Kolbe Schmitt Carboxylation

Aromatic Reactions: Kolbe–Schmitt Carboxylation of Phenols

Aromatic Reactions: Kolbe–Schmitt Carboxylation (Phenol → Hydroxybenzoic Acid)

The Kolbe–Schmitt reaction converts phenols into hydroxybenzoic acids by deprotonating the phenol, pushing CO₂ into the activated ring at ortho and/or para positions under heat/pressure, rearomatizing under basic conditions, and finally protonating every oxygen during acidic workup. Classic NaOH/CO₂ conditions favor salicylic acid, while blocking groups, potassium cations, or hotter profiles bias para. Because the electrophile is CO₂ (not a carbocation), no rearrangements occur; orientation is controlled entirely by the phenoxide ion pair and any pre-existing substituents.


Key Emphasis (Teaching Pivots)

  • Activation first: Phenol must be deprotonated to the phenoxide/alkali-metal ion pair before CO₂ insertion is feasible.
  • CO₂ is the electrophile: The aryl carbon donates into CO₂ (no carbocation); the σ-complex is purely polar.
  • Ortho vs para control: Na⁺ plus moderate T pushes ortho; bulkier cations, hotter/longer profiles, or ortho blocking boost para.
  • Rearomatization/base relay: Phenoxide (or another base) removes the benzylic proton to reform the aromatic system while leaving a carboxylate salt.
  • Acidic workup is mandatory: Both the phenoxide and the carboxylate must be protonated to isolate hydroxybenzoic acids.

Quick Summary

  • Reagents/conditions: NaOH or KOH (dry) → phenoxide; CO₂ (several bar, 100–170 °C) → σ-complex; aqueous acid workup → hydroxybenzoic acids.
  • Outcome: o-Hydroxybenzoic acid (salicylic acid) dominates for free ortho positions; para product grows if ortho sites are blocked or K⁺/higher T is used.
  • Mechanism: Base deprotonation → CO₂ attack (σ-complex) → base-assisted rearomatization → protonate both oxygens on workup.
  • Orientation knobs: Metal cation, temperature/pressure profile, and steric blocking near ortho positions.
  • Common pitfalls: Forgetting to form phenoxide, drawing carbocation-style intermediates, or omitting the final protonations.

Mechanism — Kolbe–Schmitt (5 Frames)

Step 1: Base deprotonates phenol to phenoxide
Step 1 — Base deprotonation. NaOH (or KOH) removes the phenolic proton, leaving an ion-paired phenoxide that directs incoming CO₂ toward ortho/para sites.
Step 2: CO₂ addition to the ring
Step 2 — CO₂ addition to form the σ-complex. The activated ring carbon donates π electrons into CO₂ at ortho or para, generating a cyclohexadienyl carboxylate σ-complex (aromaticity temporarily lost).
Step 3: Base removes benzylic proton
Step 3 — Base-assisted rearomatization. Phenoxide (or another base) removes the benzylic proton to reform the aromatic π system, leaving an aryl carboxylate salt.
Step 4: Acidic workup
Step 4 — Acidic workup. Aqueous acid protonates the phenoxide and the carboxylate, releasing neutral o- and/or p-hydroxybenzoic acids.
Kolbe–Schmitt product set
Step 5 — Product set & orientation. The router highlights both ortho and para hydroxybenzoic acids; Na⁺/moderate temperatures bias ortho salicylic acid, whereas blocking groups or K⁺/higher T swing the outcome toward para.

Mechanistic Checklist

  • Start from phenoxide (show the base removing the phenolic proton).
  • Depict CO₂ insertion at ortho/para, not O-carboxylation.
  • Include the σ-complex and base-assisted rearomatization step.
  • Finish with protonation of both the phenoxide and the carboxylate.
  • Mention the selectivity knobs (cation, temperature/pressure, blocking groups).

Worked Examples

1. Phenol → Salicylic Acid (major) + p-Hydroxybenzoic Acid (minor)

Phenol reactant
Phenol
Kolbe–Schmitt reagent button
CO₂, base → acid workup
Phenol Kolbe–Schmitt products
Salicylic acid (major) + para minor

Classic Kolbe–Schmitt conditions (NaOH → CO₂, 100–150 °C, pressure) furnish salicylic acid predominantly, with a para co-product.

