The carboxylate ion is the conjugate base of a carboxylic acid, written RCOO⁻, formed when the acid loses its acidic proton. It matters because its stability is the reason carboxylic acids are acidic at all: the negative charge is not confined to one oxygen but is shared equally between both oxygens by resonance. This delocalisation makes the carboxylate far more stable, and the parent acid far more acidic, than a corresponding alcohol and its alkoxide ion.

羧酸根离子是羧酸的共轭碱,写作 RCOO⁻,由酸失去其酸性质子而生成。它之所以重要,是因为其稳定性正是羧酸具有酸性的原因:负电荷并不局限于一个氧上,而是通过共振由两个氧平均分担。这种离域使羧酸根远比相应的醇及其烷氧负离子稳定,也使母体酸的酸性强得多。