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) \(\mathrm{COCl}_{2}^{2}\)

Step-by-Step Solution

Answer

The electrophile is cCCl_{2} (option c).

Step 1: Identify the Reaction Type

The reaction in question is the "Reimer-Tiemann Reaction," which is specific to aromatic compounds like phenol reacting with chloroform and a base.

Step 2: Role of Chloroform

In the Reimer-Tiemann Reaction, chloroform (CHCl_{3}) is used along with a strong base like NaOH to generate a reactive species.

Step 3: Formation of Electrophile

When CHCl_{3} is treated with a strong base (NaOH), it undergoes deprotonation to form the dichlorocarbene (:CCl_{2}), which is the true electrophile in this reaction.

Step 4: Electrophile Attacks Aromatic Ring

The formed :CCl_{2}, being electron deficient, can attack the electron-rich phenolic do not remove pleasebenzene ring to form a new carbon-carbon bond, leading to the addition of a chloroformyl group nearby.

Key Concepts

Electrophile

In a chemical reaction, an electrophile is an atom or molecule that seeks out and accepts electrons. This eagerness stems from having a positive charge, a partial positive charge, or an empty orbital ready to be filled.
Electrophiles are crucial players in organic chemistry, where they typically interact with nucleophiles, which are electron-rich and eager to donate electrons.

• An electrophile often has fewer electrons, leading to a natural attraction to electron-rich centers.
• In the Reimer-Tiemann Reaction, the electrophile is vital as it attacks the aromatic ring of phenol.

Understanding the behavior of electrophiles helps predict the outcomes of many organic reactions, allowing chemists to control reaction pathways effectively.

Dichlorocarbene

Dichlorocarbene ( :CCl_{2} ) is a highly reactive intermediate with significant applications in organic synthesis. It is a notable example of a carbene, which is a molecule that contains a neutral carbon atom with two non-bonded electrons.
In the Reimer-Tiemann Reaction, dichlorocarbene serves as the real electrophile, coveting the abundant electrons found in aromatic compounds.

• Dichlorocarbene is created by deprotonating chloroform ( CHCl_{3} ) using a strong base like NaOH .
• This reaction strips the hydrogen from chloroform, leaving a reactive species eager to latch onto electron-rich sites.

Due to its neutral state and the need to satisfy its electron deficiency, dichlorocarbene quickly forms bonds, facilitating transformative chemical reactions.

Aromatic Compounds

Aromatic compounds are a fascinating class of organic molecules characterized by their unique stability and structure. The term "aromatic" refers to the delightful odors of some of these compounds, but in chemistry, it denotes specific structural traits:

• They typically have rings that follow Huckel's Rule, a set of conditions that impart significant stability.
• This stability arises from a conjugated system of delocalized electrons, often visualized as a circle inside the hexagonal benzene ring.

In the context of the Reimer-Tiemann Reaction, the phenol involved possesses an aromatic benzene ring, naturally housing excess electrons that attract electrophiles like dichlorocarbene. Understanding the unique stability and reactivity of aromatic compounds like phenolics helps in designing and predicting outcomes in synthetic organic chemistry.

Phenol

Phenol is an aromatic compound with the molecular formula C_6H_5OH , where a hydroxyl group ( OH ) is directly attached to a benzene ring. This structure confers distinct chemical properties:

• The presence of the OH group makes phenol more reactive than benzene itself, providing an electron-donating effect and enhancing the aromatic ring's reactivity towards electrophiles.
• In chemical reactions, phenol often acts as a more versatile and valuable substrate due to its modified electronic properties.

In the Reimer-Tiemann Reaction, the OH group in phenol activates the ring, allowing the dichlorocarbene electrophile to attach more readily compared to other less functionalized aromatic compounds.

Chloroform

Chloroform ( CHCl_{3} ) is a simple chlorinated compound often utilized as a reagent in various chemical processes due to its ability to form reactive intermediates like dichlorocarbene. It bears several key characteristics:

• Chloroform can be deprotonated in the presence of a strong base, like NaOH , which facilitates the generation of dichlorocarbene.
• This process involves removing a proton ( H^+ ), transforming chloroform into a reactive species capable of participating in reactions such as the Reimer-Tiemann Reaction.

Understanding chloroform's behavior under these conditions is essential to utilize its properties effectively in chemical synthesis, where it acts as a precursor to active electrophilic entities.