The Reimer Tiemann Reaction is an organic chemical reaction that is used for ortho-formylation of the phenols (C6H5OH). It is named after Karl Reimer and Ferdinand Tiemann. It is essentially a substitution reaction. Ortho-formylation is a chemical process in which a formyl group replaces one hydrogen atom in the ortho position of phenol. In this article, we will understand Reimer and Tiemann reactions, the reaction conditions, applications, and more. Let us start by knowing more about the Reimer Tiemann reaction in the coming section.
Reimer Tiemann Reaction
The Reimer Tiemann reaction is the chemical reaction that is used for the ortho-formylation of phenols, with the simplest example being the conversion of phenol to salicylaldehyde. Karl Reimer and Ferdinand Tiemann had discovered this reaction.
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This reaction is said to be a type of reaction that substitutes like it is a substitution reaction, this reaction is used for the ortho-formylation reaction of C6H5OH. The Reimer and Tiemann reaction can be altered to yield phenolic acids by substituting the chloroform with carbon tetrachloride.
The altered reaction for phenol for instance would yield salicylic acid rather than the expected product which is salicylaldehyde.
By the virtue of two electrons withdrawing from the chlorine group carbene(3), is highly electron-deficient and is attracted to the electron-rich phenoxide(5) the interaction favors selective ortho formylation. Aromatic substitution of the Reimer Tiemann reaction that occurs under the basic conditions is a unique reaction in many aspects. It comes under one of the few organic reactions whose industrial example seems to increase yearly. Not neglecting the environmental hazards associated with chloroform use while the academic interest remains considerable. We focus here consequently on the recent development.
Reimer Tiemann Mechanism
The Reimer and Tiemann reaction mechanism begins with the deprotonation of chloroform by a strong base to form a chloroform carbanion this chloroform carbanion quickly starts to undergo alpha elimination giving rise to dichlorocarbene the principal reactive species for this reaction which is required is this. The Reimer and Tiemann reaction is the reaction which is also known as the organic chemical reaction where phenol is converted into an ortho benzaldehyde using a chloroform base and an acidic mixture or workup. This named reaction can also be defined as the chemical reaction used for the ortho-formylation of phenols.
The mechanism of Reimer Tiemann can be explained in 9 major steps that are listed below:
The chloroform is deprotonated by the strongly basic aqueous hydroxide solutions which give the chloroform carbenium.
This chloroform carbanion undergoes alpha elimination giving the product as dichlorocarbene which is the main reactive species in the reaction.
The aqueous hydroxide then deprotonates the reactant of phenol, giving a negatively charged phenoxide.
The negative charge is now delocalized into the benzene ring forming it a nucleophilic.
This later results in the attack of nucleophilic on the dichlorocarbene substituted phenol.
This is subjected immediately to the basic hydrolysis to finally achieve the formation of the desired ortho hydroxybenzaldehyde.
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Hydroxides are not generally readily soluble in chloroform, a biphasic solvent system is employed to carry out the reaction. This biphasic system can consist of an aqueous hydroxide solution with an organic phase that contains chloroform.
The two reagents are therefore separated and must be again brought together for the particular reaction to take place. This whole process can be achieved by rapid mixing phase transfer crystals or an emulsifying agent, the use of 1,4-dioxane as a solvent is one example. This reaction typically needs to be heated to initiate the process however, once we start the reaction this reaction proves to be highly exothermic this combination makes it prone to thermal runways.
For the other hydroxy-aromatic compounds such as naphthols, the Reimer and Tiemann reaction is effective others. Heterocycles that are electrons rich such as pyrroles and indoles are also known to react. Dichlorocarbonese can react with amines and alkanes to form respectively isocyanides and dichlorocyclopropane. The Reimer and Tiemann reaction as such may be unstable for the substrates bearing these functional groups; many compounds, in addition, cannot withstand being heated in the presence of hydroxide.
Applications of the Reimer Tiemann Reaction
By replacing the chloroform with carbon tetrachloride, this process can be slightly altered to produce phenolic acids like salicylic acid.
The ortho-formylation of phenols is the most common application.
Direct formulation of aromatic compounds can be done in a variety of ways. But this reaction is the safest and easiest because it is the only approach that does not require acidic or anhydrous conditions.
Reimer and Tiemann Details
Since in the chloroform the hydroxides are not readily soluble, a biphasic solvent system is involved to carry out the reaction. This biphasic system can consist of an aqueous hydroxide solution with an organic phase which has the chloroform. The two reagents that are separated are both together for the reaction to occur. To bring these two reagents together the technique includes rapid mixing, phase transfer catalysts, or the use of the emulsifying agent.
When other hydroxy aromatic compounds are used, the reaction is quite effective for naphthols for example.
Organic heterocyclic compounds that are quite rich in electrons such as pyrroles and indoles also can undergo this reaction.
To initiate the process the reaction needs to be provided with heat, and once the reaction is begun then it can prove to be a highly exothermic reaction and so it can further increase the reaction rate. To thermal runaways, this is the reason why the Reimer and Tiemann reaction is preferred.
Note: By the virtue of two electrons withdrawing from their own to the chlorine group carbine (3) is highly deficient and it gets attracted to electron-rich phenoxide (5).