JEE Challenger
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ChemEasyAdv 2007 • Paper 2

Electrophilic Intermediate in Reimer-Tiemann Reaction

Comprehension Passage

Reimer-Tiemann reaction introduces an aldehyde group onto the aromatic ring of phenol, ortho to the hydroxyl group. This reaction involves electrophilic aromatic substitution. This is a general method for the synthesis of substituted salicylaldehydes as depicted below:

p-cresol→reagentintermediate (I)→aq. HCl2-hydroxy-5-methylbenzaldehyde\text{p-cresol} \xrightarrow{\text{reagent}} \text{intermediate (I)} \xrightarrow{\text{aq. HCl}} \text{2-hydroxy-5-methylbenzaldehyde}

The electrophile in this reaction is

Options

A

:CHCl:\text{CHCl}

B

+CHCl2^+\text{CHCl}_2

C

:CCl2:\text{CCl}_2

Correct
D

⋅CCl3\cdot\text{CCl}_3

Topics & Concepts

Alcohols, Phenols and EthersCarbenesReaction IntermediatesReimer-Tiemann Reaction

Step-by-Step Solution

Generation of the electrophile in the Reimer-Tiemann reaction:

  1. Deprotonation of chloroform by hydroxide:
    CHCl3+OH−⇌:CˉCl3+H2O\text{CHCl}_3 + \text{OH}^- \rightleftharpoons :\bar{\text{C}}\text{Cl}_3 + \text{H}_2\text{O}

  2. Spontaneous α\alpha-elimination of chloride ion:
    :CˉCl3→:CCl2+Cl−:\bar{\text{C}}\text{Cl}_3 \rightarrow :\text{CCl}_2 + \text{Cl}^-

The resulting neutral species, dichlorocarbene (:CCl2:\text{CCl}_2), has an empty pp-orbital with an open sextet on carbon, acting as a powerful neutral electrophile.

Correct Answer: C

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