Reimer Tiemann

Reimer-Tiemann Reaction

The Reimer-Tiemann reaction introduces an aldehyde group (–CHO) at the ortho position of phenol using chloroform (CHCl₃) and aqueous NaOH, followed by acid hydrolysis. The product from phenol is salicylaldehyde (2-hydroxybenzaldehyde).

Mechanism in brief:

1. CHCl₃ reacts with strong base (NaOH) to generate dichlorocarbene (:CCl₂), an electrophilic carbene intermediate. This is the rate-determining step.
2. Dichlorocarbene attacks the electron-rich ortho position of the phenoxide ion (phenol deprotonated by NaOH).
3. The intermediate dichloromethyl phenol undergoes hydrolysis in aqueous alkali to yield the –CHO group.

Key points to lock in:

• The reactive species is dichlorocarbene (:CCl₂), not trichloromethyl anion or free CHCl₃.
• The reaction is specific to phenols — ordinary alcohols or aromatic hydrocarbons without –OH do not undergo it.
• Substitution occurs preferentially at the ortho position relative to –OH. If both ortho positions are blocked, the para product (4-hydroxybenzaldehyde) forms.
• If CCl₄ is used instead of CHCl₃, the product is a salicylic acid derivative (Kolbe-type carboxylation path) — this is a common NEET swap distractor.
• The reaction proceeds under alkaline conditions (aqueous NaOH), not acidic.

NEET trap to watch: Questions may offer "trichloromethyl carbanion" or "CHCl₃ directly" as the electrophile. The correct intermediate is always dichlorocarbene (:CCl₂). A second common confusion: swapping the CHCl₃ product (aldehyde) with the CCl₄ product (carboxylic acid).

Reference: NCERT Class 12 Chemistry, Chapter 7 (Alcohols, Phenols and Ethers), Part 2, page 16.

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