Reimer Tiemann Reaction: Explanation and Reactions

Reimer-Tiemann reaction, which was named after two chemists Karl Reimer and Ferdinand Tiemann, is a chemical reaction used for the ortho-formylation of phenols (C₆H₅OH). One of the simplest examples being the conversion of phenol to salicylaldehyde. It is a type of substitution reaction. Here we will discuss the concept in detail along with some important questions.According to the Reimer-Tiemann reaction, when phenol is refluxed with chloroform and sodium hydroxide at 340 K followed by hydrolysis, produces o-hydroxy benzaldehyde as main product and p-hydroxy benzaldehyde as minor product.

Key Terms: Reimer and Tiemann's reaction, biphasic solvent, dichlorocarbene,biphasic system, chemical reaction, chloroform, sodium hydroxide,benzaldehyde  

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Detailed Explanation of Reimer and Tiemann

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As the hydroxides that are present in chloroform are not readily soluble, there requires a biphasic solvent to carry out the entire reaction. This biphasic system mostly consists of an aqueous hydroxide solution with an organic phase that has the chloroform in it. The two reagents that were separated, are brought together so that the reaction can take place. Several techniques are used to bring these reagents together. A few of them are- rapid mixing, phase transfer catalysts, or the use of the emulsifying agent.

The same reaction becomes even more effective when hydroxy-aromatic compounds like naphthol are used. Other organic heterocyclic compounds that are very rich in electrons like pyrroles and indoles can also experience Reimer and Tiemann reactions with ease.

To start the process the reaction needs heat and once the reaction starts, it can come out as a highly exothermic reaction. This in turn further increases the reaction rate. This is the reason why Reimer and Tiemann's reactions are always preferred in thermal runaways.

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).

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Reimer and Tiemann Mechanism

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The mechanism in Reimer and Tiemann's reaction begins with the deprotonation of chloroform with the help of a strong base to form chloroform carbanion. This chloroform carbanion undergoes rapid alpha elimination giving rise to dichlorocarbene which is the principal reactive species in this reaction.

Reimer and Tiemann reaction is also known as an organic chemical reaction where phenol is converted into an ortho benzaldehyde using chloroform as a base and an acidic mixture or workup.

This chemical reaction can also be defined as the ortho-formylation of phenols.

• The chloroform is deprotonated by the strongly basic aqueous hydroxide solution and produces the chloroform carbanion.
• This chloroform carbanion easily goes through alpha elimination, and gives dichlorocarbene as the product. As already mentioned, dichlorocarbene is the primary reactive species.
• Then the aqueous hydroxide deprotonates the phenol reactant as well, and gives a negatively charged phenoxide.
• This negative charge is now delocalized into the benzene ring that causes it to be far more nucleophilic.
• As a result, this causes a nucleophilic attack on the dichlorocarbene, forming an intermediate dichloromethyl substituted phenol.
• This intermediate is exposed to basic hydrolysis in order to achieve the formation of the desired ortho-hydroxybenzaldehyde.

ortho-hydroxybenzaldehyde

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Reimer and Tiemann Condition

Mostly all hydroxides are not soluble in chloroform. A biphasic solvent is required to carry out this reaction. A biphasic system consists of an aqueous hydroxide solution with an organic phase that contains chloroform.

The two reagents are first separated and again reacted together to make sure the Reimer and Tiemann reaction takes place. This entire process can be achieved by rapid mixing phase transfer crystals or an emulsifying agent, the best example for this is the use of 1,4-dioxane as a solvent.

4-dioxane

This kind of reaction just needs to be initiated first by heating, once we start this reaction it turns out to be highly exothermic. This combination makes it prone to thermal runways.

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Things to Remember

• The topic “Reimer-Tiemann Reaction'' is covered in Chapter 11 Alcohols, Phenols, and Ethers of NCERT Class 12 Chemistry.
• The chapter will carry around 3-4 marks in the board examination.
• Reimer-Tiemann reaction is an organic chemical reaction used for the ortho-formylation of the phenols.
• It is a substitution reaction.
• One common example of this reaction is the conversion of phenol to salicylaldehyde

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