por What is sodium hydride?
He sodium hydride It is an inorganic compound with the formula NaH. It has an ionic bond between sodium and hydride. It is representative of salt hydrides, meaning that it is a salt-like hydride, composed of Na+ and H- ions, in contrast to more molecular hydrides such as borane, methane, ammonia, and water.
The crystalline structure has a coordination number 6, where each sodium molecule is surrounded by 8 hydride ions, presenting an octahedral shape.
The compound is prepared by the direct reaction between sodium and hydrogen gas as follows:
H2 + 2Na → 2NaH
Sodium hydride is sold commercially as a 60% w/w (percent weight to weight) dispersion in mineral oil for safe handling.
Physical and chemical properties of sodium hydride
Appearance: Sodium hydride is a white solid when pure, although gray or silver is usually available.
Molecular weight: 23.99771 g/mol.
Density: 1.396 g/ml.
Melting point: 800 °C.
It is insoluble in ammonia, benzene, carbon tetrachloride, and carbon disulfide.
The compound is extremely unstable. Pure NaH can easily ignite in air. When it comes into contact with water in the air, it releases highly flammable hydrogen. When open to air and moisture, NaH is also readily hydrolyzed into the strong corrosive base of sodium hydroxide (NaOH) according to the reaction:
NaH + H₂O → NaOH + H2
In this reaction it can be seen that sodium hydride behaves like a base. This is due to electronegativity.
Sodium has a significantly lower electronegativity (≈1.0) than hydrogen (≈2.1), meaning that hydrogen draws electron density toward itself, away from sodium to generate a sodium cation and anion of hydride.
For a compound to be a Brønsted acid it needs to separate the electron density of hydrogen, that is, connect it to an electronegative atom such as oxygen, fluorine, nitrogen, etc. Only then can it be formally described as H+ and dissociated as such. A hydride is much better described as H- and has a lone pair of electrons. As such, it is a Brønsted base, not an acid. In fact, if you extend the Brønsted acid/base definition the way Lewis did, he will conclude that sodium (Na+) is the acid species here.
The Brønsted acid/base reaction product of H- base and H+ acid becomes H2. Because acidic hydrogen is extracted directly from water, hydrogen gas can bubble up, shifting the equilibrium even if the reaction is not thermodynamically favoured. OH- ions may remain, which can be written with the remainder of the Na+ cation to give sodium hydroxide.
Reactivity and hazards
The compound is a powerful reducing agent. Attacks SiO2 in glass. It ignites on contact with gaseous F2, Cl2, Br2 and I2 (the latter at temperatures above 100 °C), especially in the presence of moisture, to form HF, HCl, HBr and HI.
It reacts with sulfur to give Na2S and H2S. May react explosively with dimethyl sulfoxide. Reacts vigorously with acetylene, even at -60 °C. It is spontaneously flammable in fluorine.
Starts a polymerization reaction in ethyl-2,2,3-trifluoropropionate, such that the ester decomposes violently. The presence of diethyl succinate and ethyl trifluoroacetate in the reaction has caused explosions.
Sodium hydride is considered corrosive to the skin or eyes, due to the potential for caustic byproducts of reactions with water.
In case of contact with the eyes, they should be rinsed with large amounts of water under the eyelids for at least 15 minutes and seek medical attention immediately.
In case of contact with the skin, brush immediately and rinse the affected area with water. Seek medical attention if irritation persists.
Harmful if swallowed due to reaction to water. Do not induce vomiting. Medical attention should be sought immediately and the victim transferred to a medical facility.
The dispersion of sodium hydride in oil is not powdery. However, the reacting material may emit a fine caustic mist. In case of inhalation, rinse the mouth with water and move the victim to fresh air. Medical attention should be sought.
Applications
The main use of sodium hydride is to carry out condensation and alkylation reactions that proceed through the formation of a carbanion (catalyzed by the base).
Sodium hydride in oil resembles sodium alcoholates and sodium metal in its ability to function as a deprotonating agent in acetoacetic ester, Claisen, Stobbe, Dieckmann, and other related reactions. It has marked advantages over other condensing agents in that:
It is a stronger base, which results in a more direct deprotonation.
No excess is needed.
The H2 produced gives a measure of the extent of the reaction.
Side reactions such as reductions are eliminated.
Alkylations of aromatic and heterocyclic amines such as 2-aminopyridine and phenothiazine are easily achieved in high yield using toluene-methylformamide mixtures. The concentration of dimethylformamide is a variable used to control the reaction rate.
The use of sodium hydride for the storage of hydrogen for use in fuel cell vehicles has been proposed, the hydride being enclosed in plastic pellets which are crushed in the presence of water to release the hydrogen.
References
HINCKLEY, MD Manufacture, Handling, and Uses of Sodium Hydride. Advances in Chemistry.
Mark Winter. Sodium: sodium hydride. Recovered from webelements.com.
Sodium Hydride 60% Dispersion in Oil. Retrieved from dow.com.
Sodium hydride. Retrieved from chemspider.com.