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Concept
Carboxylate
Résumé
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and n is 1, 2,.... Carboxylate esters have the general formula (also written as ), where R and R′ are organic groups.
Carboxylate ions can be formed by deprotonation of carboxylic acids. Such acids typically have pKa of less than 5, meaning that they can be deprotonated by many bases, such as sodium hydroxide or sodium bicarbonate.
RCOOH + NaOH -> RCOONa + H2O
Carboxylic acids easily dissociate into a carboxylate anion and a positively charged hydrogen ion (proton), much more readily than alcohols do (into an alkoxide ion and a proton), because the carboxylate ion is stabilized by resonance. The negative charge that is left after deprotonation of the carboxyl group is delocalized between the two electronegative oxygen atoms in a resonance structure. If the R group is an electron-withdrawing group (such as –CF3), the basicity of the carboxylate will be further weakened.
This delocalization of the electron means that both of the oxygen atoms are less strongly negatively charged: the positive proton is therefore less strongly attracted back to the carboxylate group once it has left; hence, the carboxylate ion is more stable and less basic as a result of resonance stabilization of the negative charge. In contrast, an alkoxide ion, once formed, would have a strong negative charge localized on its lone oxygen atom, which would strongly attract any nearby protons (indeed, alkoxides are very strong bases). Because of resonance stabilization, carboxylic acids have much lower pKa values (and are therefore stronger acids) than alcohols. For example, the pKa value of acetic acid is 4.8, while ethanol has a pKa of 16. Hence acetic acid is a much stronger acid than ethanol. This in turn means that for equimolar solutions of a carboxylic acid and an alcohol, the carboxylic acid would have a much lower pH.
Carboxylate ions are good nucleophiles.
Source officielle
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