The Kolbe-Schmitt Reaction

What the Kolbe-Schmitt reaction does

The Kolbe-Schmitt reaction takes a phenol, deprotonates it to a phenoxide, and staples a carbon-dioxide molecule directly onto the ring carbon next to the oxygen. The result is an aromatic ring carrying both an -OH and a -COOH group ortho to each other: salicylic acid. It is one of the very few industrial reactions in which CO₂ acts as a carbon feedstock rather than a waste gas.

    C₆H₅-ONa  +  CO₂  ──125 °C, 5 atm──→  2-HO-C₆H₄-COONa  ──H⁺──→  2-HO-C₆H₄-COOH
     sodium              carbon              sodium                    salicylic acid
    phenoxide            dioxide             salicylate               (2-hydroxybenzoic acid)

The chemistry is an electrophilic aromatic substitution with the roles inverted from what you usually expect. Normally you make an aggressive electrophile (an acylium ion, a nitronium ion) to attack an unreactive arene. Here the arene is turbo-charged into a nucleophile by the phenoxide's negative charge, and the electrophile is a mild, neutral, everyday gas — CO₂. The oxygen's lone pairs push electron density onto the ortho and para ring carbons, and one of those carbons reaches out to the electron-poor carbon of carbon dioxide.

The mechanism, arrow by arrow

Track the electrons through four moves:

1. Deprotonate the phenol. Treating phenol with NaOH (or Na metal) gives sodium phenoxide. The negative charge on oxygen delocalizes into the ring; the resonance structures place partial negative charge on the two ortho carbons and the para carbon. Those become the nucleophilic sites.
2. The sodium templates the CO₂. The Na⁺ counterion sits on the phenoxide oxygen and, under CO₂ pressure, chelates the incoming carbon dioxide by its oxygen. This ion-pair bridge parks the electrophilic carbon of CO₂ directly above an ortho ring carbon — this templating is the origin of the ortho selectivity.
3. C-C bond forms; the ring goes non-aromatic. The ortho carbon's electron density attacks the central carbon of CO₂. A new C-C bond forms and the ring becomes a cyclohexadienone-type intermediate — a keto (dearomatized) intermediate bearing a carboxylate on the sp³-like carbon and a C=O where the phenol oxygen used to be. Aromaticity is momentarily lost, exactly as in the σ-complex (arenium/Wheland intermediate) of any electrophilic aromatic substitution.
4. Rearomatize by a 1,3-proton shift (tautomerization). The proton on the newly substituted carbon migrates back to the oxygen. The ring restores its aromatic sextet, giving the aromatic sodium salicylate. Acidic workup (H₂SO₄ or HCl) protonates the carboxylate to free salicylic acid.

   step 1:  PhOH  +  NaOH   →   PhO⁻ Na⁺   +  H₂O        (make the nucleophile)
   step 2:  PhO⁻ Na⁺  +  O=C=O   →   [Na⁺ bridges O(ring)···O=C=O]   (chelate/template)
   step 3:  ortho-C attacks CO₂ carbon  →  keto (dearomatized) carboxylate intermediate
   step 4:  1,3-H shift  →  aromatic 2-hydroxybenzoate  →  (H⁺) salicylic acid

The keto intermediate is the crux. Because the added carboxylate and the ring oxygen end up cis and adjacent, the ortho salicylate can form a strong intramolecular hydrogen bond (the O-H of the phenol to the C=O of the carboxylate). That hydrogen bond is worth roughly 25-30 kJ/mol of stabilization and is a big part of why the ortho product is thermodynamically preferred once sodium has templated it there.

