Problem 73 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction of phenol with \(\mathrm{CHCl}_{3}\) and aqueous \(\mathrm{NaOH}\) at \(70^{\circ} \mathrm{C}(343 \mathrm{~K})\), the electrophile attacking the ring is (a) \(\mathrm{CHCl}_{3}\) (b) \(\mathrm{CHCl}_{2}\) (c) \(\mathrm{CCl}_{2}\) (d) \(\mathrm{COCl}_{2}^{*}\)

Step-by-Step Solution

Answer

The electrophile is \(\mathrm{CCl}_{2}\).

Step 1: Understanding the Reaction

We are dealing with the reaction of phenol with \(\mathrm{CHCl}_{3}\) and aqueous \(\mathrm{NaOH}\). The key to solving this problem is to identify the electrophile that attacks the benzene ring in phenol. When \(\mathrm{CHCl}_{3}\) is treated with a strong base like \(\mathrm{NaOH}\), it forms a reactive species that can act as an electrophile.

Step 2: Formation of the Electrophile

The reaction of \(\mathrm{CHCl}_{3}\) with aqueous \(\mathrm{NaOH}\) leads to the generation of \(\mathrm{CCl}_{2}\)carbene. \(\mathrm{CCl}_{2}\) is a highly reactive species known as dichlorocarbene, which is formed by the deprotonation of \(\mathrm{CHCl}_{3}\) in the presence of \(\mathrm{NaOH}\).

Step 3: Identifying the Electrophile

In the presence of phenol, the electrophile \(\mathrm{CCl}_{2}\) generated from the reaction of \(\mathrm{CHCl}_{3}\) and \(\mathrm{NaOH}\) will attack the benzene ring of phenol. This is because \(\mathrm{CCl}_{2}\) is a neutral but highly electrophilic species due to its electron-deficient nature.

Step 4: Conclusion

Based on the chemical reaction and the steps involved, the correct electrophile attacking the benzene ring in this reaction is \(\mathrm{CCl}_{2}\), known as dichlorocarbene.

Key Concepts

Phenol

Phenol is an aromatic compound featuring a hydroxyl group \((-\mathrm{OH})\) bonded to a benzene ring. This configuration makes phenol quite reactive, especially toward electrophilic aromatic substitution reactions. In such reactions, the electrons from the aromatic ring are used to attack an electrophile.

• The presence of the hydroxyl group makes phenol more susceptible to such reactions because it increases the electron density of the benzene ring, making it more nucleophilic.
• Phenol's enhanced nucleophilicity stems from the lone pairs of electrons on the oxygen atom, which can be delocalized into the aromatic ring. This increases the ring's reactivity toward electrophiles.

When phenol participates in a reaction with chloroform and sodium hydroxide, it plays a crucial role by being the aromatic ring that the electrophile eventually attacks. The resulting product often then undergoes further electrophilic aromatic substitution, based on the reaction's conditions and other reactants present.

Dichlorocarbene

Dichlorocarbene is a highly reactive species formed during the reaction between chloroform and aqueous sodium hydroxide. Its chemical formula is \( \mathrm{CCl}_{2}\). This species is known as a carbene, which is a molecule that contains a neutral carbon atom with two unshared valence electrons.

• Dichlorocarbene is electrophilic in nature due to its electron deficiency. This makes it keen to react with electron-rich species, such as phenol's benzene ring.
• In the context of the reaction with phenol, dichlorocarbene is generated when sodium hydroxide deprotonates trichloromethane \((\mathrm{CHCl}_{3})\).

Dichlorocarbene, being unstable and highly reactive, attacks phenol's benzene ring. This results from the carbene's need to satisfy its electron deficiency by forming bonds with the electron-dense ring of phenol.

Aqueous NaOH

Aqueous sodium hydroxide \((\mathrm{NaOH})\) is a strong base and plays a pivotal role in the generation of dichlorocarbene from chloroform \((\mathrm{CHCl}_{3})\). In this reaction, NaOH provides hydroxide ions \((\mathrm{OH}^{-})\) that perform a critical deprotonation step.

• The hydroxide ions remove a proton from \(\mathrm{CHCl}_{3}\), leading to the formation of \(\mathrm{CCl}_{3}^{-}\), which is unstable and subsequently loses a chloride ion \((\mathrm{Cl}^{-})\) to produce the dichlorocarbene \(\mathrm{CCl}_{2}\).
• The usage of aqueous NaOH enables the reaction to occur under relatively mild conditions, facilitating the selective formation of \(\mathrm{CCl}_{2}\) without overreacting or decomposing the reactants prematurely.

Thus, aqueous sodium hydroxide is essential for the transformation of chloroform into the reactive carbene intermediate, leading to the product formation when phenol is involved.

Chloroform reaction

The reaction of chloroform \((\mathrm{CHCl}_{3})\) with phenol in the presence of aqueous sodium hydroxide is a classic example of an electrophilic aromatic substitution reaction. Here, chloroform acts as a precursor to produce the electrophilic species needed for the reaction.

• Initially, chloroform is deprotonated by the hydroxide ions from \(\mathrm{NaOH}\), leading to the formation of the dichlorocarbene \((\mathrm{CCl}_{2})\).
• This dichlorocarbene then acts as the electrophile that seeks out electron-rich areas, like the aromatic ring of phenol, to form a new product by attaching itself to the ring.

This reaction is a superb example of chemistry where molecular components are transformed through a sequence of steps to yield valuable and interesting compounds, demonstrating the potential of simple reagents to undergo complex transformations in organic synthesis.