Editing Chemistry of ascorbic acid (section)

==Chemical properties==

===Acidity===
Ascorbic acid is a [[furan]]-based [[lactone]] of [[2-ketogluconic acid]]. It contains an adjacent [[enediol]] adjacent to the [[carbonyl]]. This −C(OH)=C(OH)−C(=O)− structural pattern is characteristic of [[reductone]]s, and increases the acidity of one of the enol [[hydroxyl group]]s. The deprotonated [[conjugate base]] is the [[ascorbate]] anion, which is stabilized by electron delocalization that results from [[resonance (chemistry)|resonance]] between two forms:

: [[File:Ascorbate resonance.png|400px|class=skin-invert-image]]

For this reason, ascorbic acid is much more acidic than would be expected if the compound contained only isolated hydroxyl groups.

===Salts===
The ascorbate anion forms [[salt (chemistry)|salts]], such as [[sodium ascorbate]], [[calcium ascorbate]], and [[potassium ascorbate]].

===Esters===
Ascorbic acid can also react with organic acids as an [[alcohol (chemistry)|alcohol]] forming [[ester]]s such as [[ascorbyl palmitate]] and [[ascorbyl stearate]].

===Nucleophilic attack===
[[Nucleophile|Nucleophilic attack]] of ascorbic acid on a proton results in a 1,3-diketone:

:[[Image:Ascorbic diketone.png|class=skin-invert-image]]

===Oxidation===
{{More citations needed section|date=March 2024}}
[[File:L-Semidehydroascorbinsäure.svg|right|thumb|220px|class=skin-invert-image|Semidehydroascorbate acid radical]]
[[File:Dehydroascorbic_acid_2.svg|right|thumb|220px|class=skin-invert-image|Pseudodehydroascorbate<!--Image incorrectly named as dehydroascorbic acid-->]]
{{Image frame|width=220|content=<div class="skin-invert-image">{{CSS image crop|Image = Ascorbic_acid_all.svg|bSize = 1250|cWidth = 220|cHeight = 142|oTop = 328|oLeft = 1006}}</div><!--Using cropped image until we can generate a new one-->|align=right|caption=Dehydroascorbate}}
The ascorbate ion is the predominant species at typical biological pH values. It is a mild [[reducing agent]] and [[antioxidant]], typically reacting with oxidants of the [[reactive oxygen species]], such as the [[hydroxyl radical]].

Reactive oxygen species are damaging to animals and plants at the molecular level due to their possible interaction with [[nucleic acid]]s, proteins, and lipids. Sometimes these radicals initiate chain reactions. Ascorbate can terminate these chain radical reactions by [[electron transfer]]. The oxidized forms of ascorbate are relatively unreactive and do not cause cellular damage.

Ascorbic acid and its sodium, potassium, and calcium [[Salt (chemistry)|salt]]s 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-[[Solubility|soluble]] [[ester]]s of ascorbic acid with long-chain [[fatty acid]]s (ascorbyl palmitate or ascorbyl stearate) can be used as antioxidant food additives. Sodium-dependent active transport process enables absorption of Ascorbic acid from the intestine.<ref>{{cite web |title=Re-evaluation of ascorbic acid, sodium ascorbate and calcium ascorbate as food additives {{!}} EFSA |url=https://www.efsa.europa.eu/en/efsajournal/pub/4087 |website=www.efsa.europa.eu |publisher=European Food Safety Authority |language=en |date=6 May 2015}}</ref>

Ascorbate readily donates a hydrogen atom to [[radical (chemistry)|free radicals]], forming the [[radical anion]] semidehydroascorbate (also known as monodehydroascorbate), a resonance-stabilized [[semitrione]]:<ref name=Njus2020>{{cite journal |last1=Njus |first1=David |last2=Kelley |first2=Patrick M. |last3=Tu |first3=Yi-Jung |last4=Schlegel |first4=H. Bernhard |title=Ascorbic acid: The chemistry underlying its antioxidant properties |journal=Free Radical Biology and Medicine |date=November 2020 |volume=159 |pages=37–43 |doi=10.1016/j.freeradbiomed.2020.07.013|pmid=32738399 }}</ref>
:{{chem2|C6H7O6- + L• -> C6H6O6<sup>•</sup>- + LH}}

Loss of an electron from semidehydroascorbate to produce the 1,2,3-tricarbonyl pseudodehydroascorbate is thermodynamically disfavored, which helps prevent propagation of free radical chain reactions such as [[autoxidation]]:<ref name=Njus2020/>
:{{chem2|C6H6O6<sup>•</sup>- + O2}} <math>\not\rightarrow</math> {{chem2|C6H6O6 + O2<sup>•</sup>-}}

However, being a good electron donor, excess ascorbate in the presence of free metal ions can not only promote but also initiate free radical reactions, thus making it a potentially dangerous pro-oxidative compound in certain metabolic contexts.

