Decarboxylation

Decarboxylation is the loss of CO₂ from a carboxylic acid. It is generally not favorable for simple carboxylic acids (they just sit stable), but it occurs readily for beta-keto acids and beta-dicarboxylic acids via a 6-membered cyclic transition state. Decarboxylation is the central mechanism of the acetoacetic and malonic ester syntheses (Chapter 7) and the oxidative decarboxylations in glycolysis and the Krebs cycle.

Beta-Keto Acids Decarboxylate Easily

A beta-keto acid has a ketone two carbons away from the carboxylic acid, giving the pattern:

R-CO-CH₂-COOH (alpha-C between the ketone and the COOH)

When heated, beta-keto acids lose CO₂ via a concerted mechanism through a 6-membered cyclic transition state:

1. The acidic H of the COOH is transferred to the ketone oxygen (not the alpha-C).
2. Simultaneously, the C-COOH bond breaks.
3. CO₂ leaves.
4. The result is an enol, which rapidly tautomerizes to the ketone (the final product).

The 6-membered cyclic TS involves atoms: COOH’s O-H, its C=O, the alpha-C, the ketone’s C=O, and the ketone’s O. A geometrically convenient 6-membered ring.

Malonic Acid Also Decarboxylates

Malonic acid and malonic acid derivatives (like malonate esters) decarboxylate similarly. The mechanism goes through the same 6-membered cyclic TS, just using the second COOH as the proton acceptor.

Note: dicarboxylic acids with the two COOHs farther apart (like succinic acid, 4-carbon diacid) do NOT decarboxylate readily because the 6-membered cyclic TS is not possible.

Summary: Which Acids Decarboxylate?

Rule: decarboxylation is favorable whenever a 6-membered cyclic transition state can form for concerted CO₂ loss. This requires:

1. An acidic H on the COOH (present on all carboxylic acids).
2. A nearby carbonyl (ketone or second carboxylate) positioned to accept the H.

The position of the second carbonyl must be at the BETA carbon (two bonds away from the COOH carbon).

Common substrates that decarboxylate:

• Beta-keto acids (e.g., acetoacetic acid): ketone beta to COOH → decarboxylates readily.
• Malonic acid: two COOHs separated by one CH₂ → decarboxylates on strong heating.
• Beta-keto esters after saponification → same as beta-keto acids.

Non-decarboxylating:

• Simple carboxylic acids (acetic acid does NOT decarboxylate under ordinary heating).
• Succinic acid (two COOHs separated by CH₂-CH₂, too far apart for 6-ring TS).
• Acids with no beta carbonyl.

The Acetoacetic / Malonic Ester Syntheses Use This

The acetoacetic ester synthesis (Section 7.9) and the malonic ester synthesis both end with decarboxylation:

1. Deprotonate ethyl acetoacetate (or diethyl malonate) with base.
2. Alkylate at the alpha-C.
3. Hydrolyze (saponify) the ester to the free acid.
4. Heat → decarboxylation loses CO₂ → product is a methyl ketone (from acetoacetate) or a substituted acetic acid (from malonate).

The “extra” ester group is the temporary scaffolding that enables clean alpha-alkylation, and decarboxylation neatly removes it at the end.

Biological Decarboxylation

Cells perform many decarboxylations:

• Pyruvate decarboxylation: pyruvate → acetaldehyde (anaerobic, yeast) or → acetyl-CoA (aerobic, PDH). Both require thiamine pyrophosphate (TPP) as cofactor.
• Isocitrate dehydrogenase (Krebs): isocitrate → alpha-ketoglutarate + CO₂. A combined oxidation + decarboxylation.
• Alpha-ketoglutarate dehydrogenase (Krebs): alpha-ketoglutarate → succinyl-CoA + CO₂.
• Amino acid decarboxylases: convert amino acids to biogenic amines (e.g., histidine → histamine, glutamate → GABA). Requires pyridoxal phosphate (PLP) as cofactor.

All of these are beta-decarboxylations (or equivalent) with cofactor-assisted 6-membered TS chemistry.