Reagents, catalyst, and real conditions

• Substrate. Sodium phenoxide (C₆H₅ONa), rigorously dry. Substituted phenoxides work too — naphthols, cresols, resorcinol, β-naphthol (which gives 2-hydroxy-3-naphthoic acid, a pigment intermediate known as BON acid).
• Carbon source. Carbon dioxide gas, supplied under pressure. No exotic reagent — this is why the reaction scaled industrially.
• Pressure. Schmitt's key improvement was running the reaction in a sealed autoclave under pressurized CO₂. Typical figures: about 4-7 atm (roughly 100 psi), though modern plants push higher to accelerate the sluggish gas-solid reaction.
• Temperature. 120-140 °C for the sodium/ortho pathway. Kolbe's original 1860 conditions (dry phenoxide + CO₂ at ambient pressure, heated) worked but gave low yields; Schmitt's pressurized version pushed conversion up.
• Water is the enemy. The phenoxide must be anhydrous — the salt is often pre-dried at ~130 °C under vacuum. Residual water hydrolyzes intermediates and caps the yield.
• Yield. Industrial salicylic acid runs around 60-70% on the sodium route; unreacted phenol is recovered and recycled.
• No transition-metal catalyst. Unlike the palladium cross-couplings, the Kolbe-Schmitt needs no precious metal — the alkali cation is the only "catalyst," and it is consumed stoichiometrically as the salt.

Regioselectivity: sodium ortho, potassium para

The single most tested fact about this reaction is the cation effect on regiochemistry:

• Sodium phenoxide → ortho (salicylic acid). The small, hard Na⁺ chelates tightly between the phenoxide oxygen and CO₂, holding the electrophile over the adjacent (ortho) carbon. Ortho dominates.
• Potassium phenoxide → para (p-hydroxybenzoic acid). The larger K⁺ coordinates the oxygen more loosely and cannot pin CO₂ over the ortho carbon. Carboxylation drifts to the para position, which is the least hindered and still electron-rich. K⁺ also tends to favor the para route more strongly at higher temperature.
• Temperature tilts it too. Higher temperatures generally favor the thermodynamically stabler para product (or the more substituted product on complex phenols), because the ortho salicylate can retro-carboxylate and re-form. The ortho product is kinetically templated; the para is often the thermodynamic sink for the larger cations.

This is not a stereochemistry story — salicylic acid is planar and achiral, so there is no enantioselectivity to discuss. The selectivity that matters here is regiochemistry: which ring carbon gets the -COOH, decided almost entirely by the templating cation.

Kolbe-Schmitt vs other ways to install a -COOH

	Kolbe-Schmitt	Grignard + CO₂	Friedel-Crafts acylation + hydrolysis
Carbon source	CO₂ gas (cheap, waste-derived)	CO₂ gas (quenched onto RMgX)	Acyl chloride / anhydride
Nucleophile	Phenoxide ring carbon	Carbanion (R-MgX)	Arene (activated)
Needs a metal catalyst?	No — alkali cation templates	Mg metal to form the reagent	Stoichiometric AlCl₃
Water tolerance	Must be anhydrous	Strictly anhydrous	Anhydrous
Regiocontrol	Ortho (Na) / para (K), cation-set	Wherever the C-Mg bond was	Directed by ring substituents
Installs -OH too?	Yes — starts from a phenol	No — only the acid	No — gives a ketone first
Scale	Multi-thousand-ton (aspirin)	Lab / fine-chemical	Bulk aromatics
Green credential	Uses CO₂ as a C₁ building block	Uses CO₂, but needs Mg	Chloride waste, AlCl₃ sludge

Worked example: phenol to aspirin in two steps

The Kolbe-Schmitt reaction is the first half of the classic two-step aspirin synthesis. Start from phenol, finish at acetylsalicylic acid.

  Step A — Kolbe-Schmitt carboxylation
    C₆H₅OH  --NaOH-->  C₆H₅ONa  --CO₂, 125 °C, 5 atm-->  2-NaO₂C-C₆H₄-OH (sodium salicylate)
    then  H₂SO₄  →  2-HOOC-C₆H₄-OH   (salicylic acid, ~65% yield)

  Step B — acetylation to aspirin
    salicylic acid  +  (CH₃CO)₂O  --cat. H₂SO₄ or H₃PO₄, 90 °C-->  acetylsalicylic acid  +  CH₃COOH
                       acetic anhydride                              (ASPIRIN)

• Step A is the Kolbe-Schmitt: the ortho carbon of sodium phenoxide attacks CO₂; acidic workup liberates salicylic acid.
• Step B acetylates the phenolic -OH (not the -COOH) with acetic anhydride. The acid catalyst protonates the anhydride; the phenol oxygen does a nucleophilic acyl substitution, expelling acetate. The carboxylic acid installed in Step A is a spectator.
• Why it matters. Salicylic acid itself is a potent keratolytic and anti-inflammatory but is harsh on the stomach; capping the phenol as the acetate gives the milder pro-drug aspirin, which Bayer commercialized in 1899. Global aspirin production is on the order of tens of thousands of tons per year, and the Kolbe-Schmitt step feeds essentially all of it.

