Reimer Tiemann Reaction Mechanism

The Reimer-Tiemann reaction is an organic transformation that converts phenols into ortho-hydroxy benzaldehydes using chloroform, a base, and subsequent acid treatment. It is named after Karl Reimer and Ferdinand Tiemann, the chemists who developed the method.

1.0The Reimer-Tiemann reaction

The Reimer-Tiemann reaction is a well-known chemical process used for the ortho-formylation of phenols. The simplest example is the conversion of phenol to salicylaldehyde. Karl Reimer and Ferdinand Tiemann discovered this reaction.                 

This substitution reaction is used explicitly for the ortho-formylation of phenol (C₆H₅OH). The Reimer-Tiemann reaction can also be modified to yield phenolic acids by substituting chloroform with carbon tetrachloride. 

For instance, when phenol is used in the modified reaction, the product is salicylic acid rather than the expected salicylaldehyde.

2.0Conditions for Reimer-Tiemann Reaction:

• Biphasic Solvent System: The reaction must be carried out in a biphasic solvent system, an aggregate of immiscible phases, typically including an organic solvent and an aqueous phase.
• Mixing and Emulsification: The two reagents are combined by rapid mixing, phase transfer catalysts, or 1,4-dioxane, which acts as an emulsifying agent.
• Versatility: This reaction is effective when other hydroxy-aromatic compounds, such as naphthols, are used.
• Heat Requirement: Heat is necessary to initiate the reaction process.
• Exothermic Nature: Once the reaction begins, it is highly exothermic.

3.0Reimer-Tiemann Reaction Mechanism

The Reimer-Tiemann reaction is an organic chemical process that converts phenol into ortho-hydroxybenzaldehyde. This reaction involves chloroform, a base, and an acid workup, and it can also be characterized as a method to ortho-formylate phenols. Here is a step-by-step breakdown of the process:

• Formation of Chloroform Carbanion:
• The process starts with the deprotonation of chloroform by a strong base, forming a chloroform carbanion.

     ${\mathrm{CHCl}}_3+{\mathrm{OH}}^{-}\to {\mathrm{CCl}}_3^{-}+{\mathrm{H}}_2\mathrm{O}$

• Production of Dichlorocarbene:
• The chloroform carbanion undergoes alpha elimination to produce dichlorocarbene, the primary reactive species.

    ${\mathrm{CCl}}_3^{-}\to {\mathrm{CCl}}_2^{-}+{\mathrm{Cl}}^{-}$

• Formation of Phenoxide Ion:
• The aqueous hydroxide deprotonates Phenol to produce a negatively charged phenoxide ion.

      ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}+{\mathrm{OH}}^{-}\to {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{O}-+{\mathrm{H}}_2\mathrm{O}$

• Increase in Nucleophilicity:
• The negative charge on the phenoxide ion delocalizes within the benzene ring, significantly increasing its nucleophilicity.
• Nucleophilic Attack:
• The phenoxide ion attacks the dichlorocarbene, forming an intermediate dichloromethyl-substituted phenol.

     ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{O}}^{-}+{\mathrm{CCl}}_2\to {\mathrm{C}}_6{\mathrm{H}}_4-{\mathrm{CCl}}_2-\mathrm{OH}$

• Formation of Ortho-Hydroxybenzaldehyde:
• Basic hydrolysis of the intermediate leads to the formation of the desired ortho-hydroxybenzaldehyde.

    ${\mathrm{C}}_6{\mathrm{H}}_4-{\mathrm{CCl}}_2-\mathrm{OH}+{\mathrm{OH}}^{-}\to {\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{CHO}+2{\mathrm{Cl}}^{-}+{\mathrm{H}}_2\mathrm{O}$

4.0Comparison of the Reimer-Tiemann reaction :

Given below is a brief comparison of the Reimer-Tiemann reaction with other famous aromatic substitution reactions:

5.0Application of Reimer-Tiemann reaction

The Reimer-Tiemann reaction indeed finds diverse applications across various industries and research fields.

The Reimer-Tiemann reaction's ability to modify aromatic compounds by adding formyl groups in a controlled manner makes it invaluable in organic synthesis, catering to various industrial and commercial applications.

Here are some key applications :

• Formation of Aromatic Aldehydes: It introduces formyl groups (-CHO) onto phenol rings, synthesizing aromatic aldehydes. This is crucial in the production of various aromatic compounds used in industry.
• Building Blocks for Pharmaceuticals, Agrochemicals, and Materials: The reaction products are essential intermediates in synthesizing pharmaceuticals, agrochemicals (pesticides, herbicides), and various materials.
• Production of Ortho-Hydroxy Benzaldehydes: By selectively introducing a formyl group at the ortho position relative to the hydroxyl group on phenols, this reaction facilitates the production of ortho-hydroxy benzaldehydes. These compounds are used in the manufacture of dyes and pigments.
• Vanillin Production: One notable application is the synthesis of vanillin using the Reimer-Tiemann reaction. Vanillin is widely used in the fragrance industry for perfumes, scents, deodorants, and food flavorings.

6.0Solved Questions

Ques.1 What is the main objective of the Reimer-Tiemann reaction in organic chemistry?

Ans. The primary purpose of the Reimer-Tiemann reaction in organic chemistry is the ortho-formylation of phenols. This reaction introduces a formyl group (-CHO) at the ortho position relative to the hydroxyl group (-OH) on a phenol ring. This selective Formylation is valuable for synthesizing aromatic aldehydes, important intermediates in producing various chemical compounds, including pharmaceuticals, agrochemicals, dyes, pigments, and fragrances like vanillin.

Ques.2  What is the nature of the Reimer-Tiemann reaction, and what intermediate is formed during this reaction?

Ans.  The Reimer-Tiemann reaction is an electrophilic aromatic substitution reaction.This reaction generally occurs under primary conditions and involves the generation of a reactive carbene intermediate.

The key intermediate formed during the Reimer-Tiemann reaction is dichlorocarbene (CCl₂). This highly reactive species is generated in situ from chloroform and base and then reacts with the Phenol to produce the formylated product.

Ques.3  Why is ortho-hydroxybenzaldehyde a major product in the reaction?

Ans.  Ortho-hydroxybenzaldehyde is the primary product in the Reimer-Tiemann reaction due to intramolecular hydrogen bonding, the electron-donating effect of the hydroxyl group activating ortho and para positions, and the selective reactivity of dichlorocarbene towards the ortho position on the phenol ring. These factors collectively favour its formation.