2. o-Methylphenol (one ortho blocked) → Para-biased product

o-Methylphenol reactant
o-Methylphenol
Kolbe–Schmitt reagent button
Kolbe–Schmitt set
Para-hydroxy-m-methylbenzoic acid
Para-hydroxy-m-methylbenzoic acid

Blocking one ortho site pushes CO₂ to the para position, boosting the p-isomer at the expense of salicylic acid.

3. 2,6-Dimethylphenol (both ortho blocked) → Para only

2,6-dimethylphenol reactant
2,6-Dimethylphenol
Kolbe–Schmitt reagent button
Kolbe–Schmitt set
Para-only product
Para product only

With both ortho sites blocked, para is the sole accessible site—exactly what the router enforces.

4. 3,5-Dimethylphenol → Single accessible site

3,5-dimethylphenol reactant
3,5-Dimethylphenol
Kolbe–Schmitt reagent button
Kolbe–Schmitt set
p-Hydroxy-3,5-dimethylbenzoic acid
p-Hydroxy-3,5-dimethylbenzoic acid

Only one para site is open, so the router funnels CO₂ there—useful when teaching steric gating.

5. Potassium Phenoxide + Higher T → Para-leaning mixture

Potassium phenoxide reactant
K⁺ phenoxide
Kolbe–Schmitt reagent button
Hot Kolbe–Schmitt
Potassium phenoxide products
Para-leaning mixture

Bulkier cations plus higher temperature bias para even without steric blocks—handy for process knobs questions.


Scope & Limitations

  • Substrates: Phenols and phenoxide salts; aromatic rings must retain at least one ortho/para hydrogen.
  • Selectivity tuning: Na⁺ + 120–140 °C → ortho; K⁺/Rb⁺, 150 °C+, or bulky ortho substituents → para.
  • Functional tolerance: Strongly deactivating groups (–NO₂, –CF₃, –SO₃H) slow or halt the sequence. Protect amines/alcohols that would be protonated or destroyed under base/heat.
  • Steric blocking: Two ortho blocks enforce para; one block makes para competitive; para block (e.g., p-cresol) funnels reaction to the remaining ortho slot.
  • Process constraints: Requires dry base, pressure-rated CO₂ equipment, and carefully staged acid workup.

Edge Cases & Exam Traps

  • Forgetting phenoxide formation → no carboxylation.
  • Drawing a carbocation intermediate → incorrect (CO₂ is electrophile; ring is nucleophile).
  • Claiming ortho attack after both ortho sites are blocked.
  • Skipping acid workup → leaves salts (ArO⁻/ArCO₂⁻) rather than the named hydroxybenzoic acids.
  • Assuming para is always minor: with K⁺ or strongly hindered ortho sites, para can dominate.

Practical Tips

  • Dry the phenoxide salt and purge moisture before charging with CO₂.
  • Pre-pressurize with CO₂ before heating to avoid solvent bumping.
  • Monitor ortho/para ratio by NMR (intramolecular H-bond of salicylic acid gives a diagnostic downfield OH).
  • Cool and depressurize before acidifying; then protonate both oxygens with mineral acid to ensure neutral product isolation.

Exam-Style Summary

Phenol + strong base → phenoxide. The ring carbon donates into CO₂ to give an ortho/para σ-complex. Base removes the adjacent proton to restore aromaticity, leaving an aryl carboxylate salt. Acidic workup protonates both oxygens to release salicylic acid (ortho) and/or p-hydroxybenzoic acid. Ortho is favored under Na⁺/moderate T; para grows when ortho is blocked or when hotter K⁺ conditions are used. No rearrangements or carbocations are involved.


Interactive Toolbox

  • Use Mechanism Solver to step through phenoxide activation, CO₂ insertion, base-induced rearomatization, and acid workup with tunable cation/temperature controls.
  • Use Reaction Solver to predict whether ortho, para, or “no reaction” is routed for a given phenol, including warnings for deactivation or blocked sites.
  • Use IUPAC Namer to confirm names such as 2-hydroxybenzoic acid, 4-hydroxybenzoic acid, or p-hydroxy-3,5-dimethylbenzoic acid.