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Carboxylation is a chemical reaction in which a carboxyl group (-COOH) is introduced into a substrate, typically by reaction with carbon dioxide (CO2) or by transfer of CO2 in enzymatic processes. In biological systems, carboxylation plays a central role in metabolism and CO2 fixation. Enzymes such as RuBisCO fix CO2 in photosynthesis, combining CO2 with ribulose-1,5-bisphosphate to form two molecules of 3-phosphoglycerate in the Calvin cycle. Other biotin-dependent carboxylases, including pyruvate carboxylase and acetyl-CoA carboxylase, incorporate CO2 into organic substrates as part of energy metabolism and fatty acid synthesis. These reactions are often anaplerotic, replenishing four-carbon intermediates in the citric acid cycle or providing building blocks for biosynthesis.

Chemical carboxylation methods extend the concept to synthetic chemistry. Classic approaches use CO2 as the electrophile for nucleophilic carbanions or organometallic reagents, yielding carboxylic acids after hydrolysis. The Kolbe–Schmitt reaction is a notable example in which CO2 is introduced into a phenoxide under high pressure to give salicylic acid. Modern methods include transition-metal-catalyzed carboxylation of aryl or vinyl halides with CO2 to form aryl and vinyl carboxylic acids, as well as hydro- or oxidative carboxylation of alkenes with CO2 to produce carboxylated products. These strategies are explored for carbon capture and utilization (CCU) to convert CO2 into value-added chemicals.

Applications and considerations: Carboxylation provides routes to pharmaceuticals, agrochemicals, and polymers, and it supports efforts to valorize CO2. Challenges include controlling regio- and chemoselectivity, the need for high CO2 pressure or strong reagents, and the development of efficient, scalable catalysts. See also carboxylation in organic synthesis and carbon dioxide fixation.