Sodium hydride

Description

Sodium hydride, is a binary salt that has a specific hazard of releasing hydrogen upon contact with water. It is an odorless powder that is violently water reactive. The four-digit UN identification number is 1427. The NFPA 704 designation is health 3, flammability 3, and reactivity 2. The white space at the bottom of the diamond has a W with a slash through it, indicating water reactivity.

Description

Sodium hydride (NaH) is a strong base commonly used in organic chemistry to deprotonate alcohols, amine, amides, and other sufficiently acidic protons. Pure sodium hydride is prone to spontaneous ignition in moist air. For this reason, the most common form of sodium hydride used in labs is 60% sodium hydride dispersed on mineral oil.

Chemical properties

Grey solid. Pure sodium hydride forms colorless cubic crystals; however, the commercial product contains traces of sodium metal, giving it a light gray color. At atmospheric pressure, sodium hydride slowly evolves hydrogen above 300 ℃. At 420 ℃ decomposition is rapid but melting does not take place. Sodium hydride is a salt and therefore insoluble in inert organic solvents. It dissolves in molten sodium hydroxide, in sodium – potassium alloys and in molten LiCl – KCl eutectic mixtures (352 ℃). Sodium hydride is stable in dry air but ignites above 230 ℃, burning to form sodium oxide. It is hydrolyzed rapidly in moist air and as a dry powder it is spontaneously flammable. Sodium hydride reacts extremely violently with water, the heat of hydrolysis being sufficient to ignite the liberated hydrogen. It reacts with carbon dioxide to form sodium formate.

Physical properties

Silvery needles; refractive index 1.470; density 0.92 g/cm³; decomposes at 800°C; decomposes explosively in water; reacts violently with lower alcohols; dissolves in molten sodium and molten sodium hydroxide; insoluble in liquid ammonia, benzene, carbon tetrachloride and carbon disulfide. Sodium hydride is available commercially as 80 % and 60 % dispersions in mineral oil. The light-gray pourable material has an average particle size of 10 mm (Degussa, Chemetall, Ventron). The oil protects the surface of the highly reactive sodium hydride, which easily becomes passivated by the slightest atmospheric exposure or, worse, can ignite on longer contact with air.

The Uses of Sodium hydride

Sodium hydride is used to enhance the condensation reactions of carbonyl compounds through the Dieckmann condensation, Stobbe condensation, Darzens condensation and Claisen condensation. It acts as a reducing agent used to prepare diborane from boron trifluoride. It is also used in fuel cell vehicles. Further, it is used to dry some organic solvents. In addition to this, it is involved in the preparation of sulfur ylides, which is utilized for the conversion of ketones into epoxides.

The Uses of Sodium hydride

At low temperatures where reducing properties of sodium are undesirable as in the condensation of ketones and aldehydes with acid esters; in solution with molten sodium hydroxide for the reduction of oxide scale on metals; at high temperatures as a reducing agent and reduction catalyst.

The Uses of Sodium hydride

Direct intercalation into C₆₀ results in the superconducting material (NaH)₄C₆₀.

Definition

sodium hydride: A white crystallinesolid, NaH; cubic; r.d. 0.92; decomposesabove 300°C (slow);completely decomposed at 800°C.Sodium hydride is prepared by thereaction of pure dry hydrogen withsodium at 350°C. Electrolysis ofsodium hydride in molten LiCl/KClleads to the evolution of hydrogen;this is taken as evidence for the ionicnature of NaH and the presence ofthe hydride ion (H–). It reacts violentlywith water to give sodium hydroxideand hydrogen, with halogensto give the halide and appropriatehydrogen halide, and ignites spontaneouslywith oxygen at 230°C. It is apowerful reducing agent with severallaboratory applications.

Production Methods

Sodium hydride, reactive with water yielding hydrogen gas and NaOH solution, formed by reaction of sodium and hydrogen at about 360 °C (680 °F). Used as a powerful reducing agent.

Reactions

The most important chemical property of sodium hydride is its basicity. Sodium hydride reacts irreversibly with a large number of compounds containing acidic hydrogen atoms, including alcohols, phenols, thiols, and amines, but also with weak carbon acids such as ketones, carboxylic esters, and acetylenic hydrocarbons. Sodium hydride is a poor reducing agent because of its high basicity and its insolubility in inert organic solvents. If the sodium hydride is complexed with alcoholates and metallic salts, complex reducing agents are formed that can reduce halides, alkenes, and ketones.

General Description

A silvery to whitish powder or slurry in oil. Used to make other chemicals.

Air & Water Reactions

Highly flammable. Ignites or explodes in contact with air of high humidity [Bretherick 1979 p. 107]. Reacts violently with water producing a caustic solution (NaOH) and hydrogen (H2). Heat of reaction may ignite the hydrogen.

Reactivity Profile

Sodium hydride is a powerful reducing agent. Attacks SiO2 in glass. Ignites on contact with gaseous F2, Cl2, Br2, and I2 (the last at temperatures exceeding 100°C), especially in the presence of moisture, to form HF, HCl, HBr, and HI [Mellor 2:483 1946-47]. Reacts with sulfur to give Na2S and H2S [Bretherick 1979 p. 107]. Can react explosively with dimethyl sulfoxide [Chem. Eng. News 44(24):7 1966]. Reacts vigorously with acetylene, even at -60°C [Mellor 2:483 1946-47]. Spontaneously flammable in fluorine. Reaction with dimethylformamide, when heated, runs away [Chem. Eng. News, 1982, 60(28), 5]. Initiates a polymerization reaction in ethyl-2,2,3-trifluoropropionate such that the ester decomposed violently [Bretherick 5th ed. 1995]. Presence in the reaction of diethyl succinate and ethyl trifluoroacetate, has twice caused explosions [Chem. Brit., 1983, 19, 645].

Hazard

Dangerous fire risk, reacts violently with water evolving hydrogen. Irritant.

Health Hazard

SOLID: Will burn skin and eyes. Harmful if swallowed.

Fire Hazard

FLAMMABLE. MAY EXPLODE ON CONTACT WITH WATER. Accidental contact with water used to extinguish surrounding fire will result in the release of hydrogen gas and possible explosion.

Flammability and Explosibility

Highly flammable

Safety Profile

The powder ignites spontaneously in air. Flammable when exposed to heat or flame. Potentially explosive reaction with water, diethyl succinate + ethyltrifluoroacetate (above 60℃), dimethyl sulfoxide + heat, sulfur dioxide. Ignition or violent reaction with dimethylformamide (above 50℃), ethyl 2,2,3-trifluoropropionate, oxygen (at 230℃). Incompatible with acetylene + moisture, glycerin, halogens, sulphur. Normal fire extinguishers are unsuitable, use sand, ashes, solurn chloride. The commercial material may contain traces of sodium. When heated to decomposition it emits toxic fumes of Na2O. See also HYDRIDES.