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 representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na+ and H− ions, in contrast to the more molecular hydrides such as borane, methane, ammonia and water. It is an ionic material that is insoluble in organic solvents (although soluble in molten Na), consistent with the fact that H− remains an unknown anion in solution. Because of the insolubility of NaH, all reactions involving NaH occur at the surface of the solid.




Contents





  • 1 Basic properties and structure

    • 1.1 "Inverse sodium hydride"



  • 2 Applications in organic synthesis

    • 2.1 As a strong base


    • 2.2 As a reducing agent


    • 2.3 Hydrogen storage



  • 3 Practical considerations


  • 4 Safety


  • 5 References




Basic properties and structure


NaH is produced by the direct reaction of hydrogen and liquid sodium.[4] Pure NaH is colorless, although samples generally appear grey. NaH is ca. 40% denser than Na (0.968 g/cm3).


NaH, like LiH, KH, RbH, and CsH, adopts the NaCl crystal structure. In this motif, each Na+ ion is surrounded by six H− centers in an octahedral 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.[5]



"Inverse sodium hydride"


A very unusual situation occurs in a compound dubbed "inverse sodium hydride", which contains Na− and H+ 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 adamanzane. This molecule irreversibly encapsulates the H+ and shields it from interaction with the alkalide Na−.[6] 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.[7]



Applications in organic synthesis



As a strong base


NaH is a base of wide scope and utility in organic chemistry.[8] 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 most notably is employed to deprotonate carbon acids such as 1,3-dicarbonyls and analogues 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).[9] Notably boron trifluoride reacts to give diborane and sodium fluoride:[4]


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).[10]



Hydrogen storage


The use of sodium hydride has been proposed for hydrogen storage for use in fuel cell vehicles, the hydride being encased in plastic pellets which are crushed in the presence of water to release the hydrogen.[11]



Practical considerations


Sodium hydride is sold by many chemical suppliers usually as a mixture of 60% sodium hydride (w/w) in mineral 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 oil with pentane or THF, with care being taken because the washings will contain traces of NaH that can ignite in air. Reactions involving NaH require an inert atmosphere, such as nitrogen or argon gas. Typically NaH is used as a suspension in THF, a solvent that resists deprotonation but solvates many organosodium compounds.



Safety


NaH can ignite in air, especially upon contact with water to release hydrogen, which is also flammable. Hydrolysis converts NaH into 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.[12]



References




  1. ^ ab Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X..mw-parser-output cite.citationfont-style:inherit.mw-parser-output qquotes:"""""""'""'".mw-parser-output code.cs1-codecolor:inherit;background:inherit;border:inherit;padding:inherit.mw-parser-output .cs1-lock-free abackground:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration abackground:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center.mw-parser-output .cs1-lock-subscription abackground:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registrationcolor:#555.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration spanborder-bottom:1px dotted;cursor:help.mw-parser-output .cs1-hidden-errordisplay:none;font-size:100%.mw-parser-output .cs1-visible-errorfont-size:100%.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-formatfont-size:95%.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-leftpadding-left:0.2em.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-rightpadding-right:0.2em


  2. ^ Index no. 001-002-00-4 of Annex VI, Part 3, to Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006. OJEU L353, 31.12.2008, pp 1–1355 at p 340.


  3. ^ Inc, New Environment,. "New Environment Inc. - NFPA Chemicals". www.newenv.com. Archived from the original on 2016-08-27.


  4. ^ ab Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001.
    ISBN 0-12-352651-5.


  5. ^ Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press


  6. ^ Mikhail Y. Redko; et al. (2002). ""Inverse Sodium Hydride":  A Crystalline Salt that Contains H+ and Na-". J. Am. Chem. Soc. 124 (21): 5928–5929. doi:10.1021/ja025655+.


  7. ^ Agnieszka Sawicka; Piotr Skurski & Jack Simons (2003). "Inverse Sodium Hydride: A Theoretical Study" (PDF). J. Am. Chem. Soc. 125 (13): 3954–3958. doi:10.1021/ja021136v. PMID 12656631. Archived (PDF) from the original on 2013-02-09.


  8. ^ Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.


  9. ^ Too, Pei Chui; Chan, Guo Hao; Tnay, Ya Lin; Hirao, Hajime; Chiba, Shunsuke (2016-03-07). For early examples of NaH acting as a hydride donor, see ref. [3] therein. "Hydride Reduction by a Sodium Hydride–Iodide Composite". Angewandte Chemie International Edition. 55 (11): 3719–3723. doi:10.1002/anie.201600305. ISSN 1521-3773. PMC 4797714. PMID 26878823.


  10. ^ Ong, Derek Yiren; Tejo, Ciputra; Xu, Kai; Hirao, Hajime; Chiba, Shunsuke (2017-01-01). "Hydrodehalogenation of Haloarenes by a Sodium Hydride–Iodide Composite". Angewandte Chemie International Edition: n/a–n/a. doi:10.1002/anie.201611495. ISSN 1521-3773.


  11. ^ J. Philip DiPietro; Edward G. Skolnik (October 1999). "Analysis of the Sodium Hydride-based Hydrogen Storage System being developed by PowerBall Technologies, LLC" (PDF). US Department of Energy, Office of Power Technologies. Archived (PDF) from the original on 2006-12-13. Retrieved 2009-09-01.


  12. ^ "The Dow Chemical Company - Home". www.rohmhaas.com.



Sodium hydride

Sodium hydride

Sodium-hydride-3D-vdW.png
Identifiers

CAS Number



  • 7646-69-7 ☑Y


3D model (JSmol)


  • Interactive image


ChemSpider


  • 23144 ☑Y


ECHA InfoCard

100.028.716

EC Number
231-587-3


PubChem CID


  • 24758





Properties

Chemical formula

NaH

Molar mass
23.99771 g/mol
Appearance
white or grey solid

Density
1.396 g/cm3

Melting point
 300 °C (572 °F; 573 K) decomposes

Solubility in water

Reacts with water

Solubility
insoluble in ammonia, benzene, CCl4, CS2


Refractive index (nD)

1.470
Structure

Crystal structure


fcc (NaCl), cF8

Space group

Fm3m, No. 225

Lattice constant


a = 498 pm

Coordination geometry

Octahedral (Na+)
Octahedral (H−)
Thermochemistry


Heat capacity (C)

36.4 J/mol K


Std molar
entropy (So298)

40 J·mol−1·K−1[1]


Std enthalpy of
formation (ΔfHo298)

−56.4 kJ·mol−1[1]


Gibbs free energy (ΔfG˚)

-33.5 kJ/mol
Hazards[2]
Main hazards
highly corrosive, pyrophoric in air, reacts violently with water

Safety data sheet

External MSDS

GHS pictograms

Water-react. 1

GHS signal word
DANGER

GHS hazard statements


H260

NFPA 704



Flammability code 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g., gasolineHealth code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond

3


3


2

W




Flash point
combustible
Related compounds

Other cations


Lithium hydride
Potassium hydride

Related compounds


Sodium borohydride

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).


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