 The structure of the oxalate anion | |
| Names | |
|---|---|
| IUPAC name Oxalate | |
| Systematic IUPAC name Ethanedioate[1] | |
| Identifiers | |
| |
3D model (JSmol) | |
| 1905970 | |
| ChEBI | |
| ChemSpider | |
| 2207 | |
| KEGG | |
| UNII | |
| |
| |
| Properties | |
| C2O2−4 | |
| Molar mass | 88.018 g·mol−1 |
| Conjugate acid | Hydrogenoxalate[2] |
| Structure | |
| D2h | |
| Related compounds | |
Relatedisoelectronic | Dinitrogen tetroxide |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Oxalate (systematic IUPAC name:ethanedioate) is ananion with thechemical formulaC2O2−4. Thisdianion is colorless. It occurs naturally, including in some foods. It forms a variety ofsalts, for examplesodium oxalate (Na2C2O4), and severalesters such asdimethyl oxalate ((CH3)2C2O4). It is aconjugate base ofoxalic acid. At neutralpH inaqueous solution, oxalic acid converts completely to oxalate.
The dissociation ofprotons from oxalic acid proceeds in a stepwise manner; as for otherpolyprotic acids, loss of a single proton results in the monovalenthydrogenoxalate anionHC2O−4. Asalt with thisanion is sometimes called anacid oxalate, monobasic oxalate, orhydrogen oxalate. Theequilibrium constant (Ka) for loss of the first proton is5.37×10−2 (pKa = 1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of5.25×10−5 (pKa = 4.28). These values imply, in solutions with neutralpH, no oxalic acid and only trace amounts of hydrogen oxalate exist.[3] The literature is often unclear on the distinction betweenH2C2O4,HC2O−4, andC2O2−4, and the collection of species is referred to as oxalic acid.[citation needed]
The oxalate anion exists in a nonplanar conformation where the O–C–C–Odihedrals approach 90° with approximateD2d symmetry.[4] When chelated to cations, oxalate adopts the planar, D2h conformation.[5][6] However, in the structure ofcaesium oxalateCs2C2O4 the O–C–C–O dihedral angle is 81(1)°.[7][8] Therefore,Cs2C2O4 is more closely approximated by a D2d symmetry structure because the twoCO2 planes are staggered. Two structural forms ofrubidium oxalateRb2C2O4 have been identified by single-crystal X-ray diffraction: one contains a planar and the other a staggered oxalate.
Thebarrier to rotation about this bond is calculated to be roughly 2–6 kcal/mol for the free dianion,C2O2−4.[11][12][13] Such results are consistent with the interpretation that the centralC−C bond is regarded as asingle bond with minimalπ interactions between the twoCO−2 units.[4] 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 toelectrostatic interactions as unfavorable O−O repulsion is maximized in the planar form.
Oxalate occurs in many plants, where it is synthesized by the incompleteoxidation ofsaccharides.
Several plant foods such as the root and/or leaves ofspinach,rhubarb, andbuckwheat are high in oxalic acid and can contribute to the formation of kidney stones in some individuals. Other oxalate-rich plants includefat hen ("lamb's quarters"),sorrel, and severalOxalis species (also sometimes called sorrels). The root and/or leaves of rhubarb and buckwheat are high in oxalic acid.[14] Other edible plants with significant concentrations of oxalate include, in decreasing order, star fruit (carambola),black pepper,parsley,poppy seed,amaranth,chard,beets,cocoa,chocolate, mostnuts, mostberries,fishtail palms, New Zealand spinach (Tetragonia tetragonioides), andbeans.[citation needed]Leaves of thetea plant (Camellia sinensis) contain among the greatest measured concentrations of oxalic acid relative to other plants. However, the drink derived byinfusion in hot water typically contains only low to moderate amounts of oxalic acid due to the small mass of leaves used for brewing.[citation needed]
| Food item | Serving | Oxalate content (mg) |
|---|---|---|
| Beetroot greens, cooked | 1⁄2cup | 916 |
| 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 |
| Poke greens, cooked | 1⁄2 cup | 476 |
| Almond, raw | 1 oz | 296 |
| Endive, raw | 20 long leaves | 273 |
| 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 |
| Peanuts, raw | 1⁄3 cup (1+3⁄4oz) | 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 |
| Pecans, halves, raw | 1⁄3 cup (1+1⁄4 oz) | 74 |
| Tea, leaves (4-minute infusion) | 1 leveltsp in 7fl oz water | 72 |
| Cereal germ, toasted | 1⁄4 cup | 67 |
| Gooseberries | 1⁄2 cup | 66 |
| Potato, Idaho white, baked | 1 medium | 64 |
| Carrots, cooked | 1⁄2 cup | 45 |
| Apple, raw with skin | 1 medium | 41 |
| Brussels sprouts, cooked | 6–8 medium | 37 |
| Strawberries, raw | 1⁄2 cup | 35 |
| Celery, raw | 2 stalks | 34 |
| Milk chocolate bar | 1 bar (1.02 oz) | 34 |
| Raspberries, black, raw | 1⁄2 cup | 33 |
| Orange, edible portion | 1 medium | 24 |
| Green beans, cooked | 1⁄2 cup | 23 |
| Chives, raw, chopped | 1tablespoon | 19 |
| Leeks, raw | 1⁄2 medium | 15 |
| Blackberries, raw | 1⁄2 cup | 13 |
| Concord grapes | 1⁄2 cup | 13 |
| Blueberries, raw | 1⁄2 cup | 11 |
| Redcurrants | 1⁄2 cup | 11 |
| Apricots, raw | 2 medium | 10 |
| Raspberries, red, raw | 1⁄2 cup | 10 |
| Broccoli, cooked | 1 large stalk | 6 |
| Cranberry juice | 1⁄2 cup (4 oz) | 6 |
Excess consumption of oxalate-rich foods has been linked tokidney stone formation of metal ions, such ascalcium oxalate, arisk factor for kidney stones.[16]
Somefungi of thegenusAspergillus produce oxalic acid.[17]
Oxalate also formscoordination compounds where it is sometimes abbreviated asox. It is commonly encountered as abidentate ligand. When the oxalate chelates to a single metal center, it always adopts the planar conformation. As a bidentate ligand, it forms a 5-memberedMC2O2 ring. An illustrative complex ispotassium ferrioxalate,K3[Fe(C2O4)3]. The drugoxaliplatin exhibits improved water solubility relative to olderplatinum-based drugs, avoiding the dose-limiting side-effect ofnephrotoxicity. 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.
An excess oxalate level in the blood is termedhyperoxalemia, and high levels of oxalate in the urine is termedhyperoxaluria.
Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such asBassia hyssopifolia, or human consumption ofwood sorrel or, specifically in excessive quantities,black tea) may result inkidney disease or evendeath 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 one gallon or 3.8 liters). 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 withoutproteinuria (an excess of protein in the urine) and with large amounts of calcium oxalate in urine sediment.[18]Oxalobacter formigenes in thegut flora may help alleviate this.[19]
Primary hyperoxaluria is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.
oxalate(2−) (CHEBI:30623) is conjugate base of oxalate(1−) (CHEBI:46904) … oxalate(1−) (CHEBI:46904) is conjugate acid of oxalate(2−) (CHEBI:30623)
