluble in all solvents (other than molten sodium metal), consistent with the fact that H− ions do not exist in solution.
NaH is colorless, although samples generally appear grey. NaH is around 40% denser than Na (0.968 g/cm3).
NaH, likeLiH,KH,RbH, andCsH, adopts theNaClcrystal structure. In this motif, each Na+ ion is surrounded by six H− centers in anoctahedral geometry. Theionic radii of H− (146 pm in NaH) and F− (133 pm) are comparable, as judged by the Na−H and Na−F distances.[8]
A very unusual situation occurs in a compound dubbed "inverse sodium hydride", which contains H+ and Na− ions. Na− is analkalide, 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]
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 250−300 °C.
The resultant suspension of NaH in mineral oil is often directly used, such as in the production ofdiborane.[11]
NaH is a base of wide scope and utility in organic chemistry.[12] As asuperbase, it is capable ofdeprotonating a range of even weakBrønsted acids to give the corresponding sodium derivatives. Typical "easy" substrates contain O-H, N-H, S-H bonds, includingalcohols,phenols,pyrazoles, andthiols.
NaH notably deprotonates carbon acids (i.e., C-H bonds) such as 1,3-dicarbonyls such asmalonic esters. The resulting sodium derivatives can be alkylated. NaH is widely used to promote condensation reactions of carbonyl compounds via theDieckmann condensation,Stobbe condensation,Darzens condensation, andClaisen condensation. Other carbon acids susceptible to deprotonation by NaH include sulfonium salts andDMSO. NaH is used to makesulfurylides, which in turn are used to convertketones intoepoxides, as in theJohnson–Corey–Chaykovsky reaction.
NaH reduces certain main group compounds, but analogous reactivity is very rare in organic chemistry (see below).[13] Notablyboron trifluoride reacts to givediborane andsodium fluoride:[14]
Si–Si and S–S bonds indisilanes anddisulfides 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]
Although not commercially significant sodium hydride has been proposed for hydrogen storage for use infuel 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]
Sodium hydride is sold as a mixture of 60% sodium hydride (w/w) inmineral oil. 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 tetrahydrofuran, with care being taken because the waste solvent will contain traces of NaH and can ignite in air. Reactions involving NaH usually requireair-free techniques.
NaH canignite spontaneously in air. It also reacts vigorously with water or humid air to releasehydrogen, which is very flammable, andsodium hydroxide (NaOH), a quite corrosivebase. In practice, most sodium hydride is sold as a dispersion inmineral oil, which can be safely handled in air.[17] Although sodium hydride is widely used inDMSO,DMF orDMAc forSN2 type reactions there have been many cases of fires and/or explosions from such mixtures.[18][19]
