Ascorbic acid
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/* Uses */ Changed for clarity
:''This article deals with the molecular aspects of ascorbic acid. For information about its purpose in nutrition, see [[Vitamin C]].''
{{Chembox new
| ImageFile = VitaminC.svg
| ImageFile1 =L-ascorbic-acid-3D-balls.png
| IUPACName = (''R'')-3,4-dihydroxy-5-((''S'')- 1,2-dihydroxyethyl)furan-2(5''H'')-one
| OtherNames =
| Section1 = {{Chembox Identifiers
| CASNo = 50-81-7
| EINECS = 200-066-2
| FEMA = 2109
| PubChem = 5785
| SMILES =
| InChI=1/C6H8O6/c7-1-2(8)5-3(9)4<br/>(10)6(11)12-5/h2,5,7-8,10-<br/>11H,1H2/t2-,5+/m0/s1
}}
| Section2 = {{Chembox Properties
| Formula = C<sub>6</sub>H<sub>8</sub>O<sub>6</sub>
| MolarMass = 176.1241 g/mol
| Appearance = White or light yellow solid
| Density = 1.65 g/cm³
| MeltingPt = 190 - 192 °C (dec)
| MeltingPtCL = 190
| MeltingPtCH = 192
| Melting_notes = decomposes
| BoilingPt =
| Solubility = 33g/100ml
| Solubility1 = 2g/100ml
| Solvent1 = ethanol
| Solubility2 = 1g/100ml
| Solvent2 = glycerol
| Solubility3 = 5g/100ml
| Solvent3 = propylene glycol
| Solubility4 = insoluble
| Solvent4 = diethyl ether, chloroform, benzene, petroleum ether, oils, fats, fat solvents
| pKa = 4.17 (first), 11.6 (second)
}}
| Section3 = {{Chembox Hazards
| ExternalMSDS = [http://www.sciencelab.com/xMSDS-Ascorbic_acid-9922972 ScienceLab.com]
| MainHazards =
| FlashPt =
| Autoignition =
| LD50 = 11.9 g/kg (oral, rat)<ref>{{cite web |url=http://physchem.ox.ac.uk/MSDS/AS/ascorbic_acid.html |title=Safety (MSDS) data for ascorbic acid |accessdate= 2007-02-21 |date= [[2005-10-09]] | publisher= [[Oxford University]] }}</ref>
}}
}}
'''Ascorbic acid''' is a [[sugar acid]] with [[antioxidant]] properties. Its appearance is white to light-yellow crystals or powder. It is water-soluble. The L-[[enantiomer]] of ascorbic acid is commonly known as [[vitamin C]]. The name is derived from the [[alpha privative]] ''a-'' (meaning no) and ''scorbuticus'' ([[scurvy]]), the disease caused by a deficiency of vitamin C. In [[1937]] the [[Nobel Prize]] for chemistry was awarded to [[Walter Haworth]] for his work in determining the structure of ascorbic acid (shared with [[Paul Karrer]], who received his award for work on [[vitamin]]s), and the prize for Physiology or Medicine that year went to [[Albert Szent-Györgyi]] for his studies of the biological functions of L-ascorbic acid. At the time of its discovery in the 1920s, it was called '''hexuronic acid''' by some researchers.<ref>Joseph Louis Svirbelf, Albert Szent-Gyorgyi [http://profiles.nlm.nih.gov/WG/B/B/G/W/_/wgbbgw.pdf The Chemical Nature Of Vitamin C], [[April 25]], [[1932]]. Part of the [[National Library of Medicine]] collection. Accessed January 2007</ref>
== Chemistry ==
===Acidity===
Ascorbic acid, the formula of which is C<sub>6</sub>H<sub>8</sub>O<sub>6</sub>, behaves as a [[vinylogous]] [[carboxylic acid]], wherein the double bond ("vinyl") transmits electron pairs between the hydroxyl and the carbonyl. There are two [[resonance structure]]s for the deprotonated form, differing in the position of the double bond.
Another way to look at ascorbic acid is to consider it as an [[enol]]. The deprotonated form is an [[enolate]], which is usually strongly basic. However, the adjacent double bond stabilizes the deprotonated form.
[[Image:Ascorbic acidity1.png|thumb|left|500px|Movement of electron pairs in deprotonation]]<br style="clear:left;">
===Tautomerism===
[[Image:Ascorbic diketone.png|frame|left|[[Nucleophile|Nucleophilic attack]] of ascorbic enol on proton to give 1,3-diketone]]
Ascorbic acid also rapidly interconverts into two unstable [[diketone]] [[tautomer]]s by [[proton]] transfer, although it is the most stable in the enol form. The proton of the enol is lost, and reacquired by electrons from the double bond, to produce a diketone. This is an [[enol]] reaction. There are two possible forms: 1,2-diketone and 1,3-diketone.
