Reimer-Tiemann Reaction: Mechanism & Examples
The Reimer-Tiemann Reaction is a classic electrophilic substitution reaction used to formylate phenols. Specifically, it involves the treatment of phenol with chloroform in the presence of an aqueous alkali (base) to synthesize Salicylaldehyde (2-Hydroxybenzaldehyde).
1. General Reaction
Phenol reacts with Chloroform ($CHCl_3$) and aqueous Sodium Hydroxide ($NaOH$) followed by acid hydrolysis to produce o-hydroxybenzaldehyde as the major product.
Note: The para-isomer is also formed as a minor product, but the ortho-isomer is major due to stability from intramolecular hydrogen bonding.
2. Detailed Mechanism
The reaction proceeds via the formation of a neutral electrophile called Dichlorocarbene ($:CCl_2$).
Step 1: Generation of Electrophile
The hydroxide ion from the alkali abstracts the acidic proton from chloroform to form the trichloromethyl anion, which loses a chloride ion to form Dichlorocarbene.
$:CCl_2$ is an electron-deficient species (incomplete octet) and acts as a neutral electrophile.
Step 2: Electrophilic Attack
Phenol is converted to the phenoxide ion by NaOH (which is more reactive towards electrophilic substitution). The phenoxide ion attacks the dichlorocarbene.
Step 3: Hydrolysis
The intermediate undergoes basic hydrolysis to form an aldehyde group, followed by acidification to yield Salicylaldehyde.
3. Variation: Synthesis of Salicylic Acid
If Carbon Tetrachloride ($CCl_4$) is used instead of Chloroform ($CHCl_3$), the reaction yields Salicylic Acid (2-Hydroxybenzoic acid) instead of the aldehyde.
4. Key Points for Exams
- Electrophile: Dichlorocarbene ($:CCl_2$).
- Reaction Type: Electrophilic Aromatic Substitution (EAS).
- Major Product: Ortho-isomer (Salicylaldehyde).
- Driving Force for Major Product: Intramolecular Hydrogen Bonding/Chelation makes the ortho isomer more stable and easier to isolate (steam volatile).
Reimer-Tiemann Quiz
Test your mastery of this reaction mechanism. 10 MCQs with explanations.
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