View source for Oxalate

{{short description|Any derivative of oxalic acid; chemical compound containing oxalate moiety}}
{{Chembox
|IUPACName = Oxalate
|SystematicName = Ethanedioate<ref name=pubchem>{{cite web | url=https://pubchem.ncbi.nlm.nih.gov/compound/Oxalate | title=Oxalate }}</ref>
|Section1 = {{Chembox Identifiers
|CASNo = 338-70-5
|CASNo_Ref = {{cascite|correct|CAS}}
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = PQ7QG47K6T
|ChEBI = 30623
|ChemSpiderID = 64235
|KEGG = C00209
|PubChem = 71081
|SMILES = C(=O)(C(=O)[O-])[O-]
|StdInChI = 1S/C2H2O4/c3-1(4)2(5)6/h(H,3,4)(H,5,6)/p-2
|StdInChIKey = MUBZPKHOEPUJKR-UHFFFAOYSA-L
|InChI = 1S/C2H2O4/c3-1(4)2(5)6/h(H,3,4)(H,5,6)/p-2
|InChIKey = MUBZPKHOEPUJKR-UHFFFAOYSA-L
|Beilstein = 1905970
|Gmelin = 2207
}}
|Section2 = {{Chembox Properties
|Formula = {{chem2|C2O4(2−)}}
|C=2|O=4
|ConjugateAcid = [[Hydrogenoxalate]]<ref>{{cite web |title=oxalate(2−) (CHEBI:30623) |url=https://www.ebi.ac.uk/chebi/searchId.do?chebiId=30623 |website=www.ebi.ac.uk |access-date=2 January 2019 |quote=oxalate(2−) (CHEBI:30623) is conjugate base of oxalate(1−) (CHEBI:46904) … oxalate(1−) (CHEBI:46904) is conjugate acid of oxalate(2−) (CHEBI:30623)}}</ref>
}}
|Section3 = {{Chembox Structure
|PointGroup = D<sub>2h</sub>
}}
|Section4 = {{Chembox Related
|OtherFunction = [[Dinitrogen tetroxide]]
|OtherFunction_label = [[isoelectronic]]
}}
|ImageFile = Structure of oxalate.svg
|ImageAlt = two capital Cs connected to each other by a solid line and each connected to two separate Os by a solid line and a dashed line next to the solid line, the whole thing in brackets with a −2 to the top-right
|ImageCaption = The structure of the oxalate anion
}}
'''Oxalate''' (systematic IUPAC name: '''ethanedioate''') is an [[anion]] with the [[chemical formula]] {{chem2|C2O4(2−)}}. This [[dianion]] is colorless. It occurs naturally, including in some foods. It forms a variety of [[salt (chemistry)|salt]]s, for example [[sodium oxalate]] ({{chem2|Na2C2O4}}), and several [[esters]] such as [[dimethyl oxalate]] ({{chem2|(CH3)2C2O4}}). It is a [[conjugate base]] of [[oxalic acid]]. At neutral [[pH]] in [[aqueous solution]], oxalic acid converts completely to oxalate.

==Relationship to oxalic acid==
The dissociation of [[proton]]s from oxalic acid proceeds in a stepwise manner; as for other [[acid#Polyprotic acids|polyprotic acids]], loss of a single proton results in the monovalent [[hydrogenoxalate]] anion {{chem2|HC2O4−}}. A [[salt (chemistry)|salt]] with this [[anion]] is sometimes called an [[Acid salt|acid oxalate]], monobasic oxalate, or [[hydrogen oxalate]]. The [[equilibrium constant]] ([[acid dissociation constant|''K''<sub>a</sub>]]) for loss of the first proton is {{val|5.37e-2}} ([[pKa|p''K''<sub>a</sub>]]&nbsp;=&nbsp;1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of {{val|5.25e-5}} (p''K''<sub>a</sub>&nbsp;=&nbsp;4.28). These values imply, in solutions with neutral [[pH]], no oxalic acid and only trace amounts of hydrogen oxalate exist.<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a18_247|chapter=Oxalic Acid|title=Ullmann's Encyclopedia of Industrial Chemistry|year=2000|last1=Riemenschneider|first1=Wilhelm|last2=Tanifuji|first2=Minoru|isbn=3-527-30673-0|title-link=Ullmann's Encyclopedia of Industrial Chemistry}}</ref> The literature is often unclear on the distinction between {{chem2|H2C2O4}}, {{chem2|HC2O4−}}, and {{chem2|C2O4(2−)}}, and the collection of species is referred to as oxalic acid.{{cn|date=January 2022}}

