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Reimer Tiemann reaction is a type of substitution reaction, named after chemists Karl Reimer and Ferdinand Tiemann. The reaction is used for the ortho-formylation of phenols.

When phenols is treated with chloroform in the presence of NaOH / KOH , an aldehyde group (-CHO) is introduced at the ortho position of the benzene ring leading to the formation of o-hydroxybenzaldehyde. The reaction is popularly known as the Reimer Tiemann reaction.