Carboxylate: Difference between revisions

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reduction part
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[[File:Carboxylate-resonance-hybrid.png|thumb|255px|Carboxylate ion]]
 
[[File:Acrylate-ion-3D-balls.png|thumb|Acrylate ion]]
 
A '''carboxylate''' is the [[Conjugate base|conjugate base]] of a [[carboxylic acid]].
''Carboxylate salts'' have the general formula M(RCOO)<sub>''n''</sub>, where M is a metal and ''n'' is 1,&nbsp;2,...; ''carboxylate esters'' have the general formula RCOOR′. R and R′ are organic groups; R′&nbsp;≠&nbsp;H.
 
''Carboxylate salts'' have the general formula M(RCOO)<sub>''n''</sub>, where M is a metal and ''n'' is 1,&nbsp;2,...; ''carboxylate esters'' have the general formula RCOOR′ (or RCO<sub>2</sub>R′). R and R′ are organic groups; R′&nbsp;≠&nbsp;H.
A '''carboxylate ion''' is the [[conjugate base]] of a carboxylic acid, RCOO<sup>−</sup>. It is an [[ion]] with [[negative charge]].
 
A '''carboxylate ion''' is the [[conjugate base]] of a carboxylic acid, RCOO<sup>−</sup> (or (or RCO<sub>2</sub><sup>−</sup>). It is an [[ion]] with [[negative charge]].
 
== Synthesis ==
A carboxylate can be easily made by deprotonation of carboxylic acids. Most of them has a pKapK<sub>a</sub> of approximately 5, which means that they can be deprotonated by many bases, such as [[Sodiumsodium hydroxide|]] or [[sodium hydroxidebicarbonate]].
 
<chem>RCO2H + Na+OH- -> RCO2- Na+ + H2O</chem>
== Resonance stabilization of the carboxylate ion==
 
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 (chemistry)|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.
== Resonance stabilization of the carboxylate ion ==
 
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 (chemistry)|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 [[Trifluoromethyl|-CF<sub>3</sub>]]), the basicity of the carboxylate will be further weakened.
 
:[[File:Carboxylate-resonance-2D.png|400px|Equivalence of the resonance forms the delocalised form of a general carboxylate anion]]
 
This [[delocalization]] of the electron cloud 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. In contrast, 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. Carboxylic acids thus have a lower [[pKa]] values than alcohols. For example, the pKa value of ethanol is 16 while acetic acid has a pKa of 4.9. Hence acetic acid is a much stronger acid than ethanol. the higher the number of protons in solution, the lower the pH.<ref>{{cite book | last1= Fox|first1= Marye Anne|last2= Whitesell|first2= James K. | year = 1997 | title = Organic Chemistry | edition = 2nd|  publisher = Jones and Bartlett Publishers | location = Sudbury, MA| isbn = 0-7637-0178-5 }}</ref>
 
== Reactions ==
=== Nucleophilic Substitution ===
 
Carboxylate ions are good [[nucleophile|nucleophiles]]. They react with [[alkyl halides]] to form [[ester]]. The following reaction shows the reaction  mechanism.
 
[[File:Ester-from-carboxylate-and-alkyl-halide-2D-skeletal.png|thumb|The [[SN2 reaction|S<sub>N</sub>2 reaction]] between [[sodium acetate]] and [[bromoethane]]. The products are [[ethyl acetate]] and [[sodium bromide]]. |alt=|none|287x287px]]
 
The nucleophilicity of carboxylate ions are much weaker than that of [[hydroxide]] and [[alkoxide]] ions, but stronger than [[halide]] anions (in a [[polar aprotic solvent]], though there are other effects such as [[solubility]] of the ion).
==Examples==
 
=== Reduction ===
Unlike the reduction of ester, the reduction of carboxylate is different, due to the lack of the [[leaving group]] and the relatively electron-rich carbon atom (due to the negative charge on the oxygen atoms) . With small amount of acid, the reaction occurs with [[lithium aluminium hydride]] by changing the LAH into a [[Lewis acids and bases|Lewis acid]], [[Aluminium hydride|AlH<sub>3</sub>]] in the process, converting the oxyanion to 4 Al-O bonds.
 
[[File:Reaction of carboxylic acids reduction - part two.svg|alt=|500x500px]]
[[File:Reaction of carboxylic acids reduction - part three.svg|alt=|frameless|507x507px]]
 
==Examples==
* [[Formate]] ion, HCOO<sup>−</sup>
* [[Acetate]] ion, CH<sub>3</sub>COO<sup>−</sup>
* [[Oxalate]] ion, {{chem|(COO)|2|2−}}
* [[Citrate]] ion, {{chem|C|3|H|5|O(COO)|3|3−}}
 
==See also==
* [[Carboxylic acid]]
 
==References==
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