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

Step 1: Recall the Reimer-Tiemann mechanism

The Reimer-Tiemann reaction converts phenol to salicylaldehyde (o-hydroxybenzaldehyde) using $\ce{CHCl3}$ and $\ce{NaOH}$.

Step 2: Step-by-step mechanism

1. $\ce{CHCl3 + NaOH -> :CCl2 + NaCl + H2O}$ → Dichlorocarbene ($\ce{:CCl2}$) is formed.

2. $\ce{C6H5OH + NaOH -> C6H5O^-Na^+}$ (sodium phenoxide — the ring is now more electron-rich).

3. Dichlorocarbene ($\ce{:CCl2}$) is an electrophile — it attacks the electron-rich ortho position of phenoxide: $\ce{C6H5O^-Na^+ + :CCl2 -> o-(NaO)C6H4-CHCl2}$

   Intermediate = sodium phenoxide with -CHCl2 at ortho position ✓

4. Alkaline hydrolysis of $\ce{-CHCl2}$ group: $\ce{-CHCl2 + 2NaOH -> -CHO + 2NaCl + H2O}$

5. Acidification → 2-hydroxybenzaldehyde (salicylaldehyde).

Step 3: Common mistakes

• Option (c) shows -CHO at ortho — that is the final product, not the intermediate.
• Option (b) is neutral phenol, not the sodium phenoxide form.
• The intermediate must be the sodium salt form with -CHCl2 (before hydrolysis).

Key Points to Remember:

• Reimer-Tiemann intermediate: sodium o-(dichloromethyl)phenoxide.
• Dichlorocarbene (:CCl2) is the key electrophile, generated from CHCl3 + NaOH.
• After formation of -CHCl2 intermediate → hydrolysis → -CHO → salicylaldehyde.