==Structure==
The oxalate anion exists in a nonplanar conformation where the O–C–C–O [[Dihedral angle|dihedrals]] approach 90° with approximate [[Molecular symmetry#Common point groups|D<sub>2d</sub> symmetry]].<ref name="ReferenceA">{{cite journal |doi=10.1021/ed200202r|title=The Oxalate Dianion, C2O42-: Planar or Nonplanar?|journal=Journal of Chemical Education|volume=89|issue=3|pages=417–418|year=2012|last1=Dean|first1=Philip A. W.|bibcode=2012JChEd..89..417D}}</ref> When chelated to cations, oxalate adopts the planar, D<sub>2h</sub> conformation.<ref>{{cite journal |doi=10.1107/S0567740881004676 |title=Sodium oxalate structure refinement |journal=Acta Crystallographica Section B |volume=37 |issue=4 |pages=938–939 |year=1981 |last1=Reed |first1=D. A. |last2=Olmstead |first2=M. M.|bibcode=1981AcCrB..37..938R |url=http://journals.iucr.org/b/issues/1981/04/00/a20052/a20052.pdf}}</ref><ref>{{cite journal |doi=10.1107/S0365110X64002079|title=The structure of lithium oxalate|journal=Acta Crystallographica|volume=17|issue=6|pages=783–788|year=1964|last1=Beagley|first1=B.|last2=Small|first2=R. W. H.|bibcode=1964AcCry..17..783B |doi-access=free}}</ref> However, in the structure of [[caesium oxalate]] {{chem2|Cs2C2O4}} the O–C–C–O dihedral angle is 81(1)°.<ref>In the figure 81(1)°, the (1) indicates that 1° is the standard uncertainty of the measured angle of 81°</ref><ref name="Dinnebier">{{cite journal |doi=10.1021/ic0205536|pmid=12611516|title=Crystal and Molecular Structures of Alkali Oxalates: First Proof of a Staggered Oxalate Anion in the Solid State|journal=Inorganic Chemistry|volume=42|issue=5|pages=1499–1507|year=2003|last1=Dinnebier|first1=Robert E.|last2=Vensky|first2=Sascha|last3=Panthöfer|first3=Martin|last4=Jansen|first4=Martin}}</ref> Therefore, {{chem2|Cs2C2O4}} is more closely approximated by a D<sub>2d</sub> symmetry structure because the two {{chem2|CO2}} planes are staggered. Two structural forms of [[rubidium oxalate]] {{chem2|Rb2C2O4}} have been identified by single-crystal X-ray diffraction: one contains a planar and the other a staggered oxalate.

