Problem 74 - 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 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 \(\mathrm{CCl}_{2}\), option (c).

Step 1: Understanding the Reaction Context

Phenol reacts with chloroform (\(\mathrm{CHCl}_{3}\)) in the presence of aqueous sodium hydroxide (\(\mathrm{NaOH}\)) at 70°C. This reaction is known as the Reimer-Tiemann reaction, and is mainly used to introduce a formyl group \(-CHO\) to an aromatic ring.

Step 2: Identifying the Electrophile

In the Reimer-Tiemann reaction, the base \(\mathrm{NaOH}\) deprotonates chloroform \(\mathrm{CHCl}_{3}\), facilitating the formation of a dichlorocarbene \(\mathrm{CCl}_{2}\) intermediate. This intermediate is highly reactive and acts as an electrophile, attacking the aromatic ring in the phenol. Therefore, the electrophile in this reaction is the dichlorocarbene \(\mathrm{CCl}_{2}\).

Step 3: Selecting the Correct Option

Now that we know the electrophile formed during the Reimer-Tiemann reaction is \(\mathrm{CCl}_{2}\), we can select the correct option from the provided choices. Comparing with the given options, the correct choice is (c) \(\mathrm{CCl}_{2}\).

Key Concepts

Electrophile

In chemistry, an **electrophile** is a species that loves electrons. It's a molecule or atom that seeks out electrons to react with, generally because it is electron-deficient. This makes electrophiles positively charged or at least partially positive because they are missing electrons. In the context of the Reimer-Tiemann reaction, the electrophile is critical for the process.

• **Nature of Electrophiles**: Electrophiles tend to be attracted to nucleophiles, which are electron-rich species. This is like how a magnet is attracted to metal. The electrophiles work by taking electrons from the nucleophiles to form a bond.
• **Types of Electrophiles**: They can be simple cations like \(\text{H}^+\), neutral molecules like CH₂O, or molecules with polarized bonds, like \(\text{CCl}_2\) in our reaction.
• **Role in Reimer-Tiemann Reaction**: In this specific reaction, the electrophile formed is dichlorocarbene, \(\text{CCl}_2\). It acts on phenol, role-playing in a bit of molecule theater, ensuring that the chemical transformation occurs.

Phenol

**Phenol** is a crucial player in the Reimer-Tiemann reaction. It's an aromatic compound with a hydroxyl group \(\text{-OH}\) attached to a benzene ring. This structure is not just for show—it's highly reactive, particularly susceptible to electrophilic substitution reactions due to its rich electron cloud.

• **Structure and Reactivity**: Phenol's electrons make it highly attractive to electrophiles. This is because the \(\text{-OH}\) group increases electron density on the aromatic ring, making it easy for the reaction.
• **In Reimer-Tiemann reaction**: Here, phenol's aromatic ring is attacked by the \(\text{CCl}_2\) electrophile. The reaction positions the incoming group ortho (next to) to the phenol's \(\text{-OH}\) group, which is typical behavior due to stability factors.
• **Result**: This transformation introduces a complex pattern capable of further chemical modification.

Dichlorocarbene

**Dichlorocarbene (CCl₂)** is a fascinating and pivotal intermediate in the Reimer-Tiemann reaction. This tiny but mighty species plays the lead role once it's created in the reaction process.

• **Formation**: It's generated when chloroform \(\text{CHCl}_3\) undergoes action by sodium hydroxide \(\text{NaOH}\). The base causes the chloroform to deprotonate and form \(\text{CCl}_2\) along with water and chloride ions.
• **Characteristics**: Dichlorocarbene is highly reactive due to its electron-deficient center. With only six electrons available, it desperately seeks out other electrons to stabilize itself.
• **Role in Reaction**: It seeks out the rich electrons on phenol's aromatic ring and binds quickly. This highlights its nature as an excellent electrophile and its utility in forming complex aromatic structures through substitution reactions.