Sodium bisulfite

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Sodium hydrogen sulfite or sodium bisulfite is a chemical compound with the chemical formula NaHSO₃. This sulfite is a white powder with a melting point 150 °C at which it decomposes. Solubility in water @ 20 °C is 515 g/l and the pH-value @ 54 g/l and 20 °C is 3.5 to 5.0. Sodium bisulfite is a food additive with E-number E-222. Sodium bisulfite can be prepared by bubbling sulfur dioxide in a solution of sodium carbonate in water.

[edit] Uses in chemistry

In organic chemistry sodium bisulfite has several uses. It forms a bisulfite adduct with aldehyde groups and with certain cyclic ketones to a sulfonic acid. This reaction has limited synthetic value but it is used in purification procedures. Contaminated aldehydes in a solution precipitate as the bisulfate adduct which can be isolated by filtration. The reverse reaction takes place in presence of a base such as sodium bicarbonate or sodium hydroxide and the bisulfite is liberated as sulfur dioxide. Procedures in Organic Syntheses are described for benzaldehyde (OS 5:437), tetralone (OS 4:903), citral (OS 3:747), the ethyl ester of pyruvic acid (OS 4:467) and glyoxal (OS 3:438). In the ring-expansion reaction of cyclohexanone with diazald, the bisulfite reaction is reported to be able to differentiate between the primary reaction product cycloheptanone and the main contaminant cyclooctanone (OS 4:221).

The other main use of sodium bisulfite is as a mild reducing agent in organic synthesis in particular in purification procedures. It can efficiently remove traces or excess amounts of chlorine, bromine, iodine, hypochlorite salts, osmate esters, chromium trioxide and potassium permanganate.

A third use of sodium bisulfite is as a decoloration agent in purification procedures obviously because it can reduce strongly colorized oxidizing agents and conjugated alkenes and carbonyl compounds.

Sodium bisulfite is also the key ingredient in the Bucherer reaction. In this reaction an aromatic hydroxyl group is replaced by an aromatic amine group en vice versa because it is a reversible reaction. The first step in this reaction is an addition reaction of sodium bisulfite to an aromatic double bond. The Bucherer carbazole synthesis is a related organic reaction.