Problem 31 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When phenol is reacted with \(\mathrm{CHCl}_{3}\) and \(\mathrm{NaOH}\) followed by acidification, salicylaldehyde is obtained. Which of the following species are involved in the above mentioned reaction as intermediates?

Step-by-Step Solution

Answer

The intermediates involved are dichlorocarbene and a dichlorocyclopropene intermediate.

Step 1: Identify the reaction

The reaction described here is the Reimer-Tiemann reaction, where phenol reacts with chloroform (\(\mathrm{CHCl}_3\)) and sodium hydroxide (\(\mathrm{NaOH}\)), followed by acidification to produce salicylaldehyde.

Step 2: Understand the mechanism

In the Reimer-Tiemann reaction mechanism, chloroform reacts with sodium hydroxide to generate dichlorocarbene (:\(\mathrm{CCl}_2\)), a highly reactive intermediate.

Step 3: Reaction of dichlorocarbene with phenol

The generated dichlorocarbene (\(:\mathrm{CCl}_2\)) reacts with the phenol, particularly at the ortho position, forming an intermediate dichlorocyclopropene ring.

Step 4: Rearrangement and hydrolysis

This intermediate forms another intermediate after opening of the three-membered ring and rearrangement, which ultimately gets hydrolyzed to form salicylaldehyde.

Key Concepts

Dichlorocarbene formation

In the Reimer-Tiemann reaction, one of the crucial steps is the formation of dichlorocarbene (:\(\mathrm{CCl}_2\)). This species is generated when chloroform (\(\mathrm{CHCl}_3\)) is treated with a strong base like sodium hydroxide (\(\mathrm{NaOH}\)).
Upon treatment with \(\mathrm{NaOH}\), the chloroform loses a hydrogen atom from its structure, producing trichloromethide anion, which then expels chloride ions to form dichlorocarbene.
The resulting dichlorocarbene is a neutral species that contains two carbon-chlorine bonds and a highly reactive electron-deficient carbon atom. This makes it an excellent electrophile, ready to partake in further chemical reactions, especially with nucleophilic sites in organic molecules. Understanding the formation of this intermediate is key to mastering the Reimer-Tiemann reaction.

Phenol reactivity

Phenol is a fascinating organic compound, prominently involved in various chemical reactions due to its reactive hydroxyl group. In the Reimer-Tiemann reaction, its reactivity is crucial as the dichlorocarbene generated in the earlier step attacks the phenol.
Phenol's structure allows it to be very reactive because of its aromatic ring, which stabilizes intermediates formed during reactions. In this specific reaction, dichlorocarbene prefers to attack the ortho position (positions adjacent to the hydroxyl group) on the benzene ring of phenol, thanks to increased electron density there.
The reactivity of phenol is further enhanced by the resonance stabilization it provides to potential cationic intermediates. This makes phenol not just a participant in the reaction, but a guiding structure that directs where the dichlorocarbene adds.

Intermediate species in organic reactions

Understanding intermediates in organic chemistry is essential, as they represent the actual forms of molecules that transform from reactants to products. In the Reimer-Tiemann reaction, a sequence of intermediate species arises automatically as the reaction proceeds.
Once dichlorocarbene attaches to the ortho position of phenol, a transient three-membered ring, known as a dichlorocyclopropene, is formed. This structure is inherently unstable and does not remain long. Instead, it quickly undergoes a rearrangement, eventually leading to another intermediate.
This rearrangement primes the species for hydrolysis, which converts it to the final product, salicylaldehyde. Each intermediate represents crucial transformation stages, enabling chemists to understand and manipulate reactions for desired outcomes. Understanding these intermediates allows for greater control over the reaction kinetics and outcomes.