Chapter 19: Carboxylic Acids

Summary

• Loss of carbon dioxide is called decarboxylation.
• Simple carboxylic acids rarely undergo decarboxylation.
• Carboxylic acids with a carbonyl group at the 3- (or b-) position readily undergo thermal decarboxylation, e.g. derivatives of malonic acid.

• The reaction proceeds via a cyclic transition state giving an enol intermediate that tautomerises to the carbonyl.

DECARBOXYLATION
Step 1: Remember curly arrows flow.... Start at the protonation of the carbonyl, break the O-H bond and form the p bond, break the C-C and make the C=C. Note the concerted nature of this reaction and the cyclic transition state.
Step 2: Tautomerisation of the enol of the carboxylic acid leads to the acid product (not shown here).