Semidehydroascorbate oxidation instead occurs in conjunction with hydration, yielding the bicyclic [[hemiketal]] [[dehydroascorbic acid|dehydroascorbate]]. In particular, semidehydroascorbate undergoes disproportionation to ascorbate and dehydroascorbate:<ref name=Njus2020/>
:{{chem2|C6H6O6<sup>•</sup>- + L• + H2O + H+ -> C6H8O7 + LH}}
:{{chem2|2 C6H6O6<sup>•</sup>- + H2O + H+ -> C6H8O7 + C6H7O6-}}

Aqueous solutions of dehydroascorbate are unstable, undergoing hydrolysis with a half-life of 5–15&nbsp;minutes at {{convert|37|C}}. Decomposition products include [[diketogulonic acid]], [[xylonic acid]], [[threonic acid]] and [[oxalic acid]].<ref>{{cite book |url=https://books.google.com/books?id=45I3EQAAQBAJ&q=diketogulonic&pg=PA311 |title = Ingredient Interactions: Effects on Food Quality, Second Edition|isbn = 9781420028133|last1 = Gaonkar|first1 = Anilkumar G.|last2 = McPherson|first2 = Andrew |date = 2016-04-19| publisher=CRC Press }}</ref><ref>{{cite journal |last1=Linster |first1=Carole L. |author-link1=:lb:Carole Linster |last2=Van Schaftingen |first2=Emile |title=Vitamin C: Biosynthesis, recycling and degradation in mammals |journal=The FEBS Journal |date=January 2007 |volume=274 |issue=1 |pages=1–22 |doi=10.1111/j.1742-4658.2006.05607.x|pmid=17222174 }}</ref>{{rp|p=14}}

===Other reactions===
It creates volatile compounds when mixed with [[glucose]] and [[amino acid]]s at 90&nbsp;°C.<ref>{{cite journal |author1=Seck, S. |author2=Crouzet, J. | title = Formation of Volatile Compounds in Sugar-Phenylalanine and Ascorbic Acid-Phenylalanine Model Systems during Heat Treatment | journal = Journal of Food Science | year = 1981 | volume = 46 | issue = 3 | pages = 790–793 | doi = 10.1111/j.1365-2621.1981.tb15349.x }}</ref>

It is a cofactor in [[tyrosine]] [[oxidation]], though because a crude extract of animal liver is used, it is unclear which reaction catalyzed by which enzyme is being helped here.<ref>{{cite journal | vauthors = Sealock RR, Goodland RL, Sumerwell WN, Brierly JM | title = The role of ascorbic acid in the oxidation of <small>L</small>-Tyrosine by guinea pig liver extracts | journal = The Journal of Biological Chemistry | volume = 196 | issue = 2 | pages = 761–7 | date = May 1952 | doi = 10.1016/S0021-9258(19)52407-3 | pmid = 12981016 | url = http://www.jbc.org/content/196/2/761.full.pdf | doi-access = free }}</ref> For known roles in enzymatic reactions, see {{section link|Vitamin C#Pharmacodynamics}}.

Because it reduces iron(III) and chelates iron ions, it enhances the oral absorption of non-heme iron.<ref name="pmid28189173">{{cite journal |vauthors=DeLoughery TG |title=Iron deficiency anemia |journal=Med Clin North Am |volume=101 |issue=2 |pages=319–32 |date=March 2017 |pmid=28189173 |doi=10.1016/j.mcna.2016.09.004 |type=Review}}</ref> This property also applies to its enantiomer.<ref>{{cite journal | title = Erythorbic acid is a potent enhancer of nonheme-iron absorption | last = Fidler | first = MC |author2=Davidsson L |author3=Zeder C |author4=Hurrell RF  | journal = American Journal of Clinical Nutrition |date=January 2004 | volume = 79 | issue = 1 | pmid = 14684404 | pages = 99–102 | doi=10.1093/ajcn/79.1.99| doi-access = free }}</ref>

