Kolbe–Schmitt Reaction

Introduction

Carbon dioxide is a weak electrophile, yet it can add a carboxyl group to an activated aromatic ring. In the Kolbe–Schmitt reaction, phenol is converted to phenoxide and treated with CO₂ under suitable pressure and heat. The usual sodium phenoxide route favours a carboxyl group ortho to the oxygen substituent. Acid work-up gives salicylic acid, an important precursor to acetylsalicylic acid.

Core explanation

First remove the phenolic O–H proton with base to form phenoxide, C₆H₅O⁻. The negative charge is not confined to oxygen in resonance descriptions: oxygen donation increases electron density at ortho and para positions of the aromatic ring. This activation enables attack on CO₂, whose carbon is electrophilic because both C–O bonds are polarised toward oxygen. Ordinary benzene is insufficiently electron rich for the same easy carboxylation, so making phenoxide is mechanistically important.

Under classical Kolbe–Schmitt conditions, sodium phenoxide and pressurised CO₂ are heated. The aromatic ring forms a bond to the carbon of CO₂ at an ortho position, temporarily disrupting aromaticity. Proton transfer and rearomatisation produce a hydroxybenzoate salt. The sodium salt of salicylic acid is a common major product. Treat the placement as a product rule tied to the usual sodium phenoxide conditions, not as proof that para carboxylation can never occur: cation and temperature can alter regioselectivity in some variants.

The carbon atom added to the ring comes from CO₂, and both oxygen atoms of that CO₂ become part of the carboxylate group. The original phenolic oxygen remains attached to the ring. Tracking those atoms distinguishes C-carboxylation at an aromatic carbon from simply attaching CO₂ to oxygen as a carbonate. Some mechanistic discussions consider oxygen-bound intermediates, but the named synthetic target is a new ring C–C bond to a carboxyl group.

The product under basic conditions is a carboxylate salt. Acidification protonates it to salicylic acid, 2-hydroxybenzoic acid, HO–C₆H₄–COOH with the OH and COOH adjacent. If a question stops before acid work-up, write sodium salicylate or its ionic form. If it includes H₃O⁺, write the free carboxylic acid. Phenolic OH and carboxylic-acid OH are separate functional groups in that final structure.

The salicylic-acid framework matters beyond an exercise. Its phenolic OH can be acetylated to make acetylsalicylic acid, while its carboxylic acid remains. The Kolbe–Schmitt step installs the carbonyl-containing group; the later acetylation modifies oxygen. Recognising these two distinct bond changes prevents an answer that confuses the product with aspirin itself.

Compare this reaction with Reimer–Tiemann formylation. Both begin with phenoxide activation and often put a new carbon-containing group ortho to OH. Kolbe–Schmitt uses CO₂ to install –COOH after acid work-up. Reimer–Tiemann uses a dichlorocarbene-derived electrophile and eventually gives –CHO. The common substrate does not imply the same final oxidation state of the new carbon group.

Step-by-step reasoning

Draw phenol and convert it to phenoxide. Mark ortho carbons as activated positions. Treat CO₂ carbon as the electrophile and connect one ortho carbon to it, then restore aromaticity. Write the carboxylate salt under basic reaction conditions. If acid work-up appears, protonate carboxylate and identify 2-hydroxybenzoic acid.

Visual explanation

Draw the phenoxide ring with O⁻ at carbon 1 and highlighted ortho carbons 2 and 6. Put CO₂ beside carbon 2 and draw a new ring-C-to-CO₂-carbon bond. Redraw the final ring with adjacent OH and COO⁻ labels; after a separate H⁺ arrow, change COO⁻ to COOH.

Real-world analogy

Think of phenoxide as a ring with two nearby docking positions made more welcoming by an oxygen donor. CO₂ supplies a single new carbon-containing module that docks at one of those positions. Acid work-up changes the module's charge but not where it is attached. The analogy distinguishes building the C–C bond from simply protonating the product.

Real-world example

The salicylic acid framework produced after Kolbe–Schmitt carboxylation is a precursor to aspirin. A later acetylation of its phenolic oxygen gives acetylsalicylic acid. In a synthesis diagram, the CO₂ step adds the carboxyl carbon, whereas the acetylation step adds a different two-carbon acyl fragment to oxygen.

Why?

Why is phenoxide used instead of benzene? Phenoxide donates electron density into its aromatic ring, activating ortho and para positions sufficiently to react with weakly electrophilic CO₂ under pressure and heat. Benzene lacks that strong donor and does not undergo the same useful carboxylation under comparable conditions.

Common misconception

"The CO₂ bonds to phenolic oxygen and the product is an aromatic carbonate." The named Kolbe–Schmitt product has a new bond between an aromatic carbon and the CO₂ carbon. After acid work-up it is hydroxybenzoic acid, with separate phenolic OH and carboxylic acid groups.

Worked example

Question: Name the principal acid-work-up product when sodium phenoxide reacts with CO₂ under usual Kolbe–Schmitt conditions.

Reasoning: Sodium phenoxide activates its ortho ring position. CO₂ supplies a carboxyl carbon, and carboxylation gives sodium salicylate. Acid work-up protonates carboxylate without moving the new C–C bond.

Answer: Salicylic acid, also called 2-hydroxybenzoic acid.

Quick check

1. Which reagent supplies the new carboxyl carbon in Kolbe–Schmitt chemistry? Answer: Carbon dioxide supplies the carbon of the new ring-bound carboxyl group.

Exam focus

Show phenoxide formation, CO₂ as electrophile, and ortho C-carboxylation in the usual sodium system. Label reaction conditions separately from acid work-up. Draw adjacent OH and COOH in salicylic acid and avoid confusing it with aspirin or an oxygen carbonate.

Advanced insight

The exact ortho/para balance can depend on counterion, temperature and other conditions, so the standard sodium-salicylate outcome is a specified synthetic case rather than an absolute aromatic directing law. The original O⁻ activates the ring electronically, while ion pairing and reaction conditions influence which activated position reacts most efficiently.

Summary

Kolbe–Schmitt reaction carboxylates phenoxide with CO₂, commonly at the ortho ring carbon under sodium phenoxide conditions. The reaction makes a new C–C bond and first gives a salicylate salt. Acid work-up yields salicylic acid, which can be further acetylated to aspirin. CO₂ provides the carboxyl carbon while phenolic oxygen stays on the ring.

Practice questions

1. What changes when phenol is treated with base before CO₂ exposure? Answer: It becomes phenoxide, whose oxygen donor activates the aromatic ring. 2. What is the usual ring relationship between OH and COOH in salicylic acid? Answer: They are ortho, on adjacent aromatic carbons. 3. What should be drawn before acid work-up? Answer: A salicylate carboxylate salt rather than neutral salicylic acid. 4. How does the Kolbe–Schmitt product differ from a Reimer–Tiemann product? Answer: It has a ring-bound carboxyl group, whereas Reimer–Tiemann typically installs an aldehyde group.