{{multiple image
 | align = none
 | direction = horizontal
 | total_width = 300
 | image1 = Anion-from-caesium-oxalate-xtal-3D-bs-17.png
 | caption1 = Nonplanar conformation found in [[caesium oxalate]] {{chem2|Cs2C2O4}}<ref name="Dinnebier" /><ref>{{Cite journal | title = CSD Entry WUWTIR: Di-cesium oxalate | journal = Cambridge Structural Database: Access Structures | date = 2003 | publisher = [[Cambridge Crystallographic Data Centre]] | doi = 10.5517/cc6fzf0 | last1 = Dinnebier | first1 = R.E. | last2 = Vensky | first2 = S. | last3 = Panthofer | first3 = M. | last4 = Jansen | first4 = M.}}</ref>
 | image2 = Anion-from-potassium-oxalate-xtal-3D-bs-17.png
 | caption2 = Planar conformation found in potassium oxalate<!--The link "potassium oxalate" misleads to the "Oxalate" article. Someone should create an article about potassium oxalate.--> {{chem2|K2C2O4}}<ref name="Dinnebier" /><ref>{{Cite journal | title = CSD Entry QQQAZJ03: Di-potassium oxalate | journal = Cambridge Structural Database: Access Structures | date = 2003 | publisher = [[Cambridge Crystallographic Data Centre]] | doi = 10.5517/cc6fzcy | last1 = Dinnebier | first1 = R.E. | last2 = Vensky | first2 = S. | last3 = Panthofer | first3 = M. | last4 = Jansen | first4 = M.}}</ref>}}

The [[Conformational isomerism|barrier to rotation]] about this bond is calculated to be roughly 2–6&nbsp;kcal/mol for the free dianion, {{chem2|C2O4(2−)}}.<ref>{{cite journal |doi=10.1002/jcc.540020106|title=Conformational preferences of 34 valence electron A<sub>2</sub>X<sub>4</sub> molecules: An ''ab initio'' Study of B<sub>2</sub>F<sub>4</sub>, B<sub>2</sub>Cl<sub>4</sub>, N<sub>2</sub>O<sub>4</sub>, and {{chem|C|2|O|4|2−}}|journal=Journal of Computational Chemistry|volume=2|pages=20–29|year=1981|last1=Clark|first1=Timothy|last2=Schleyer|first2=Paul von Ragué|s2cid=98744097}}</ref><ref>{{cite journal |doi=10.1016/0166-1280(90)85053-P|title=Structure of the oxalate ion|journal=Journal of Molecular Structure: THEOCHEM|volume=209|issue=1–2|pages=157–162|year=1990|last1=Dewar|first1=Michael J.S.|last2=Zheng|first2=Ya-Jun}}</ref><ref>{{cite journal |doi=10.1021/jp002657c|title=Ab Initio Investigation of Electron Detachment in Dicarboxylate Dianions|journal=The Journal of Physical Chemistry A|volume=104|issue=50|pages=11786–11795|year=2000|last1=Herbert|first1=John M.|last2=Ortiz|first2=J. V.|bibcode=2000JPCA..10411786H}}</ref> Such results are consistent with the interpretation that the central [[carbon–carbon bond|C−C bond]] is regarded as a [[single bond]] with minimal [[pi interaction|π interactions]] between the two {{chem2|CO2−}} units.<ref name="ReferenceA"/> This barrier to rotation about the C−C bond (which formally corresponds to the difference in energy between the planar and staggered forms) may be attributed to [[electrostatic interaction]]s as unfavorable O−O repulsion is maximized in the planar form.

==Occurrence in nature==
Oxalate occurs in many plants, where it is synthesized by the incomplete [[oxidation]] of [[saccharide]]s.

Several plant foods such as the root and/or leaves of [[spinach]], [[rhubarb]], and [[buckwheat]] are high in oxalic acid and can contribute to the formation of kidney stones in some individuals. Other oxalate-rich plants include [[Chenopodium album|fat hen]] ("lamb's quarters"), [[sorrel]], and several ''[[Oxalis]]'' species (also sometimes called sorrels). The root and/or leaves of rhubarb and buckwheat are high in oxalic acid.<ref name="heathcock">{{cite book |author1-link=Andrew Streitwieser|last1=Streitweiser|first1=Andrew Jr. |author2-link=Clayton Heathcock|last2=Heathcock|first2=Clayton H.|title=Introduction to Organic Chemistry|url=https://archive.org/details/introductiontoor00stre|url-access=registration|publisher=Macmillan|date=1976|page=[https://archive.org/details/introductiontoor00stre/page/737 737]|isbn=9780024180100 }}</ref> Other edible plants with significant concentrations of oxalate include, in decreasing order, star fruit ([[carambola]]), [[black pepper]], [[parsley]], [[poppy seed]], [[amaranth]], [[chard]], [[beet]]s, [[cocoa solids|cocoa]], [[chocolate]], most [[nut (fruit)|nuts]], most [[berry|berries]], [[caryota|fishtail palms]], New Zealand spinach (''[[Tetragonia tetragonioides]]''), and [[bean]]s.{{Citation needed|date=January 2010}}
Leaves of the [[tea]] plant (''[[Camellia sinensis]]'') contain among the greatest measured concentrations of oxalic acid relative to other plants. However, the drink derived by [[infusion]] in hot water typically contains only low to moderate amounts of oxalic acid due to the small mass of leaves used for brewing.{{cn|date=May 2022}}