====Conversion to oxalate====
In 1958, it was discovered that ascorbic acid can be converted to [[oxalate]], a key component of calcium oxalate [[kidney stone]]s.<ref name="pmid13525409">{{cite journal |vauthors=Hellman L, Burns JJ |title=Metabolism of L-ascorbic acid-1-C14 in man |journal=The Journal of Biological Chemistry |volume=230 |issue=2 |pages=923–30 |date=February 1958 |doi=10.1016/S0021-9258(18)70515-2 |doi-access=free |pmid=13525409 |url=https://www.jbc.org/article/S0021-9258(18)70515-2/pdf}}</ref><ref name="pmid27002809">{{cite journal |vauthors=Knight J, Madduma-Liyanage K, Mobley JA, Assimos DG, Holmes RP |title=Ascorbic acid intake and oxalate synthesis |journal=Urolithiasis |volume=44 |issue=4 |pages=289–97 |date=August 2016 |pmid=27002809 |pmc=4946963 |doi=10.1007/s00240-016-0868-7}}</ref><ref name="Kayis 2024">{{cite book |vauthors=Kayis C |title=Ascorbic Acid - Biochemistry and Functions |chapter=Effect of Ascorbic Acid on the Kidneys |publisher=IntechOpen |year=2024 |isbn=978-1-83768-562-2 |doi=10.5772/intechopen.111913 |doi-access=free |url=https://www.intechopen.com/citation-pdf-url/87322 |access-date=12 January 2025 |page=}}</ref> The process begins with the formation of [[dehydroascorbic acid]] (DHA) from the ascorbyl radical. While DHA can be recycled back to ascorbic acid, a portion irreversibly degrades to 2,3-diketogulonic acid (DKG), which then breaks down to both oxalate and the sugars [[erythrulose|L-erythrulose]] and [[threosone]].<ref name="pmid27002809"/><ref name="Kayis 2024"/><ref name="pmid38089442">{{cite journal |vauthors=Bao D, Wang Y, Zhao MH |title=Oxalate Nephropathy and the Mechanism of Oxalate-Induced Kidney Injury |journal=Kidney Diseases |volume=9 |issue=6 |pages=459–468 |date=December 2023 |pmid=38089442 |pmc=10712969 |doi=10.1159/000533295}}</ref> Research conducted in the 1960s suggested ascorbic acid could substantially contribute to urinary oxalate content (possibly over 40%), but these estimates have been questioned due to methodological limitations.<ref name="pmid27002809"/><ref name="Kayis 2024"/><ref name="pmid14217884">{{cite journal |vauthors=Atkins GL, Dean BM, Griffin WJ, Watts RW |title=Quantitative Aspects Of Ascorbic Acid Metabolism In Man |journal=The Journal of Biological Chemistry |volume=239 |issue= 9|pages=2975–80 |date=September 1964 |doi=10.1016/S0021-9258(18)93840-8 |doi-access=free |pmid=14217884 |url=https://www.jbc.org/article/S0021-9258(18)93840-8/pdf}}</ref> Subsequent large cohort studies have yielded conflicting results regarding the link between vitamin C intake and kidney stone formation. The overall clinical significance of ascorbic acid consumption to kidney stone risk, however, remains inconclusive, although several studies have suggested a potential association, especially with high-dose supplementation in men.<ref name="pmid27002809"/><ref name="Kayis 2024"/><ref name="pmid36839235">{{cite journal |vauthors=Cupisti A, Giannese D, D'Alessandro C, Benedetti A, Panichi V, Alfieri C, Castellano G, Messa P |title=Kidney Stone Prevention: Is There a Role for Complementary and Alternative Medicine? |journal=Nutrients |volume=15 |issue=4 |date=February 2023 |page=877 |pmid=36839235 |pmc=9959749 |doi=10.3390/nu15040877|doi-access=free }}</ref><ref name="pmid30178451">{{cite journal |vauthors=Jiang K, Tang K, Liu H, Xu H, Ye Z, Chen Z |title=Ascorbic Acid Supplements and Kidney Stones Incidence Among Men and Women: A systematic review and meta-analysis |journal=Urology Journal |volume=16 |issue=2 |pages=115–120 |date=May 2019 |pmid=30178451 |doi=10.22037/uj.v0i0.4275}}</ref>