Carboxylic acids — common mistakes

The reaction between a carboxylic acid and a carbonate/hydrogencarbonate is a crucial test. If you are asked to distinguish a carboxylic acid from an alcohol or a phenol, this is the go-to reaction. Only the carboxylic acid is acidic enough to produce effervescence ($CO_2$ gas) with a weak base like $Na_2CO_3$.

The carboxylate anion ($RCOO^-$) formed when a carboxylic acid loses a proton is stabilised by resonance. The negative charge is delocalised over the two oxygen atoms, spreading the charge out. The alkoxide ion ($RO^-$) from an alcohol has its negative charge localised on the single oxygen atom, making it less stable and thus less likely to form. This greater stability of the conjugate base makes the carboxylic acid a stronger acid.

A carboxylic acid has a hydroxyl group ($-OH$) attached to the carbonyl carbon. An ester has an alkoxy group ($-OR'$) attached to the carbonyl carbon, where R' is an alkyl group. Carboxylic acids are acidic and can act as hydrogen bond donors, leading to high boiling points. Esters are neutral, generally have pleasant smells, and cannot act as hydrogen bond donors (though they can be acceptors), so their boiling points are lower than corresponding acids.

Both are weakly acidic and will react with strong bases like NaOH. However, carboxylic acids are strong enough to react with weak bases like sodium carbonate ($Na_2CO_3$) or sodium hydrogencarbonate ($NaHCO_3$) to produce carbon dioxide gas (effervescence). Phenols are generally not acidic enough to do this. This reaction is a key distinguishing test.