Sodium hydride

Sodium hydride is a chemical compound represented by the empirical formula NaH. It is classified as an alkali metal hydride. This substance is a saline hydride, which means it is salt-like in nature. It consists of sodium ions and hydride ions. This structure makes it very different from molecular hydrides like methane or ammonia. Instead of forming discrete molecules, it is an ionic material. Because of this ionic nature, it is insoluble in all solvents. The only exception is molten sodium metal, which can dissolve it.

The physical structure of NaH is quite specific. It adopts the NaCl crystal structure, which is the same pattern found in rock salt. In this motif, each ion is surrounded by six centers. This arrangement is known as an octahedral geometry. The ions in the crystal are very tightly packed. In fact, NaH is approximately 40% denser than pure sodium. While the compound is technically colorless, most physical samples appear grey to the eye.

A very unusual version of this compound is called "inverse sodium hydride." This version contains Na+ and H- ions in a different way. It is an alkalide, which is a type of chemical compound. This version has a much higher energy content than ordinary sodium hydride. This happens because two electrons are moved from hydrogen to sodium. Scientists have found that a derivative of this can exist with a molecule called adamanzane. This molecule shields the ions from interacting with the alkalide. Without this shield, the reaction would not be as stable.

Industrial production of sodium hydride requires precise conditions. To prepare it, manufacturers introduce molten sodium into mineral oil. They add hydrogen gas at atmospheric pressure during this process. The mixture is then stirred very vigorously at about 8000 rpm. This reaction becomes especially rapid when the temperature reaches 150 degrees Celsius. The final product is a suspension of NaH within the mineral oil. This oil suspension is often used directly for other tasks, such as making diborane.

In the field of organic synthesis, NaH is a highly valued tool. It functions as a strong base with a wide scope of utility. Because it is a superbase, it can deprotonate many weak Brønsted acids. This means it can remove a proton from an acid to create a sodium derivative. It works easily on substrates with O-H, N-H, or S-H bonds. This includes substances like alcohols, phenols, pyrazoles, and thiols. It is also effective at deprotonating carbon acids, such as 1,3-dicarbonyls. These derivatives can then undergo alkylation.

Sodium hydride also drives several important chemical transformations. It promotes condensation reactions of carbonyl compounds. These include the Dieckmann, Stobbe, Darzens, and Claisen condensations. Additionally, NaH can act as a reducing agent for certain main group compounds. For example, it reacts with boron trifluoride to produce diborane and sodium fluoride. It can also reduce Si-Si and S-S bonds in disilanes and disulfides. When mixed with an alkali metal iodide, it can reduce imines to amines and amides to aldehydes.

Despite its uses, sodium hydride requires strict safety protocols. It is a combustible material that can ignite spontaneously in the air. It reacts vigorously when it touches water or humid air. This reaction releases hydrogen gas, which is highly flammable. It also produces sodium hydroxide, which is a corrosive base. To make handling safer, NaH is often sold as a 60% mixture in mineral oil. This dispersion is much easier to weigh and handle safely. However, chemists must still use air-free techniques to prevent dangerous fires or explosions.