He Sodium hydride Is an inorganic compound of the formula NaH. It has an ionic bond between sodium and hydride. Its structure is illustrated in figure 1.
NaH is representative of saline hydrides, which means 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.
Structure of sodium hydride.
The crystal structure has a coordination number 6, where each sodium molecule is surrounded by 8 hydride ions having an octahedral shape and is illustrated in Figure 2 (Mark Winter [The University of Sheffield and WebElements Ltd, 1993-2016]).
Figure 2. Crystalline 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 (wt%) dispersion form in mineral oil for safe handling (SODIUM HYDRIDE, s.f.).
You can also see Calcium Hydride: Properties, Reactivity, Storage and Uses .
Physical and chemical properties of sodium hydride
Sodium hydride is a white solid when it is pure although generally it is achieved in gray or silver. Their 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 Y Carbon disulfide (National Center for Biotechnology Information, s.f.).
The compound is extremely unstable. Pure NaH can easily ignite in the air. When it comes into contact with water present in the air, it releases highly flammable hydrogen. When open to air and moisture, NaH is also easily hydrolyzed in 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 as a base. This is due to electronegativity.
Sodium has a significantly lower electronegativity (≈1.0) than hydrogen (≈2,1), which means that hydrogen extracts the electron density towards itself, moving away from sodium to generate a sodium cation and an anion Of hydride.
For a compound to be a Brønsted's acid It needs to separate the density of electrons from hydrogen, ie, to 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 has a free electron pair. As such, it is a Brønsted base, not an acid. In fact, if the Brønsted acid / base definition is extended in the form Lewis He did, will come to the conclusion that sodium (Na +) is the acid species here.
The Brønsted acid / base reaction product of the H- base and H + acid becomes H2. Because the acidic hydrogen is extracted directly from the water, the hydrogen gas can bubble, shifting the equilibrium even if the reaction is not thermodynamically favored.
There may be OH- ions which can be written with the remainder of the Na + cation to give sodium hydroxide (Why is solid sodium hydride based and not acid when reacted with water?, 2016).
Reactivity and hazards
The compound is a powerful reducing agent. Attacks the SiO2 in glass. It is ignited on contact with F2, Cl2, Br2 and I2 gaseous (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 Dimethylsulfoxide . Reacts vigorously with acetylene, even at -60 ° C. It is spontaneously flammable in fluorine.
Initiates a polymerization reaction in ethyl-2,2,3-trifluoropropionate, in such a way 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 caustic byproducts of water reactions.
In case of contact with 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, immediately brush 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. Seek medical attention immediately and transfer the victim to a medical facility.
The dispersion of sodium hydride in oil is not dust. 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 care 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 their ability to function as a deprotonating agent in the acetoacetic ester, Condensations of Claisen , Stobbe, Dieckmann And other related reactions. It has marked advantages over other condensing agents in which:
- It is a stronger base, resulting in more direct deprotonation.
- No excess is needed.
- The produced H2 gives a measure of the extent of the reaction.
- Secondary reactions such as reductions are eliminated.
Alkylations of aromatic and heterocyclic amines such as 2-aminopyridine and Phenothiazine Are readily achieved in high yield using mixtures of toluene-methylformamide. The concentration of Dimethylformamide Is a variable used to control the rate of reaction (HINCKLEY, 1957).
It has been proposed to use sodium hydride for the storage of hydrogen for use in fuel cell vehicles, the hydride being enclosed in plastic granules which are ground in the presence of water to release the hydrogen.
References
- HINCKLEY, M. D. (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. (S.f.). PubChem Compound Database; CID = 24758. Retrieved from PubChem: pubchem.ncbi.nlm.nih.gov.
- Rohm and Haas 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. (S.f.). Retrieved from chemicalland21: chemicalland21.com.
- Why is solid sodium hydride based and not acid when reacted with water? (2016, April 20). Retrieved from stackexchange: chemistry.stackexchange.com.