{| class="wikitable sortable toccolours collapsible collapsed"
|+Common high-oxalate foods<ref>{{cite book |first1=Martin I. |last1=Resnick |last2=Pak|first2= Charles Y. C. |year=1990 |title=Urolithiasis, A Medical and Surgical Reference |pages=158 |publisher=W.B. Saunders Company |isbn=0-7216-2439-1}}</ref>
|-
! Food item
! Serving<br />
! Oxalate<br />content<br />(mg)
|-
| [[Beetroot]] greens, cooked
| {{1/2}} [[cup (unit)|cup]]
| 916
|-
| [[Portulaca oleracea|Purslane]], leaves, cooked
| {{1/2}} cup
| 910
|-
| [[Rhubarb]], stewed, no sugar
| {{1/2}} cup
| 860
|-
| [[Spinach]], cooked
| {{1/2}} cup
| 750
|-
| [[Beet]], cooked
| {{1/2}} cup
| 675
|-
| [[Chard]], Swiss, leaves cooked
| {{1/2}} cup
| 660
|-
| [[Rhubarb]], canned
| {{1/2}} cup
| 600
|-
| [[Spinach]], frozen
| {{1/2}} cup
| 600
|-
| [[Beet]], pickled
| {{1/2}} cup
| 500
|-
| [[Pokeweed|Poke]] greens, cooked
| {{1/2}} cup
| 476
|-
| [[Almond]], raw
| 1 oz
| 296
|-
| [[Endive]], raw
| 20 long leaves
| 273
|-
| [[Cocoa solids|Cocoa]], dry
| {{1/3}} cup
| 254
|-
| [[Dandelion]] greens, cooked
| {{1/2}} cup
| 246
|-
| [[Okra]], cooked
| 8–9 pods
| 146
|-
| [[Sweet potato]], cooked
| {{1/2}} cup
| 141
|-
| [[Kale]], cooked
| {{1/2}} cup
| 125
|-
| [[Peanut]]s, raw
| {{1/3}} cup ({{frac|1|3|4}} [[Ounce|oz]])
| 113
|-
| [[Turnip]] greens, cooked
| {{1/2}} cup
| 110
|-
| [[Chocolate]], unsweetened
| 1 oz
| 91
|-
| [[Parsnips]], diced, cooked
| {{1/2}} cup
| 81
|-
| [[Collard greens]], cooked
| {{1/2}} cup
| 74
|-
| [[Pecan]]s, halves, raw
| {{1/3}} cup ({{frac|1|1|4}} oz)
| 74
|-
| [[Tea]], leaves (4-minute infusion)
| 1 level [[teaspoon|tsp]] in 7 [[fluid ounce|fl oz]] water
| 72
|-
| [[Cereal germ]], toasted
| {{1/4}} cup
| 67
|-
| [[Gooseberry|Gooseberries]]
| {{1/2}} cup
| 66
|-
| [[Potato]], Idaho white, baked
| 1 medium
| 64
|-
| [[Carrot]]s, cooked
| {{1/2}} cup
| 45
|-
| [[Apple]], raw with skin
| 1 medium
| 41
|-
| [[Brussels sprout]]s, cooked
| 6–8 medium
| 37
|-
| [[Strawberry|Strawberries]], raw
| {{1/2}} cup
| 35
|-
| [[Celery]], raw
| 2 stalks
| 34
|-
| [[Milk chocolate]] bar
| 1 bar (1.02&nbsp;oz)<br />{{clarify|date=December 2021}}
| 34
|-
| [[Raspberry|Raspberries]], black, raw
| {{1/2}} cup
| 33
|-
| [[Orange (fruit)|Orange]], edible portion
| 1 medium
| 24
|-
| [[Green bean]]s, cooked
| {{1/2}} cup
| 23
|-
| [[Chive]]s, raw, chopped
| 1 [[tablespoon]]
| 19
|-
| [[Leek]]s, raw
| {{1/2}} medium
| 15
|-
| [[Blackberry|Blackberries]], raw
| {{1/2}} cup
| 13
|-
| [[Concord grape]]s
| {{1/2}} cup
| 13
|-
| [[Blueberry|Blueberries]], raw
| {{1/2}} cup
| 11
|-
| [[Redcurrant]]s
| {{1/2}} cup
| 11
|-
| [[Apricot]]s, raw
| 2 medium
| 10
|-
| [[Raspberry|Raspberries]], red, raw
| {{1/2}} cup
| 10
|-
| [[Broccoli]], cooked
| 1 large stalk
| 6
|-
| [[Cranberry]] juice
| {{1/2}} cup (4&nbsp;oz)
| 6
|}

