Question

In Reimer - Tiemann reaction, phenol is converted into salicylaldehyde through an intermediate. The structure of intermediate is ______.

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is D

In the Reimer-Tiemann reaction, phenol reacts with chloroform (CHCl$_3$) and aqueous sodium hydroxide (NaOH) to form salicylaldehyde via the following mechanism:
Phenol undergoes ortho-substitution due to the electrophilic attack of dichlorocarbene (:CCl$_2$), generated in situ from CHCl$_3$ and NaOH.
The intermediate formed is the sodium salt of dichlorophenol.
On further hydrolysis and protonation, salicylaldehyde is obtained.
The reaction proceeds as:

Answer 2

The question asks to identify the structure of the key intermediate formed during the Reimer-Tiemann reaction, where phenol is converted into salicylaldehyde (2-hydroxybenzaldehyde).

Concept Used:

The Reimer-Tiemann reaction is an organic chemical reaction used for the ortho-formylation of phenols. The reaction proceeds through the following key steps:

1. Deprotonation of Phenol: Phenol is treated with a strong base (like NaOH) to form the sodium phenoxide ion. The phenoxide ion is more electron-rich and thus more susceptible to electrophilic attack than phenol itself.
2. Generation of Electrophile: The base also reacts with chloroform (\(CHCl_3\)) to generate a highly reactive electrophile, dichlorocarbene (\(:CCl_2\)). \[ CHCl_3 + OH^- \rightleftharpoons :CCl_3^- + H_2O \] \[ :CCl_3^- \rightarrow :CCl_2 + Cl^- \]
3. Electrophilic Aromatic Substitution: The electron-rich phenoxide ring attacks the dichlorocarbene, primarily at the ortho position due to the directing effect of the \(-\text{O}^-\) group. This results in the formation of a dichloromethyl-substituted phenoxide intermediate.
4. Hydrolysis: This intermediate is then hydrolyzed in the presence of the base to form a formyl group (\(-CHO\)).
5. Acidification: Finally, acidification of the reaction mixture protonates the phenoxide to yield the final product, salicylaldehyde.

The question specifically asks for the structure of the intermediate formed after the attack of dichlorocarbene and before the hydrolysis step.

Step-by-Step Solution:

Step 1: Phenol reacts with sodium hydroxide (NaOH) to form sodium phenoxide. This is the active species that will undergo electrophilic substitution.

 

Step 2: Dichlorocarbene (\(:CCl_2\)) is generated from chloroform and NaOH, which acts as the electrophile.

Step 3: The sodium phenoxide ion attacks the dichlorocarbene. The attack occurs at the ortho-position, which is electron-rich. This leads to the formation of an intermediate where the dichloromethyl group (\(-CHCl_2\)) is attached to the ring. The species present in the basic medium is the sodium salt.

 

This intermediate is sodium 2-(dichloromethyl)phenoxide.

Step 4: This intermediate then undergoes hydrolysis with NaOH to form sodium salicylate, which upon acidification gives the final product, salicylaldehyde. However, the structure formed in Step 3 is the key intermediate asked for in the question.

 

Step 5: We now compare the structure of the intermediate identified in Step 3 with the given options.

• Option (1) is incorrect as it shows a methyl group.
• Option (2) shows the protonated form (phenol derivative), but in the strongly basic reaction medium, the phenoxide form is the stable intermediate.
• Option (3) is the salt of the final product, which is formed after the hydrolysis step.
• Option (4) correctly shows the sodium phenoxide ion with a dichloromethyl (\(-CHCl_2\)) group attached at the ortho position. This matches our derived intermediate.

Final Computation & Result:

The intermediate formed in the Reimer-Tiemann reaction after the electrophilic attack of dichlorocarbene on the phenoxide ion is the dichloromethyl substituted phenoxide ion.

This corresponds to the structure shown in option (4).