Imagine you have a benzene ring with an –OH group attached — that's phenol. Now, phenol is weakly acidic, so when you treat it with a strong base like NaOH, you get sodium phenoxide. That oxygen now carries a full negative charge. That negative charge is the key.
The oxygen's lone pairs are strongly electron-donating. They push electron density into the ring, making the ortho and para positions much more reactive toward electrophiles. The ortho position, being right next to the oxygen, gets the most activation. This is the intuition: the negatively charged oxygen "wakes up" the ring, especially the ortho carbon, making it hungry for a positive or electron-deficient species.
Now, enter carbon dioxide. CO₂ is a weak electrophile — the carbon is partially positive because of the two oxygens pulling electrons away. Under normal conditions, CO₂ is too weak to attack phenol directly. But under high pressure (about 4–7 atm) and moderate heat (125–150°C), the activated ortho position of sodium phenoxide can attack the carbon of CO₂.
Important
The Kolbe reaction is the carboxylation of sodium phenoxide using CO₂ under pressure, followed by acidification, to give salicylic acid (2-hydroxybenzoic acid). The carboxyl group (–COOH) attaches exclusively at the ortho position relative to the –OH group.
The precise steps
Formation of sodium phenoxide
C6H5OH+NaOH→C6H5ONa+H2O
Carboxylation under pressure
The ortho carbon of the phenoxide ion attacks CO₂. A tetrahedral intermediate forms, which then rearranges to give sodium salicylate.
Acidification
Treating with dilute HCl gives the free carboxylic acid.
o-HO-C6H4-COONa+HCl→o-HO-C6H4-COOH+NaCl
Note
Why only ortho? The para position is also activated, but steric hindrance from the bulky –ONa group and the incoming CO₂ molecule makes para attack unfavourable. The ortho position is both electronically favoured and sterically accessible.
Why this reaction matters
Salicylic acid is the precursor to aspirin (acetylsalicylic acid). The Kolbe reaction is the industrial route to salicylic acid — cheap, simple, and high-yielding. Without it, aspirin would be far more expensive. …
Kolbe's reaction carboxylates the electron-rich phenoxide ring using CO₂ as the electrophile, and acidifying the resulting salt then releases the free acid. …
Phenoxide's electron-rich ring is carboxylated at the ortho position by CO2 acting as an electrophile, and acidifying the resulting carboxylate salt releases the free acid.
Sodium phenoxide (C₆H₅ONa, formed by treating phenol with NaOH) is heated with CO₂ gas under pressure at about 400 K (4–7 atm). The electron-rich phenoxide ring is attacked by CO₂ (acting as an electrophile) predominantly at the ortho position (favoured by chelation of the incipient carboxylate with the adjacent O⁻ in the transition state), giving sodium salicylate:
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2023Set ANNUAL1 mark
Q.Write chemical equation to prepare salicylic acid from phenol.
›Reveal solutionSolution
Salicylic acid is made from phenol via Kolbe's reaction: phenol is first converted to sodium phenoxide, which reacts with CO2 and is then acidified.
Step 1: Phenol is treated with NaOH to form sodium phenoxide (more nucleophilic than phenol itself):
C6H5OH + NaOH -> C6H5ONa + H2O
Step 2 (Kolbe's reaction): Sodium phenoxide is heated with CO2 at about 400 K under 4-7 atm pressure. The electrophilic carbon of CO2 is attacked at the ortho position (electrophilic substitution), and after tautomerisation, sodium salicylate is obtained:
C6H5ONa + CO2 --(400 K, 4-7 atm)--> sodium salicylate (2-hydroxybenzoate, Na salt)