Bisulfite reaction

The bisulfite reaction is a nucleophilic addition of sodium hydrogen sulfite (bisulfite) to carbonyls and ketones . A solid, water-soluble adduct is formed from the liquid starting materials used. The reaction is reversible and can therefore be used to separate numerous carbonyl compounds from other organic substances. It is therefore an effective method for purifying and separating the starting materials and impurities used if they have chemical properties similar to those of the carbonyl compound.

Mechanism of the bisulfite reaction

Course of the reaction

The electrophilic carbonyl carbon is first attacked by the lone pair of electrons on the sulfur atom, and the sulfite ion is added to the nucleophile. After intramolecular proton migration, the product is created, a sulfonic acid that precipitates as a solid from organic solvents.

However, the reaction is almost completely reversible. If the sulfonate dissolved in water is mixed with 20% sulfuric acid and heated under reflux, the actual carbonyl compound separates out as an organic phase with the evolution of sulfur dioxide.

Reactivity

Like any nucleophilic addition to the electrophilic carbonyl carbon, the reaction is dependent on steric and electronic factors.

• the greater the steric hindrance on the radicals R ¹ and R ² , the more unreactive the carbonyl compound becomes
• If R ¹ and R ² correspond to residues with + I and + M effects, the compound is less electrophilic and therefore less reactive

The more electrophilic the carbonyl compound, the more exothermic the reaction that takes place.

literature

• Latscha, Katzmaier, Klein: Organic Chemistry: Chemistry-Basiswissen II 6th Edition, Springer April 2008, ISBN 978-3-540-77106-7