Problem 143 - A Complete Resource Book in Chemistry for JEE Main

Question

When phenol is treated 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? 1 . Oc1ccccc1C(Cl)(Cl)Cl 2 Oc1ccccc1C(Cl)Cl 3 . [O-]c1ccccc1C(Cl)Cl 4 . [O-]c1ccccc1C(Cl)Cl (a) 2,4 (b) 2,3 (c) 1,3 (d) 1,4

Step-by-Step Solution

Answer

(b) 2,3

Step 1: Understand the Reaction

The reaction described is the Reimer-Tiemann reaction in which phenol is converted into ortho-hydroxybenzaldehyde or salicylaldehyde when treated with chloroform (\(\mathrm{CHCl}_{3}\)) and sodium hydroxide (\(\mathrm{NaOH}\)), followed by acidification.

Step 2: Recognizing Key Intermediates

In the Reimer-Tiemann reaction, initially, a dichlorocarbene (\(:CCl_{2}\)) is formed as an intermediate from \(\mathrm{CHCl}_{3}\) in the presence of a strong base. This dichlorocarbene then reacts with phenol. Therefore, the first intermediate to recognize is where dichlorocarbene might form a bond with phenol.

Step 3: Identifying the Enolate Ion Formation

The phenoxide ion (\([O^-]c_1ccccc_1\)) is formed after the deprotonation of phenol by \(\mathrm{NaOH}\). This phenoxide ion then attacks the dichlorocarbene to form an intermediate where a chlorine replaces a hydrogen on the aromatic ring, and a dichloromethyl group attaches instead, forming an enolate ion.

Step 4: Recognizing the Correct Intermediates

From the given options, the correct intermediates are those that involve the phenoxide ion attached to a dichloromethyl group. The structures in the options can be translated from SMILES: 2 and 3 match this description. They represent structures that have undergone deprotonation of the hydroxyl group followed by a reaction involving \(:CCl_{2}\).

Key Concepts

Phenol Chemistry

Phenol chemistry revolves around the interactions and reactions phenol can undergo due to its unique structure. Phenol has a hydroxyl group directly attached to a benzene ring. The presence of this group alters the chemical characteristics of phenol significantly, making it more reactive than benzene. This reactivity is primarily due to two factors:

• Hydrogen bonding: The hydroxyl group can form hydrogen bonds, which affect the solubility and boiling point of phenol.
• Acidity: Phenol is more acidic than alcohols because the phenoxide ion is resonance-stabilized, allowing it to easily lose a proton.

In the context of the Reimer-Tiemann reaction, phenol's acidity comes into play when it interacts with alkalis such as sodium hydroxide (NaOH). This interaction results in the formation of the phenoxide ion, which is crucial for further reactions. Understanding how phenol behaves under different conditions is essential for predicting the outcomes of organic chemistry reactions that involve phenol.

Intermediate Species

In organic chemistry, reactions often proceed through a series of intermediate species, which are transitional structures between reactants and products. During the Reimer-Tiemann reaction, the creation and transformation of intermediates are crucial for understanding the reaction mechanism.
One key intermediate in this reaction is the phenoxide ion, formed by deprotonating phenol with NaOH. This anionic species is important because it is highly reactive.

• The phenoxide ion facilitates nucleophilic attacks due to its electron-rich nature.
• It acts as a nucleophile, attacking the dichlorocarbene generated during the reaction, which leads to further transformation of the molecule.

These intermediates highlight the dynamic and reactive nature of organic molecules during transformations, allowing synthetic chemists to exploit them for creating desired compounds.

Dichlorocarbene

Dichlorocarbene is a fascinating reactive intermediate in organic chemistry, particularly in the Reimer-Tiemann reaction. It is represented as \[ :CCl_{2} \] and is a highly reactive electrophilic species due to its divalent carbon, possessing two lone pairs and lacking full octet stabilization.

This makes it seek out electron-rich environments to stabilize itself.

• Dichlorocarbene is generated in situ from chloroform (\( \mathrm{CHCl}_{3} \)) when treated with a strong base like NaOH.
• This generation involves a base-induced elimination of hydrogen chloride, creating this transient carbene.

Its role in the Reimer-Tiemann reaction is pivotal as it interacts with phenoxide ions. This interaction helps incorporate a dichloromethyl group onto the phenoxide structure, playing a key role in the formation of salicylaldehyde.

Organic Chemistry Reactions

Organic chemistry reactions form the backbone of synthetic chemistry, enabling the transformation of simple molecules into complex structures. A prime example is the Reimer-Tiemann reaction, which elegantly combines multiple principles of organic reactions.

• The reaction type: This is a substitution reaction where a substituent on the aromatic ring is replaced with an aldehyde group.
• Key Mechanism: The process showcases the generation of a carbene and its subsequent reactions. This involves generating dichlorocarbene which then adds to the phenoxide ion.

These reactions are not just about creating new bonds; they involve understanding the electronic and steric properties of the involved species. This knowledge allows chemists to predict reaction outcomes and design pathways to synthesize targeted compounds. Mastering such reactions is essential for developing advanced applications in pharmaceuticals, materials science, and more.