ReimerTiemann

Reimer–Tiemann reaction is an organic chemical reaction used to introduce an aldehyde group ortho to a phenolic ring. It converts phenols into o-formylphenols, most commonly yielding salicylaldehyde after hydrolysis. The method uses chloroform and a strong aqueous base, typically sodium hydroxide, at low temperature. The reaction is named after the chemists Reimer and Tiemann, who described the procedure in the 19th century.

The reaction mechanism involves generation of dichlorocarbene from chloroform under basic conditions. The highly reactive :CCl2 species adds to the activated phenoxide ring, leading to an ortho-formylated intermediate. Hydrolysis and further processing then furnish salicylaldehyde. Because the phenoxide ion directs electrophilic attack to the ortho position relative to the hydroxyl group, the major product is typically the ortho-formylated phenol.

Scope and limitations: The Reimer–Tiemann reaction works best with electron-rich phenols. Substituents that donate electrons can accelerate the reaction, while steric hindrance or strongly deactivating groups reduce yields or suppress the reaction. The method is mainly used to prepare salicylaldehyde, a key intermediate in the synthesis of salicylic acid and related compounds. The process employs hazardous reagents and generates chlorinated byproducts, so it requires careful handling and waste management. Compared with modern formylation methods, the Reimer–Tiemann reaction is relatively specific for ortho-formylation of phenols but is limited in substrate scope and practicality for complex or sensitive substrates.

See also: formylation, Gattermann–Kohlhauser reaction, chloromethylation.