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A carboxylate anion, RCO2, is an ion with negative charge that contains the group -COO. It is the conjugate base of a carboxylic acid.

Carboxylic acids dissociate into a carboxylate ion and a positively-charged hydrogen ion (proton) much more readily than alcohols dissociate (into an alkoxide ion and a proton), because the carboxylate ion is more stable: the negative charge that is left behind when a proton leaves the carboxylate group is 'shared' between the two electronegative oxygen atoms in a resonance structure.

Equivalence of the resonance forms the delocalised form of a general carboxylate anion

This delocalization of the electron cloud means that neither of the oxygen atoms is so strongly negatively charged; the positively-charged proton is therefore less readily attracted back to the carboxylate group once it has left. On the other hand, an alkoxide ion, once formed, would have a strong negative charge on the oxygen atom, which would make it difficult for the proton to escape. Thus, the carboxylate ion is more stable - this is also why carboxylic acids have a lower pH than alcohols: the higher the number of protons that escape into solution, the lower the pH. [1]

Examples

Discussion

References

  1. ^ Fox, Marye Anne; Whitesell, James K. (1997). Organic Chemistry (2 ed.). Sudbury, Massachusetts: Jones and Bartlett Publishers. ISBN 0-7637-0178-5.  

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