In chemistry, carboxylic acids (also called alkanoic acids) are organic acids characterized by the presence of a carboxyl group. Carboxylic acids thus have the structure
where R is a hydrogen or an organic group. In chemical formulas, this is also written as RCOOH.
Acidity, electron distribution and resonance
Carboxylic acids are typically weak acids, with only about 1% of RCOOH molecules dissociated into H+ cation and RCOO- anions at room temperature in aqueous solution. The anion RCOO- is usually named with the suffix "-ate", so acetic acid, for example, becomes acetate ion.
The two electronegative oxygen atoms tend to pull the electron away from the hydrogen of the hydroxyl group, and the remaining proton H+ 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 presence of electronegative groups (such as _OH or -Cl) next to the carboxylic group increases the acidity. So for example, trichloroacetic acid (three -Cl groups) is a stronger acid than lactic acid (one -OH group) which in turn is stronger than acetic acid (no helping group).
Carboxylic acids can be made by the complete oxidation of primary alcohols.
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 acid (the same as acetic acid) 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.
Carboxylic acids can be reduced by LiAlH4 to form primary alcohols:
Some carboxylic acids include: