Reimer-Tiemann reaction

Reimer Tiemann
Named after	Karl Reimer Ferdinand Tiemann
Reaction type	Substitution reaction
Identifiers
RSC ontology ID	RXNO:0000072

The Reimer–Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols; with the simplest example being the conversion of phenol to salicylaldehyde. The reaction was discovered by Karl Reimer () and Ferdinand Tiemann. The Reimer in question was Karl Reimer (1845-1883) not the less known Carl Ludwig Reimer (1856-1921).

Chloroform (1) is deprotonated by a strong base (normally hydroxide) to form the chloroform carbanion (2) which will quickly alpha-eliminate to give dichlorocarbene (3); this is the principal reactive species. The hydroxide will also deprotonate the phenol (4) to give a negatively charged phenoxide (5). The negative charge is delocalised into the aromatic ring, making it far more nucleophilic. Nucleophilic attack of the dichlorocarbene gives an intermediate dichloromethyl substituted phenol (7). After basic hydrolysis, the desired product (9) is formed.

By virtue of its 2 electron-withdrawing chlorine groups, carbine (3) is highly electron deficient and is attracted to the electron rich phenoxide (5). This interaction favors selective ortho-forymation.

Hydroxides are not readily soluble in the chloroform, thus the reaction is generally carried out in a biphasic solvent system. In the simplest sense this consists of an aqueous hydroxide solution and an organic phase containing the chloroform. The two reagents are therefore separated and must be brought together for the reaction to take place. This can be achieved by rapid mixing, phase-transfer catalysts, or an emulsifying agent (the use of 1,4-Dioxane as a solvent is an example).

The reaction typically needs to be heated to initiate the process, however once started the Reimer-Tiemann Reaction can be highly exothermic; this combination makes it prone to thermal runaways.

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