The anion derived by deprotonation of a carboxylic acid is the carboxylate.

Physical Properties:

• The polar nature of both the O-H and C=O bonds (due to the electonegativity difference of  the atoms) results in the formation of strong hydrogen bonds with other carboxylic acid molecules or other H-bonding systems (e.g. water). The implications are:
• higher melting and boiling points compared to analogous alcohols
• high solubility in aqueous media
• hydrogen bonded dimers in gas phase and dimers or aggregates in pure liquid

• The CO₂H unit is planar and consistent with sp² hydridization and a resonance interaction of the lone pairs of the hydroxyl oxygen with the p system of the carbonyl.

• Carboxylic acids are the most acidic simple organic compounds (pKa ~ 5).
• But they are only weak acids compared to acids like HCl or H₂SO₄. (Remember the lower the pKa, the stronger the acid)
• Resonance stabilization of the carboxylate ion allows the negative charge to be delocalized between the two electronegative oxygen atoms (compare with alcohols, pKa ~ 16).
• Adjacent electron withdrawing substituents increase the acidity by further stabilizing the carboxylate.

Reactivity:
 

The image shows the electrostatic potential for acetic acid (ethanoic acid). The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  There is low electron density (blue) on H atom of the -CO2H group alcohol, i.e. H+ character.  The H atom of the RCO2H is acidic (pKa ~ 5). The most important reactions of carboxylic acids converts them into carboxylic acid derivatives such as acyl halides, esters and amides via nucleophilic acyl substitution reactions.

The image shows the electrostatic potential for the acetate ion (ethanoate ion) The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  There is high electron density (red) on both O atoms of the -CO2- group alcohol, i.e. resonance and basic or nucleophilic behavior

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