===Determination===
The concentration of a solution of ascorbic acid can be determined in many ways, the most common ways involving [[titration]] with an [[oxidizing agent]].
;DCPIP
A commonly-used oxidising agent is the dye 2,6-dichlorophenol-indophenol, or [[DCPIP]] for short. The blue dye is run into the ascorbic acid [[solution]] until a faint pink colour persists for 15 seconds.
;Iodine
Another method involves using [[iodine]] and a [[starch]] [[indicator]], wherein iodine reacts with ascorbic acid, and, when all the ascorbic acid has reacted, the iodine is then in excess, forming a blue-black complex with the starch indicator. This indicates the end-point of the titration. As an alternative, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as an indicator.
;Iodate and iodine
The above method involving iodine requires making up and standardising the iodine solution. One way around this is to generate the iodine in the presence of the ascorbic acid by the reaction of [[iodate]] and iodide ion in [[acid]] solution.
;N-Bromosuccinimide
A much-less-common oxidising agent is [[N-Bromosuccinimide|''N''-bromosuccinimide]], (NBS). In this titration, the NBS oxidises the ascorbic acid (in the presence of [[potassium iodide]] and starch). When the NBS is in excess (i.e., the reaction is complete), the NBS liberates the iodine from the potassium iodide, which then forms the blue/black complex with starch, indicating the end-point of the titration.
== Uses ==
<div class="thumb tright" style="background-color: #f9f9f9; border: 1px solid #CCCCCC; margin:0.5em;">
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|[[Image:Ascorbic acid structure.png|150px]]
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{|border="0" width=150px border="0" cellpadding="2" cellspacing="0" style="font-size: 85%; border: 1px solid #CCCCCC; margin: 0.3em;"
|[[Image:Dehydroascorbic acid.png|150px]]
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<div style="border: none; width:150px;"><div class="thumbcaption"><small>Top: ascorbic acid<br>([[reducing agent|reduced form]] of [[Vitamin C]])<br>Bottom: [[dehydroascorbic acid]]<br>([[oxidizing agent|oxidized form]] of [[Vitamin C]])</small></div></div></div>
Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a [[preservative]].
Exposure to oxygen, metals, light, and heat destroys ascorbic acid, so it must be stored in a dark, cold, and non-metallic container.
The [[Optical isomerism|L-enantiomer]] of ascorbic acid is also known as [[vitamin C]]. The name "ascorbic" comes from its property of preventing and curing [[scurvy]]. [[Primate]]s, including humans, and a few other species in all divisions of the animal kingdom, notably the [[guinea pig]], have lost the ability to synthesize ascorbic acid, and must obtain it in their food.
Ascorbic acid and its sodium, potassium, and calcium [[salts]] are commonly used as [[antioxidant]] [[food additive]]s. These compounds are water-soluble and thus cannot protect [[fat]]s from oxidation: For this purpose, the fat-[[soluble]] [[ester]]s of ascorbic acid with long-chain [[fatty acid]]s (ascorbyl palmitate or ascorbyl stearate) can be used as food antioxidants. Eighty percent of the world's supply of ascorbic acid is produced in China. <ref>Washington Post, [http://www.washingtonpost.com/wp-dyn/content/article/2007/05/19/AR2007051901273.html Tainted Chinese Imports Common], published [[May 20]], [[2007]]</ref>
The relevant European food additive [[E number]]s are:
#E300 ascorbic acid,
#E301 [[sodium ascorbate]],
#E302 [[calcium ascorbate]],
#E303 [[potassium ascorbate]],
#E304 fatty acid esters of ascorbic acid (i) [[ascorbyl palmitate]] (ii) [[ascorbyl stearate]].