Where the products end up

• Salicylic acid → aspirin (acetylsalicylic acid) — the flagship application, plus salicylate esters (methyl salicylate = oil of wintergreen, used in liniments) and phenyl salicylate (salol).
• p-Hydroxybenzoic acid (the potassium/para product) → parabens, the methyl-, ethyl-, and propyl-esters used as preservatives in cosmetics and food; also a monomer for liquid-crystal polymers such as Vectra.
• β-Naphthol carboxylation → BON acid (2-hydroxy-3-naphthoic acid / 3-hydroxy-2-naphthoic acid), a key coupling component for azo pigments and lake dyes.
• 4-Hydroxysalicylic and related hydroxy-acids used as pharmaceutical intermediates (for example, in the synthesis of mesalazine/5-ASA, a bowel anti-inflammatory).
• CO₂ valorization. Because it consumes carbon dioxide as a raw material, the Kolbe-Schmitt reaction is a textbook example in "CO₂ as a C₁ feedstock" green-chemistry courses.

Limitations and side reactions

• Only electron-rich phenols react. The ring must be activated as a phenoxide; simple benzene, chlorobenzene, or electron-poor arenes are inert to CO₂ under these conditions. Strongly deactivated phenols (e.g. nitrophenols) carboxylate poorly.
• Sluggish gas-solid kinetics. The reaction runs on solid phenoxide under CO₂ gas; mass transfer is slow, which is why it needs pressure, heat, and hours. This is a major reason yields sit at 60-70% rather than near-quantitative.
• Retro-Kolbe-Schmitt / decarboxylation. The C-C bond to CO₂ is reversible. Overheating drives the salicylate to lose CO₂ and revert to phenoxide, or to shuffle from ortho toward the para/thermodynamic isomer. Time and temperature must be controlled.
• Di-carboxylation and tar. With very electron-rich substrates (resorcinol, phloroglucinol) a second carboxylation or over-reaction can occur, and prolonged heating produces colored tars that complicate purification.
• Moisture-driven yield loss. Any water hydrolyzes intermediates and consumes base, so anhydrous handling is mandatory — a real operational burden at scale.

Who discovered it, and when

The reaction is named for two German chemists. Hermann Kolbe — the same Kolbe of the Kolbe electrolysis and an early champion of structural organic chemistry — first carboxylated dry sodium phenoxide with CO₂ in 1860, obtaining salicylic acid but in modest yield. In 1885 his student Rudolf Schmitt found that running the reaction in a sealed vessel under CO₂ pressure dramatically improved the conversion and cemented the ortho selectivity for the sodium salt. The combined name Kolbe-Schmitt honors both the discovery and the practical fix.

The timing mattered commercially. Salicylic acid was already known as an antipyretic and preservative, but it was scarce and expensive when isolated from willow bark or oil of wintergreen. The Kolbe-Schmitt route turned phenol and CO₂ — two cheap bulk chemicals — into salicylic acid at industrial scale, which is precisely what let Bayer's Felix Hoffmann and Arthur Eichengrün develop and launch acetylsalicylic acid as Aspirin in 1899. A 19th-century electron-pushing curiosity became the foundation of the world's most-produced synthetic drug.

Industrial and safety notes

• Autoclave hazard. The reaction runs hot under several atmospheres of CO₂ in sealed vessels; pressure-relief and temperature control are essential. Runaway heating risks both decarboxylation and vessel overpressure.
• Corrosive workup. Liberating the free acid requires mineral acid (H₂SO₄/HCl), and the resulting salicylic acid is a skin and eye irritant. Standard acid-handling PPE applies.
• Phenol toxicity. The feedstock, phenol, is corrosive and readily absorbed through skin; closed handling is standard.
• Recycling. Modern plants recover and recycle unreacted phenol and manage the sodium sulfate byproduct stream from acidification, which is a significant part of the process economics.
• Green framing. Each ton of salicylic acid fixes roughly 0.3 tons of CO₂ into the product's carboxyl group — small on a climate scale, but a genuine example of carbon dioxide used constructively rather than emitted.