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Potassium hydride: structure, formation, properties

Hydrides are chemical compounds made up of hydrogen and one or more other elements, metallic or non-metallic in nature. Depending on their structure and characteristics, these substances can be of three classes: ionic, covalent or interstitial hydrides.

Ionic model of potassium hydride

By having the nature of an ionic compound, potassium hydride is made up of an anion (in this case, the hydride ion H  ) and a cation (the potassium ion K + ).

Chemical structure of potassium hydride

Potassium was first identified experimentally in 1807 by the British chemist Sir Humphry Davy, as well as other chemical elements (calcium, magnesium, boron, strontium and barium) using the electrolysis technique.

This scientist was also the one who discovered the chemical reaction that results in the formation of potassium hydride, which occurs in its pure form as a white solid, although commercially available reagents are gray.

The structure of this binary hydride is characterized by being crystalline, specifically of the cubic type, that is, the unit cell of this crystal is a face-centered cube, as seen in the previous figure. 

The reactions carried out by metal hydrides occur on the crystalline surface, and this hydride has shown to have the hydride radius and the optimal lattice energy for this type of reaction, even above the hydrides of other metals.

Training

Potassium hydride, whose formula is represented as KH, is an inorganic substance that is classified as alkali metal hydride because it is formed by directly combining molecular hydrogen with potassium through the following reaction:

This reaction was discovered by the same scientist who first identified potassium. He noticed how this metal vaporized when exposed to a stream of hydrogen gas, when its temperature was increased below its boiling point .

A potassium hydride that possesses superior activity can also be produced in a simple way, starting from a reaction of hydrogen and other super-basic compounds (such as potassium tert-butoxide, called t-BuOK-TMEDA), and being prepared in hexane.

Properties

Potassium hydride is not found spontaneously in nature. It is produced from the reaction described above and is found as a crystalline solid, which decomposes at a temperature around 400 ° C, before reaching its melting point.

This compound has a molar mass of approximately 40.106 g / mol due to the combination of the molar masses of its two components. In addition, its density is 1.43 g / cm 3 (taking as a reference point that of water under standard conditions, which is 1.00 g / cm 3 ).

In this sense, it is also known that this compound has pyrophoric properties; that is, it can ignite spontaneously in the presence of air, as well as oxidizing agents and certain gases.

For this reason, it should be treated with caution and contained as a suspension in a mineral oil or even paraffin wax, thus reducing its pyrophoricity and facilitating its handling.

Solubility

Regarding its solubility, this hydride is considered soluble in molten hydroxides (such as fused sodium hydroxide), as well as in saline mixtures. Instead, it is insoluble in solvents of organic origin such as diethyl ether, benzene or carbon disulfide.

In the same way, it is considered a fairly corrosive substance, which also manifests a violent reaction when it comes into contact with acidic compounds, interacting in a quantitative relationship.

This species also behaves as a “superbase” considered even stronger than the sodium hydride compound; Furthermore, it has the character of a donor of hydride ions.

Applications

The commercially available potassium hydride, formed through the reaction of molecular hydrogen with elemental potassium, has a reactivity that is related to the impurities it has (primarily potassium or its reaction products), which leads to side reactions and yields that may vary.

Its nature of extreme basicity makes it very useful to carry out certain organic syntheses, as well as in deprotonation processes of certain substances that have carbonyl groups to give rise to enolate compounds.

Likewise, potassium hydride is used in the transformation of certain amines into their corresponding amides (amides with alkyl chains of the KNHR and KNR 2 type ), by means of their deprotonation. Similarly, it carries out rapid deprotonation in tertiary alcohols.

As well as being an excellent deprotonator, this compound is also used in some elimination, cyclization-condensation and molecular rearrangement reactions, and constitutes an excellent reducing agent.

In other types of reactions, a crown ether can act as a phase transfer agent, although it can also act as a simple “pickling” agent (process to remove impurities) from the surface of the potassium hydride, through the dissolution of the inorganic salts that are formed.

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