Ch24: Phenols + CO2

Chapter 24: Phenols

Carboxylation of Phenols (Kolbe-Schmitt reaction)

reaction of a phenolate with CO2

Reaction type: Electrophilic Aromatic Substitution

Summary

Heating the nucleophilic phenolate salt with carbon dioxide under high pressure / temperature results in regioselective ortho-substitution.
This process is also known as the Kolbe-Schmitt synthesis.
o-hydroxybenzoic acid is more commonly known as salicyclic acid.


Check out the CHIME image for the intramolecular hydrogen bond by looking the the postion of the shared H atom with respect to the two oxygen atoms

QUESTIONS

Aspirin is the acetyl ester of salicylic acid (hence ASA), how would you make aspirin from salicylic acid ? ANSWER
Can you think of other reactions of nucleophilic species with carbon dioxide ? ANSWER

Study Tip:
Consider the phenolate to be an enolate, hence reactions at the a-C are typically favoured.

MECHANISM FOR CARBOXYLATION OF PHENOLS

Step 1:
The nucleophilic phenolate (reacting like an enolate) reacts with the electrophilic carbon of carbon dioxide in the ortho position (compare this with an Aldol reaction)

Step 2:
The non-aromatic cyclohexadienonecarboxylate intermediate tautomerises to the more stable aromatic enol which is further stabilised by an intramolecular hydrogen bond. An acidic work-up will generate the carboxylic acid.

MECHANISM FOR CARBOXYLATION OF PHENOLS
Step 1: The nucleophilic phenolate (reacting like an enolate) reacts with the electrophilic carbon of carbon dioxide in the ortho position (compare this with an Aldol reaction)
Step 2: The non-aromatic cyclohexadienonecarboxylate intermediate tautomerises to the more stable aromatic enol which is further stabilised by an intramolecular hydrogen bond. An acidic work-up will generate the carboxylic acid.