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Carboxylation of a phenoxide

Kolbe-Schmitt reaction

How the Kolbe-Schmitt reaction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.

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In short. A 3-step mechanism. Typical reagents: PhONa, CO₂ (5–100 atm); then H₂SO₄. Conditions: 125 °C, pressure, then acid work-up. Bonds formed: Ar–CO₂⁻ (C–C σ); bonds broken: Ar–H (ortho). Stereochemistry: Ortho-selective with Na⁺ (para with K⁺, higher T). Rate and selectivity: Needs dry phenoxide and CO₂ pressure; weak electrophile.

At a glance

ReagentsPhONa, CO₂ (5–100 atm); then H₂SO₄
Solvent / conditions125 °C, pressure, then acid work-up
Bonds formedAr–CO₂⁻ (C–C σ)
Bonds brokenAr–H (ortho)
StereochemistryOrtho-selective with Na⁺ (para with K⁺, higher T)
Rate / selectivityNeeds dry phenoxide and CO₂ pressure; weak electrophile

Step 1: Na⁺ chelates phenoxide and CO₂

+ONaOOδ⁻δ⁻1 · NA⁺ CHELATES PHENOXIDE AND CO₂sodium phenoxideC₆H₅NaONUCLEOPHILEcarbon dioxideELECTROPHILENa⁺ bridges O⁻ and CO₂, delivering the electrophile to the ortho position.

Sodium phenoxide's electron-rich ring, further activated by chelation of the sodium ion to CO₂, positions the ortho carbon to attack CO₂'s electrophilic carbon.

Step 2: Ortho carbon attacks CO₂

+Na⁺ templateOOOOOO2 · ORTHO CARBON ATTACKS CO₂phenoxideC₆H₅O⁻NUCLEOPHILEcarbon dioxideELECTROPHILEketo intermediate (carboxylate)C₇H₅O₃⁻INTERMEDIATEAromaticity is briefly lost (cyclohexadienone) and a carboxylate forms.

The ring's π electrons attack CO₂ directly — the same nucleophilic-ring logic as any EAS, just with CO₂ standing in for a typical electrophile — forming a new C–C bond and a carboxylate.

Step 3: Tautomerise, then acidify

H shift,re-aromatiseH₂SO₄work-upOOOOOOHOHOOH3 · TAUTOMERISE, THEN ACIDIFYketo intermediateC₇H₅O₃⁻INTERMEDIATEsalicylateC₇H₅O₃⁻INTERMEDIATEsalicylic acidC₇H₆O₃PRODUCTAcetylation of salicylic acid gives aspirin.

Protonation on acidic workup gives salicylic acid, the direct precursor to aspirin — industrially, this exact reaction is how salicylic acid is manufactured at scale.

Abbreviations and terms

Ar
Aryl group. Any aromatic ring substituent such as phenyl.
H2SO4
Sulfuric acid. Strong acid and dehydrating agent.
EAS
Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.

How it compares with related reactions

Kolbe-Schmitt reactionAcetic acid transfers a proton to waterAcid-catalyzed amide hydrolysis
ReagentsPhONa, CO₂ (5–100 atm); then H₂SO₄Acetic acid (pKa 4.76) + waterAmide, H₃O⁺ (HCl or H₂SO₄), H₂O
Conditions125 °C, pressure, then acid work-upAqueous solution, 25 °C6 M acid, reflux (hours)
StereochemistryOrtho-selective with Na⁺ (para with K⁺, higher T)None (achiral species)No stereocentre created; α-stereocentres retained
Rate / selectivityNeeds dry phenoxide and CO₂ pressure; weak electrophileEquilibrium: Ka = 1.8 × 10⁻⁵, lies mostly leftWater attack / C–N cleavage rate-limiting; far slower than ester hydrolysis

Test yourself

Try to answer each question before opening it.

Which reagents and conditions are typical for the Kolbe-Schmitt reaction?

PhONa, CO₂ (5–100 atm); then H₂SO₄. Conditions: 125 °C, pressure, then acid work-up.

Which bonds are formed and which are broken?

Formed: Ar–CO₂⁻ (C–C σ). Broken: Ar–H (ortho).

What is the stereochemical outcome?

Ortho-selective with Na⁺ (para with K⁺, higher T)

What controls the rate or the selectivity?

Needs dry phenoxide and CO₂ pressure; weak electrophile

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