The sodium hydride is an inorganic compound of formula NaH. It has an ionic bond between sodium and hydride. Its structure is illustrated in figure 1. It is representative of saline hydrides, which means that it is a hydride similar to salt, composed of Na + and H- ions, in contrast to the more molecular hydrides such as borane, methane, ammonia and water.
The crystal structure has a coordination number of 6, where each sodium molecule is surrounded by 8 hydride ions presenting an octahedral shape and is illustrated in figure 2 (Mark Winter [The University of Sheffield and WebElements Ltd, 1993-2016).
Structure of sodium hydride.
Figure 2. Crystal structure of sodium hydride.
The compound is prepared by the direct reaction between sodium and hydrogen gas (Sodium hydride Formula - Sodium hydride Uses, Properties, Structure and Formula, 2005-2017) as follows:
H2 + 2Na → 2NaH
Sodium hydride is sold commercially as a 60% w / w (weight to weight percentage) dispersion form in mineral oil for safe handling (SODIUM HYDRIDE, nd).
Physical and chemical properties of sodium hydride
Sodium hydride is a white solid when it is pure, although it is generally gray or silver in color. Its appearance is shown in figure 3.
Figure 3. Appearance of sodium hydride.
NaH has a molecular weight of 23.99771 g / mol, a density of 1.396 g / ml and a melting point of 800 ° C (Royal Society of Chemistry, 2015). It is insoluble in ammonia, benzene, carbon tetrachloride, and carbon disulfide (National Center for Biotechnology Information, nd).
The compound is extremely unstable. Pure NaH can easily catch fire in air. When it comes in contact with water in the air, it releases highly flammable hydrogen.
When open to air and moisture, NaH is also easily hydrolyzed to the strong corrosive base of sodium hydroxide (NaOH) based on 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 significantly lower electronegativity (≈1.0) than hydrogen (≈2.1), which means that hydrogen draws electron density towards itself, moving away from sodium to generate a sodium cation and an anion 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 can be dissociated as such.
A hydride is much better described as H- and it has a free electron pair. As such, it is a Brønsted base, not an acid. In fact, if you extend the Brønsted acid / base definition the way Lewis did, you will conclude that sodium (Na +) is the acidic 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, shifting equilibrium even if the reaction is not thermodynamically favored.
OH- ions can remain that can be written with the rest of the Na + cation to give sodium hydroxide (Why is solid sodium hydride a base and not an acid when reacted with water ?, 2016).
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.
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.
Initiates a polymerization reaction in ethyl-2,2,3-trifluoropropionate, such that the ester decomposes violently. The presence in the reaction of diethyl succinate and ethyl trifluoroacetate has caused explosions (SODIUM HYDRIDE, 2016).
Sodium hydride is considered corrosive to the skin or eyes, due to the potential for caustic by-products 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 skin contact, 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.
Sodium hydride oil dispersion is not powder. However, the reacting material can emit a fine caustic mist. In case of inhalation, rinse mouth with water and remove victim to fresh air. Medical attention should be sought (Rhom and Hass inc., 2007).
Applications
The main use of sodium hydride is to carry out condensation and alkylation reactions that develop through the formation of a carbanion (catalyzed by the base).
Sodium hydride in oil resembles sodium and sodium metal alcoholates in its ability to function as a deprotonating agent in acetoacetic ester, Claisen, Stobbe, Dieckmann condensations, and other related reactions. It has marked advantages over other condensation 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 accomplished in high yield using toluene-methylformamide mixtures. The concentration of dimethylformamide is a variable used to control the reaction rate (HINCKLEY, 1957).
The use of sodium hydride for hydrogen storage has been proposed for use in fuel cell vehicles, the hydride being enclosed in plastic granules which are crushed in the presence of water to release hydrogen.
References
- HINCKLEY, MD (1957). Manufacture, Handling, and Uses of Sodium Hydride. Advances in Chemistry, Vol. 19, 106-117.
- Mark Winter [The University of Sheffield and WebElements Ltd, U. (1993-2016). Sodium: sodium hydride. Retrieved from WebElements: webelements.com.
- National Center for Biotechnology Information. (sf). PubChem Compound Database; CID = 24758. Retrieved from PubChem: pubchem.ncbi.nlm.nih.gov.
- Rhom and Hass inc. (2007, December). Sodium Hydride 60% Dispersion in Oil. Retrieved from dow.com.
- Royal Society of Chemistry. (2015). Sodium hydride. Retrieved from ChemSpider: chemspider.com.
- SODIUM HYDRIDE. (2016). Retrieved from cameochemicals: cameochemicals.noaa.gov.
- Sodium hydride Formula - Sodium hydride Uses, Properties, Structure and Formula. (2005-2017). Retrieved from Softschools.com: softschools.com.
- SODIUM HYDRIDE. (sf). Retrieved from chemicalland21: chemicalland21.com.
- Why is solid sodium hydride a base and not an acid when reacted with water? (2016, April 20). Retrieved from stackexchange: chemistry.stackexchange.com.