In chemistry, a carboxylate is an anion with the general formula (or ). 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.
Occurrence and uses
Carboxylates are pervasive in nature, as all amino acids exist as such. The energy producing machinery supporting aerobic life -
glycolysis and the Krebs citric acid cycle - relies heavily on carboxylates, e.g.
pyruvate,
citrate, and
acetate. Even though they are often depicted as carboxylic acids, many vitamins function as carboxylates: vitamin B3 (niacin), vitamin B5 (pantothenic acid), vitamin B7 (biotin), etc.
In commercial settings, soaps are carboxylates.
Formation
Carboxylates form by
carboxylation reactions where
carbon dioxide adds to a carbon nucleophile. In the laboratory, this process is illustrated by the conversion of
to carboxylates:
In Nature, carboxylation is the first step in
carbon fixation related to photosynthesis of organic matter. The importance of the process is indicated by the estimate that
RuBisCo, the enzyme that catalyzes this reaction, is the most abundant protein.
Carboxylates form by deprotonation of . Such acids typically have pKa of less than 5, meaning that they convert to carboxylates in neutral water.
Because of their tendency to form hydrogen bonds, carboxylic acids characteristically undergo "half deprotonations":
This tendency (
homoassociation) complicates the interpretation of the
pKa's of carboxylic acids, especially in nonpolar solvents that weakly solvate carboxylate anions.
Carboxylates arise by many other routes, such as the base hydrolysis (saponification) of esters:
Carboxylates arise by the base hydrolysis anhydrides and related species:
In the Cannizzaro reaction, strong base induces disproportionation of certain (non) , leading to carboxylates.
Structure and bonding
The C-CO
2 portion of carboxylate anions is planar. According to X-ray crystallography, the two C-O distances are equal or nearly so. In tetrabutylammonium acetate, the C-O distances are both 125
.
By contrast, the C=O and C-O bonds in carboxylic acids and carboxylic esters are localized, as reflected in bond distances that differ by >10 pm.
The infrared spectrum of carboxylates is diagnostic of the partial multiple bond between carbon and oxygen. For acetate, νCO (the C-O "stretching modes") = 1578 and 1414 cm-1.
Reactions
Lewis basicity
Carboxylate anions are
, forming a variety of adducts. One class of such derivatives are transition metal carboxylate complexes.
Deprotonation
Carboxylic acid salts with a hydrogen atom in the
alpha position can be deprotonated with strong bases like lithium diisopropylamide:
The resulting organolithium compound can be then C-alkylated:
Nucleophilic substitution
Carboxylate ions react with
alkyl halides to form
:
This route to esters complements the conventional Fischer esterification.
Reduction
Carboxylate salts are less easily reduced than esters, but the reduction can be achieved with lithium aluminium hydride.
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Examples
This list is for cases where there is a separate article for the anion or its derivatives. All other organic acids should be found at their parent carboxylic acid.