It can be added to water that has been treated with iodine to make it potable, neutralizing the unpleasant iodine taste, and increasing the health benefits of drinking [[water]], although increasing the chance of [[tooth decay]].{{Fact|date=July 2008}}
In plastic manufacturing, ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.<ref>{{cite news |first= |last= |authorlink= |author=Newswise |coauthors= |title=Vitamin C, water have benefits for plastic manufacturing |url=http://www.reliableplant.com/article.asp?pagetitle=Vitamin%20C,%20water%20have%20benefits%20for%20plastic%20manufacturing&articleid=3133 |format= |work= |publisher=Reliable Plant Magazine |id= |pages= |page= |date=2007 |accessdate=2007-06-25 |language= |quote= }}</ref>
==Antioxidant mechanism==
Ascorbate acts as an antioxidant by being available for energetically favourable oxidation. Many oxidants (typically, [[reactive oxygen species]]) such as the hydroxyl radical (formed from [[hydrogen peroxide]]), contain an unpaired electron, and, thus, are highly reactive and damaging to humans and plants at the molecular level. This is due to their interaction with [[nucleic acid]], proteins, and lipids. Reactive oxygen species oxidize (take electrons from) ascorbate first to monodehydroascorbate and then dehydroascorbate. The reactive oxygen species are reduced to water, while the oxidized forms of ascorbate are relatively stable and unreactive, and do not cause cellular damage.
==Ascorbic acid synthesis in non-primates==
Ascorbic acid is found in plants, animals, and single-cell organisms.<ref>
[http://www.seanet.com/~alexs/ascorbate/197x/stone-i-orthomol_psych-1972-v1-n2-3-p82.htm ''The Natural History of Ascorbic Acid in the Evolution of Mammals and Primates'', Irwin Stone, 1972]</ref> All living animals either make it, eat it, or die from [[scurvy]] due to lack of it. Reptiles and older orders of birds make ascorbic acid in their kidneys. Recent orders of birds and most mammals make ascorbic acid in their livers where the enzyme [[L-gulonolactone oxidase]] is required to convert glucose to ascorbic acid.<ref>Stone</ref> Humans, guinea pigs, and some other primates are not able to make L-gulonolactone oxidase because of a genetic defect and are therefore unable to make ascorbic acid in their livers. This genetic mutation occurred about 63 million years ago <ref>Stone</ref> This would have had lethal consequences for the mutated primate were it not for the fact that it occurred to an arboreal animal living in a tropical environment where plenty of foodstuffs containing ascorbic acid were available throughout the year. Although ascorbic acid is a vital food nutrient for humans and is therefore termed a vitamin, it is a natural liver metabolite in most other animals.
==See also==
*[[Vitamin C]]: a discussion of the medical properties of ascorbic acid as well as its historic and social role
*[[Dehydroascorbic acid]], an oxidized form of ascorbic acid.
*[[Erythorbic acid]]: a [[diastereomer]] of ascorbic acid.
*[[Mineral ascorbates]]: salts of ascorbic acid
*[[D-erythroascorbic acid]]: yeasts do not make vitamin C (L-ascorbic acid), but a similar antioxidant known as [http://www.scri.ac.uk/webfm_send/471 D-erythroascorbic acid]
==References==
* Clayden, Greeves, Warren, Wothers. ''Organic Chemistry.'' Oxford University Press (2001), ISBN 0-19-850346-6.
* {{cite journal | author = Derek Denby | title = Vitamin C | journal = [http://www.york.ac.uk/depts/chem/chemrev/ Chemistry Review] | publisher = [[University of York]] | volume = 5 | issue = 5 |date=May 1996}}
*''Vitamin C: Its Chemistry and Biochemistry'' Michael B. Davies, John Austin, David A. Partridge. Royal Society of Chemistry. ISBN 0-85186-333-7
*''Food: The Chemistry of Its Components; Third Edition'' T.P. Coultate. Royal Society of Chemistry. ISBN 0-85404-513-9
<references/>
==External links==
*[http://ptcl.chem.ox.ac.uk/MSDS/AS/ascorbic_acid.html Safety data] from the [[University of Oxford]].
*[http://www.inchem.org/documents/pims/pharm/ascorbic.htm International Programme on Chemical Safety]
*[http://calebkruse.com/Lab%20Write-Up%2008.html The Effect of Ascorbic Acid on Bleaching Coral]
*[http://calebkruse.com/Lab%20Write-Up.html The Effect of Ascorbic Acid on Fragmented Coral]
{{vitamin}}
{{Enzyme cofactors}}
[[Category:Organic acids]]
[[Category:Antioxidants]]
[[Category:Dietary antioxidants]]
[[Category:Coenzymes]]
[[ca:Vitamina C]]
[[da:Askorbinsyre]]
[[de:Ascorbinsäure]]
[[et:C-vitamiin]]
[[es:Ácido ascórbico]]
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[[fa:ویتامین ث]]
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[[ko:비타민 C]]
[[it:Acido ascorbico]]
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[[nl:Ascorbinezuur]]
[[ja:アスコルビン酸]]
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[[zh:维生素C]]