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A carboxylate anion is the conjugate base of a carboxylic acid. It is an anion with the general formula RCOO-. The structure consists of a carboxyl group (-COOH) where the acidic proton has been removed, leaving a negatively charged oxygen atom. This negative charge is delocalized across both oxygen atoms through resonance, resulting in a more stable species than a simple alkoxide.

Carboxylate anions are formed when carboxylic acids donate a proton (H+) to a base. This is a fundamental acid-base reaction. For example, when acetic acid (CH3COOH) reacts with sodium hydroxide (NaOH), it forms sodium acetate (CH3COONa), which dissociates into sodium ions (Na+) and acetate anions (CH3COO-).

The properties of carboxylate anions are influenced by the nature of the R group. Electron-withdrawing groups attached to the carbon adjacent to the carboxylate can stabilize the negative charge, making the parent carboxylic acid stronger (more acidic). Conversely, electron-donating groups destabilize the negative charge, making the parent carboxylic acid weaker.

Carboxylate anions are important in various chemical and biological processes. They are found in many salts, esters, and amides. In biological systems, they play roles in amino acids (as part of their side chains), fatty acids, and as buffer components in blood. The formation and reactions of carboxylate anions are central to organic chemistry, particularly in reactions involving carboxylic acids and their derivatives.