===Physiological effects===
{{main|Kidney stone}}
[[File:Surface of a kidney stone.jpg|thumb|Scanning electron micrograph of the surface of a [[kidney stone]] showing tetragonal crystals of [[weddellite]] (calcium oxalate dihydrate) emerging from the amorphous central part of the stone; the horizontal length of the picture represents 0.5 mm of the figured original.]]
Excess consumption of oxalate-rich foods has been linked to [[kidney stone]] formation of metal ions, such as [[calcium oxalate]], a [[risk factor]] for kidney stones.<ref name="oxalate">{{cite journal |vauthors=Mitchell T, Kumar P, Reddy T, Wood KD, Knight J, Assimos DG, Holmes RP |title=Dietary oxalate and kidney stone formation |journal=American Journal of Physiology. Renal Physiology |volume=316 |issue=3 |pages=F409–F413 |date=March 2019 |pmid=30566003 |pmc=6459305 |doi=10.1152/ajprenal.00373.2018}}</ref>

Some [[fungus|fungi]] of the [[genus]] ''[[Aspergillus]]'' produce oxalic acid.<ref>{{cite journal |first1= Uğur |last1=Pabuççuoğlu |journal= Pathology – Research and Practice |year= 2005 |volume= 201 |issue= 5 |pages= 363–368 |title= Aspects of oxalosis associated with aspergillosis in pathology specimens |pmid= 16047945 |doi= 10.1016/j.prp.2005.03.005}}</ref>

==As a ligand for metal ions==
{{main|Transition metal oxalate complex}}
Oxalate also forms [[coordination compound]]s where it is sometimes abbreviated as '''ox'''. It is commonly encountered as a [[bidentate ligand]]. When the oxalate chelates to a single metal center, it always adopts the planar conformation. As a bidentate ligand, it forms a 5-membered {{chem2|MC2O2}} ring. An illustrative complex is [[potassium ferrioxalate]], {{chem2|K3[Fe(C2O4)3]}}. The drug [[oxaliplatin]] exhibits improved water solubility relative to older [[platinum]]-based drugs, avoiding the dose-limiting side-effect of [[nephrotoxicity]]. Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction. One of the main applications of oxalic acid is rust-removal, which arises because oxalate forms water-soluble derivatives with the ferric ion.

==Excess==
An excess oxalate level in the blood is termed ''hyperoxalemia'', and high levels of oxalate in the urine is termed [[hyperoxaluria]].

