Chemistry:Potassium hydride

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Potassium hydride
Potassium-hydride-3D-vdW.png
Identifiers
3D model (JSmol)
ChemSpider
EC Number
  • 232-151-5
Properties
KH
Molar mass 40.1062 g/mol
Appearance colourless crystals
Density 1.43 g/cm3[1]
Melting point decomposes at ~400 °C[2]
reacts
Solubility insoluble in benzene, diethyl ether, CS2
Structure
cubic, cF8
Fm3m, No. 225
Thermochemistry
37.91 J/(mol K)
-57.82 kJ/mol
Hazards
Main hazards very corrosive, pyrophoric in air, and reacts violently with acids
NFPA 704 (fire diamond)
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. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acidNFPA 704 four-colored diamond
3
3
2
Related compounds
Other cations
Lithium hydride
Sodium hydride
Rubidium hydride
Caesium hydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Potassium hydride, KH, is the inorganic compound of potassium and hydrogen. It is an alkali metal hydride. It is a white solid, although commercial samples appear gray. A powerful superbase that is useful in organic synthesis. It is sold commercially as a slurry (~35%) in mineral oil or sometimes paraffin wax to facilitate dispensing.[3]

Preparation

Potassium hydride is produced by direct combination of the metal and hydrogen:

2 K + H2 → 2 KH

This reaction was discovered by Humphry Davy soon after his 1807 discovery of potassium, when he noted that the metal would vaporize in a current of hydrogen when heated just below its boiling point.[4]:p.25

Potassium hydride is soluble in fused hydroxides (such as molten sodium hydroxide) and salt mixtures, but not in organic solvents.[5]

Reactions

KH reacts with water according to the reaction:

KH + H2O → KOH + H2

As a superbase, potassium hydride is more basic than sodium hydride. It is used to deprotonate certain carbonyl compounds to give enolates. It also deprotonates amines to give the corresponding amides of the type KNHR and KNR2.[6]

Safety

KH can be pyrophoric in air, react violently with acids, and can ignite upon contact with oxidants. As a suspension in mineral oil, KH is less dangerous.

See also

References

  1. Robert E. Gawley, Xiaojie Zhang, Qunzhao Wang, "Potassium Hydride" Encyclopedia of Reagents for Organic Synthesis 2007 John Wiley & Sons. doi:10.1002/047084289X.rp223.pub2
  2. David Arthur Johnson; Open University (12 August 2002). Metals and chemical change. Royal Society of Chemistry. pp. 167–. ISBN 978-0-85404-665-2. https://books.google.com/books?id=dzjxzsKjZGUC&pg=PA167. Retrieved 1 November 2011. 
  3. Douglass F. Taber, Christopher G. Nelson (2006). "Potassium Hydride in Paraffin: A Useful Base for Organic Synthesis". J. Org. Chem. 71 (23): 8973–8974. doi:10.1021/jo061420v. PMID 17081034. 
  4. Humphry Davy (1808), The Bakerian Lecture on some new phenomena of chemical changes produced by electricity, particularly the decomposition of fixed alkalies, and the exhibition of the new substances which constitute their bases; and on the general nature of alkaline bodies. Philosophical Transactions of the Royal Society, volume 88, pages 1–44. In The Development of Chemistry, 1789–1914: Selected essays, edited by D. Knight, pp. 17–47.
  5. Pradyot Patnaik (1 July 2007). A Comprehensive Guide to the Hazardous Properties of Chemical Substances. John Wiley and Sons. pp. 631–. ISBN 978-0-470-13494-8. https://books.google.com/books?id=WdmWkkKF1UoC&pg=PA631. Retrieved 1 November 2011. 
  6. Charles A. Brown, Prabhakav K. Jadhav (1925). "(−)-α-Pinene by Isomerization of (−)-β-Pinene". Organic Syntheses 65: 224. http://www.orgsyn.org/demo.aspx?prep=CV8P0553. ; Collective Volume, 8, pp. 553