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 is then distributed symmetrically among the two oxygen atoms, and the two carbon–oxygen bonds take on a partial double bond character (i.e., 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 weak acids, with only about 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 reacts with sodium bicarbonate (baking soda) to form sodium ethanoate (sodium acetate), carbon dioxide, and water:
- CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O
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
- butyric acid
- all amino acids
- all fatty acids, where R is an alkane in saturated acids and an alkene in unsaturated acids