Reimer- Tiemann reaction involves as intermediate:

• (A) Carbocation
• (B) Carbanion
• (C) Free radical
• (D) Carbene

The Reimer-Tiemann reaction is a fascinating organic reaction that primarily involves the ortho-formylation of phenols. To answer your question about the intermediate involved in this reaction, the correct choice is (C) Free radical. Let’s delve deeper into the mechanism and the role of the free radical in this process.

Understanding the Reimer-Tiemann Reaction

The Reimer-Tiemann reaction typically starts with a phenol and reacts with chloroform (CHCl3) in the presence of a strong base, such as sodium hydroxide (NaOH). This reaction is particularly interesting because it leads to the formation of ortho- and para-hydroxybenzaldehydes.

Mechanism Breakdown

Here’s a step-by-step breakdown of the mechanism:

• Formation of Dichlorocarbene: The strong base deprotonates chloroform, leading to the formation of dichlorocarbene (CCl2), a highly reactive species.
• Radical Formation: The dichlorocarbene then reacts with the phenol, resulting in the formation of a free radical intermediate. This is a crucial step because the free radical is what allows for the subsequent reactions to occur.
• Rearrangement and Final Product: The free radical can then rearrange and undergo further reactions to yield the final ortho- or para-hydroxybenzaldehyde products.

The Role of Free Radicals

Free radicals are species that contain an unpaired electron, making them highly reactive. In the context of the Reimer-Tiemann reaction, the free radical intermediate is essential for the ortho- and para-substitution on the aromatic ring. This reactivity allows the radical to stabilize through resonance, which is a key factor in the formation of the final products.

Comparison with Other Intermediates

To clarify why the other options (carbocation, carbanion, and carbene) are not correct:

• Carbocation: This is a positively charged species that typically forms in reactions involving nucleophilic attack, which is not the case here.
• Carbanion: A negatively charged species that usually arises in reactions where a strong base abstracts a proton, but it does not play a role in the Reimer-Tiemann mechanism.
• Carbene: While dichlorocarbene is a type of carbene, it is not the intermediate that directly participates in the rate-determining step of the reaction. Instead, it generates the free radical.

Conclusion

In summary, the Reimer-Tiemann reaction involves a free radical as an intermediate, which is crucial for the formation of the final products. Understanding this mechanism not only highlights the importance of free radicals in organic reactions but also showcases the intricate nature of chemical transformations in aromatic compounds.