Overall reaction
Phenol + CHCl₃ + NaOH (aq), 60–70 °C, then H₃O⁺ → salicylaldehyde
Dichlorocarbene from chloroform formylates a phenoxide at the ortho position.
Name reaction · mechanism
Reimer–Tiemann Reaction
The Reimer–Tiemann reaction turns phenol into salicylaldehyde (2-hydroxybenzaldehyde) with chloroform and aqueous sodium hydroxide. The electrophile is dichlorocarbene, :CCl₂, made from chloroform by the base; phenoxide attacks it mainly at the ortho carbon, and hydrolysis of the CHCl₂ group gives the aldehyde.
Mechanism step by step
- Dichlorocarbene. NaOH deprotonates CHCl₃; CCl₃⁻ then loses Cl⁻
- Attack on the carbene. The ortho carbon of phenoxide attacks
- Tautomerisation. Proton transfer restores the aromatic ring
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- First, hydroxide removes the C–H proton of chloroform to give CCl₃⁻.
- Hydroxide then hydrolyses the CHCl₂ group to CHO; acid work-up gives salicylaldehyde.
- Ortho attack dominates; some para product forms.
- Dichlorocarbene is an electron-poor (singlet) carbene: a lone pair and an empty orbital.
- With CCl₄ instead of CHCl₃ the same route gives salicylic acid.
For JEE and NEET
- Electrophile: dichlorocarbene (:CCl₂), formed by α-elimination — CHCl₃ + OH⁻ → CCl₃⁻ → :CCl₂ + Cl⁻.
- Main product is ortho (salicylaldehyde); some para-hydroxybenzaldehyde forms.
- With CCl₄ instead of CHCl₃ the product is salicylic acid.
- The carbene is singlet: an empty p orbital (electrophilic) and a lone pair.
FAQ
What is the electrophile in the Reimer–Tiemann reaction?
Dichlorocarbene, :CCl₂. Hydroxide removes the acidic proton of chloroform to give CCl₃⁻, which loses chloride (α-elimination).
Why is the ortho product major?
The phenoxide oxygen and the ortho carbon are close to the sodium counter-ion and the carbene in the two-phase system, and the ortho intermediate is stabilised by the neighbouring O⁻; para attack also happens but is minor.
How does the CHCl₂ group become CHO?
Hydroxide replaces the two chlorines (via a chlorohydrin) and the gem-diol loses water, giving the aldehyde; acidification gives neutral salicylaldehyde.
Draw it in MolDraw
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