decarboxylation

Decarboxylation is a molecule shrugging off its carboxyl group and breathing out carbon dioxide. The fizz when you bake with baking soda, the loss of CO2 that ages and flattens certain compounds, and key steps in how your cells burn sugar all involve a carboxyl group leaving as CO2 gas. It is the carboxyl family's exit reaction.

In decarboxylation a carboxylic acid (or, more often, a carboxylate) loses CO2, and the carbon that was attached to the carboxyl keeps the electron pair. Most ordinary carboxylic acids do not do this easily — plain acetic acid is happy to stay intact. The trick is having a group right next to the carboxyl that can stabilize the electron pair left behind. Beta-keto acids are the classic case: when a carbonyl sits two carbons away (a 1,3 relationship), the molecule can lose CO2 through a tidy six-membered cyclic transition state, the departing electrons flowing into the neighboring carbonyl to form a stable enol that then tautomerizes to a ketone. Malonic acids (two carboxyls on one carbon) decarboxylate similarly on heating.

This is why decarboxylation is the finishing move in two famous synthetic strategies — the malonic ester synthesis and the acetoacetic ester synthesis — where you build up a molecule around a stabilizing carboxyl group and then heat to jettison it as CO2 at the end. Biology relies on the same chemistry constantly: the citric acid cycle releases CO2 by enzyme-catalyzed decarboxylations, and decarboxylases turn amino acids into important amines (for instance, converting an amino acid into a neurotransmitter). Releasing CO2 is irreversible in practice, which makes it a reliable way to commit a synthesis to its product.