===Acquired===
Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such as ''[[Bassia hyssopifolia]]'', or human consumption of [[wood sorrel]] or, specifically in excessive quantities, [[tea|black tea]]) may result in [[kidney disease]] or even [[death]] due to oxalate poisoning. ''[[The New England Journal of Medicine]]'' reported acute oxalate nephropathy "almost certainly due to excessive consumption of iced tea" in a 56-year-old man, who drank "sixteen 8-ounce glasses of iced tea daily" (roughly {{convert|128|USfloz|L|disp=out|abbr=off|sp=us}}). The authors of the paper hypothesized that acute oxalate nephropathy is an underdiagnosed cause of kidney failure and suggested thorough examination of patient dietary history in cases of unexplained kidney failure without [[proteinuria]] (an excess of protein in the urine) and with large amounts of calcium oxalate in urine sediment.<ref>{{cite journal |last1=Syed|first1=Fahd|last2=Mena Gutiérrez|first2=Alejandra|last3=Ghaffar|first3=Umbar|title=A Case of Iced-Tea Nephropathy|journal=New England Journal of Medicine|date=2 April 2015|volume=372|issue=14|pages=1377–1378|doi=10.1056/NEJMc1414481|pmid=25830441|doi-access=free}}</ref> ''[[Oxalobacter formigenes]]'' in the [[gut flora]] may help alleviate this.<ref>{{cite journal |title=The role of Oxalobacter formigenes colonization in calcium oxalate stone disease|journal=Kidney International|date=2013|volume=83|issue=June|pages=1144–1149|doi=10.1038/ki.2013.104|pmid=23536130|last3=Sidhu|first3=H.|last4=Hönow|first4=R.|last5=von Unruh|first5=G.|last6=Hesse|first6=A.|last1=Siener|first1=R.|last2=Bangen|first2=U.|doi-access=free}}</ref>

===Congenital===
{{Main|Primary hyperoxaluria}}
[[Primary hyperoxaluria]] is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.

==References==
{{Reflist|2}}

==Further reading==
* {{Cite web |title=''K''<sub>sp</sub> Table: Solubility product constants near 25 °C |author=Euler |work=chm.uri.edu |date= |access-date=10 June 2021 |url= https://www.chm.uri.edu/weuler/chm112/refmater/KspTable.html |language=English}}
* {{cite journal|last1=Ibis|first1=Fatma|last2=Dhand|first2=Priya|last3=Suleymanli|first3=Sanan|last4=van der Heijden|first4=Antoine E. D. M.|last5=Kramer|first5=Herman J. M.|last6=Eral|first6=Huseyin Burak|title=A combined experimental and modelling study on solubility of calcium oxalate monohydrate at physiologically relevant pH and temperatures|journal=Crystals|volume=10|issue=10|year=2020|pages=924|issn=2073-4352|doi=10.3390/cryst10100924|url=https://repository.tudelft.nl/islandora/object/uuid%3Ada7c34f4-d312-457c-8960-da91b7472244/datastream/OBJ/download |doi-access=free}}
* {{cite journal|last1=Ulmgren|first1=Per|last2=Rådeström|first2=Rune|title=Solubility of calcium oxalate in the presence of magnesium ions, and solubility of magnesium oxalate in sodium chloride medium|journal=Nordic Pulp & Paper Research Journal|volume=14|issue=4|year=1999|pages=330–335|issn=2000-0669|doi=10.3183/npprj-1999-14-04-p330-335|s2cid=96834193 }}

== External links ==
* [https://oxalate.org/ Oxalate.org] - Oxalate content of 750+ foods from university and government sources
* [https://oxalatecontent.com/ Oxalatecontent.com] - Oxalate content database based on the latest trustworthy studies

{{Antithrombotics}}
{{Oxalates}}

[[Category:Oxalates| ]]
[[Category:Anticoagulants]]
[[Category:Carboxylate anions]]