The Reimer–Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols. The reaction was discovered by Karl Ludwig Reimer and Ferdinand Tiemann. In the simplest case, the product is salicylaldehyde:
Chloroform (1) reacts with strong base to form the chloroform carbanion (2), which will quickly alpha-eliminate to give dichlorocarbene (3). Dichlorocarbene will react in the ortho-position of the phenate (5) to give dichloromethyl substituted phenol (7). After basic hydrolysis, the desired product (9) is formed.