Editing Chemistry of ascorbic acid (section)

===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}}