Carboxylic acid

In chemistry, especially in organic chemistry and biochemistry, a carboxylic acid or carboxy group is a functional group of the form

(where R is a hydrogen or an organic group)

In chemical formulas, these groups are typically represented as COOH. Molecules containing such a functional group are also called carboxylic acids or organic acids.

As the name implies, they are acids; the two electronegative oxygen atoms tend to pull the electron away from the hydrogen of the hydroxyl group and the remaining proton can more easily leave. The remaining negative charge will then be distributed symmetrically among the two oxygen atoms and the two carbon-oxygen bonds take on a partial double bond character (ie, they are delocalised).

This is a result of the resonance structure created by the carbonyl component of the carboxylic acid, without which the OH group does not as easily lose its H⁺ (see alcohol). The resulting ion is typically named with the suffix "-ate", so acetic acid, for example, becomes acetate ion.

Carboxylic acids are typically only weak acids, with only around 1% of RCOOH molecules dissociated into ions at room temperature in aqueous solution.

Carboxylic acids react with bases to form carboxylate salts, in which the hydrogen of the -OH group is replaced with a metal ion. Thus, ethanoic/acetic acid (vinegar) reacts with sodium bicarbonate (baking soda) to form sodium ethanoate (sodium acetate), carbon dioxide, and water:

CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ + H₂O

Carboxyl groups also react with amine groups to form peptide bonds, and with alcohols to form esters.

Some carboxylic acids include:

• HCOOH formic acid (found in insect stings, formic refers to ants)
• CH₃COOH acetic or ethanoic acid (found in vinegar)
• CH₃CH₂COOH propanoic acid
• C₆H₅COOH benzoic acid (sodium benzoate, the sodium salt of benzoic acid is used as a food preservative)
• lactic acid
• all amino acids
• all fatty acids