Science, Tech, Math › Science Definition of Carboxyl Group in Chemistry Share Flipboard Email Print De.Nobelium Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 14, 2020 In chemistry, the carboxyl group is an organic, functional group consisting of a carbon atom that's double-bonded to an oxygen atom and singly bonded to a hydroxyl group. Another way to view it is as a carbonyl group (C=O)that has a hydroxyl group (O-H) attached to the carbon atom. The carboxyl group is sometimes referred to as the carboxy group, carboxyl functional group, or carboxyl radical. It is commonly written as -C(=O)OH or -COOH. Carboxyl groups ionize by releasing the hydrogen atom from the -OH group. The H+, which is a free proton, is released. Thus, carboxyl groups make good acids. When hydrogen leaves, the oxygen atom has a negative charge which it shares with the second oxygen atom in the group, allowing the carboxyl to remain stable even when oxidized. Carboxyl Group Example Likely the best-known example of a molecule with a carboxyl group is a carboxylic acid. The general formula of a carboxylic acid is R-C(O)OH, where R is any number of chemical species. Carboxylic acids are found in acetic acid and the amino acids that are used to build proteins. Because the hydrogen ion detaches so readily, the molecule is most commonly found as a carboxylate anion, R-COO-. The anion is named using the suffix -ate. For example, acetic acid (a carboxylic acid) becomes the acetate ion.