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

Short answer

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

Step-by-step solution

Understand the Reaction

The reaction involves phenol reacting with chloroform \(\mathrm{CHCl}_{3}\) and aqueous sodium hydroxide \(\mathrm{NaOH}\) under heated conditions. This is commonly known as the Reimer-Tiemann reaction, which is used to form ortho-hydroxybenzaldehydes.

Formation of the Electrophile

In the presence of \(\mathrm{NaOH}\) and heat, \(\mathrm{CHCl}_{3}\) undergoes deprotonation to form \(\mathrm{CHCl}_{2}^-\) which quickly loses a chloride ion to form the electrophilic dichlorocarbene \(\mathrm{CCl}_{2}\).

Identify the Electrophile

The electrophile generated in this reaction is \(\mathrm{CCl}_{2}\), which is a strong electrophilic species that attacks the aromatic ring in phenol, typically at the ortho position.

Key concepts

Phenol

Phenol is an aromatic compound that plays a vital role in the Reimer-Tiemann reaction. It is characterized by a hydroxyl group \( (\text{-OH}) \) directly bonded to a benzene ring. This structure makes phenol significantly more reactive than benzene in electrophilic aromatic substitution reactions. The hydroxyl group activates the benzene ring, making it more susceptible to attack by electrophiles, especially at the ortho and para positions. Since phenol’s hydroxyl group donates electrons into the ring, it increases the electron density at these positions.

In the context of the Reimer-Tiemann reaction, phenol acts as the substrate whose reactive sites are attacked. When phenol reacts with the electrophilic dichlorocarbene in the Reimer-Tiemann reaction, the product is typically an ortho-hydroxybenzaldehyde. This transformation highlights phenol's importance in synthetic chemistry, enabling the introduction of formyl groups \( (\text{-CHO}) \) onto the aromatic ring.

Electrophile

An electrophile is a chemical species that accepts an electron pair to form a covalent bond. They are often positively charged or neutral molecules with an electron-deficient atom. Electrophiles play a crucial role in reactions involving aromatic compounds like phenol.

In electrophilic aromatic substitution, the aromatic ring is typically attacked by an electrophile, leading to the substitution of one of the hydrogen atoms. For phenol, the electron-donating effect of the hydroxyl group makes the aromatic ring even more attractive to electrophiles. This enhanced reactivity allows for efficient reactions that are useful in generating complex aromatic compounds.

• The Reimer-Tiemann reaction is a perfect example of this interaction, where the electrophile dichlorocarbene attacks the activated aromatic ring of phenol.
• Phenol’s high reactivity with electrophiles allows these reactions to occur under relatively mild conditions compared to other aromatic compounds.

Dichlorocarbene

Dichlorocarbene \( \text{CCl}_2 \) is a reactive intermediate and a key electrophile in the Reimer-Tiemann reaction. It is generated from chloroform \( (\text{CHCl}_3) \) in the presence of a strong base like sodium hydroxide \( (\text{NaOH}) \) and heat. Here is how it is formed:

- First, under basic conditions, \( \text{CHCl}_3 \) gets deprotonated, forming \( \text{CHCl}_2^- \).- Then, it quickly undergoes elimination of a chloride ion to generate the neutral but electron-deficient species, \( \text{CCl}_2 \).

Dichlorocarbene is notable for its strong electrophilic character, allowing it to react readily with electron-rich sites, such as the aromatic ring of phenol. Although it is neutral, it seeks out the electron density offered by the benzene structure to form a stable bond. Following its formation, dichlorocarbene attacks the activated phenol ring, leading to the substitution typically at the ortho position, ultimately forming ortho-hydroxybenzaldehyde. This reaction is essential in synthetic organic chemistry for introducing functional groups into aromatic compounds.