# Chemistry of ascorbic acid
**Ascorbic acid** (vitamin C) is a water-soluble organic compound with the empirical formula C₆H₈O₆ and a molecular weight of 176.13 Da, which functions in living systems as an enzyme cofactor and a reducing agent, or antioxidant.[1](https://pubs.rsc.org/en/content/articlehtml/2025/ra/d5ra03825a) The biologically active form is the L-enantiomer, L-ascorbic acid; the D-enantiomer is pharmacologically inert.[2](https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2025.1633447/pdf) Its chemistry is dominated by the ease with which it gives up electrons, passing through a free-radical intermediate to an oxidized form, dehydroascorbic acid, and by the instability that follows.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3608474/)
| Key fact | Detail |
|---|---|
| Formula and mass | Empirical formula C₆H₈O₆; molecular weight 176.13 Da[1](https://pubs.rsc.org/en/content/articlehtml/2025/ra/d5ra03825a) |
| Acidity | pK₁ = 4.17 and pK₂ = 11.57 (Merck Index); a pKa of 4.70 is reported at 10 °C[4](https://pubchem.ncbi.nlm.nih.gov/compound/ascorbic_acid) |
| Redox behavior | Two consecutive one-electron oxidations form the ascorbate radical (Asc•−) and then dehydroascorbic acid (DHA)[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3608474/) |
| Stability window | Maximum stability in the pH range 2 to 4; both ascorbic acid and DHA are degraded by natural and UV light[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8619176/) |
| Structural work | Norman Haworth deduced the structure of Szent-Györgyi's crystals and synthesized the vitamin, work recognized by the 1937 Nobel Prize in Chemistry[6](https://www.nobelprize.org/prizes/themes/the-nobel-prize-and-the-discovery-of-vitamins/) |
| Isolation priority | King and Waugh reported the chemical nature of vitamin C in Science on April 22, 1932; Svirbely and Szent-Györgyi announced its identity with hexuronic acid on May 7, 1932[7](https://www.nationalacademies.org/read/11807/chapter/12) |
| Concentration range in the body | Oral dosing raises plasma vitamin C to at most about 220 micromoles/L, while intravenous administration can reach about 26,000 micromoles/L[8](https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/) |
## Structure and acid–base properties
Ascorbic acid is a lactone, a cyclic ester derived from a sugar-like carbon skeleton, and it contains no carboxylic acid group. Its acidity comes instead from enolic hydroxyl groups: the first proton is lost with pK₁ = 4.17 and the second with pK₂ = 11.57 according to the [Merck Index](https://www.edgepedia.org/merck-index), while the IUPAC digitized dataset gives a pKa of 4.70 at 10 °C.[4](https://pubchem.ncbi.nlm.nih.gov/compound/ascorbic_acid) Because the first dissociation lies near pH 5, the compound exists almost entirely as the ascorbate anion at pH values of 5 to 9 in water.[4](https://pubchem.ncbi.nlm.nih.gov/compound/ascorbic_acid) The molecule exists as two enantiomers, of which L-ascorbic acid is the biologically active form.[2](https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2025.1633447/pdf)
## Redox chemistry
The defining reaction of ascorbate is electron donation. It undergoes two consecutive one-electron oxidations: the first produces the ascorbate radical (Asc•−), a relatively unreactive free radical, and the second produces dehydroascorbic acid (DHA).[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3608474/) These properties make ascorbate an effective donor antioxidant.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3608474/) The cycle is reversible in biology: DHA may be reduced back to ascorbate, sequentially through the ascorbate radical, by glutathione, or directly by enzyme-dependent mechanisms.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC4959991/) As long as ascorbic acid and DHA keep interconverting, vitamin C activity is retained; loss occurs when DHA undergoes irreversible degradation.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC8773188)
The redox instability is pronounced at physiological pH: the half-life of ascorbic acid at pH 7.4 is under 2 hours, driving rapid oxidation to the ascorbate radical and DHA.[2](https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2025.1633447/pdf) This same reducing power has a second face. Ascorbic acid can exert pro-oxidant effects in vitro, usually by interaction with transition metal ions, and at millimolar plasma concentrations, or pharmacologic concentrations, it acts as a pro-oxidant.[11](https://pubmed.ncbi.nlm.nih.gov/8902542/) • [9](https://pmc.ncbi.nlm.nih.gov/articles/PMC4959991/) In mayonnaise, for example, added ascorbic acid works as a lipid antioxidant or pro-oxidant depending on the presence or absence of vitamin E.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC8773188)
## Isolation and structural determination
The compound entered chemistry as an antioxidant extracted from tissue. After detecting an antioxidant compound in the adrenal cortex, Albert von Szent-Györgyi was invited to Cambridge in 1927 and, using a simple in vitro test to measure its concentration in tissue fractions, isolated within a few months a compound he named hexuronic acid, with the empirical formula C₆H₈O₆.[6](https://www.nobelprize.org/prizes/themes/the-nobel-prize-and-the-discovery-of-vitamins/) Szent-Györgyi spent four years in possession of the isolated vitamin, unaware of the significance of his own achievement.[6](https://www.nobelprize.org/prizes/themes/the-nobel-prize-and-the-discovery-of-vitamins/)
Independent confirmation came from the United States. [Charles Glen King](https://www.edgepedia.org/charles-glen-king) and his graduate students prepared vitamin C concentrates from lemon juice between 1929 and 1931, and a crystalline compound was isolated whose chemical nature was reported in [Science](https://www.edgepedia.org/science-journal) on April 22, 1932, followed by a fuller report by Waugh and King in the [Journal of Biological Chemistry](https://www.edgepedia.org/journal-of-biological-chemistry).[7](https://www.nationalacademies.org/read/11807/chapter/12) Fifteen days later, on May 7, 1932, J. L. Svirbely, formerly King's student, and Szent-Györgyi announced that vitamin C is a single substance identical to hexuronic acid.[7](https://www.nationalacademies.org/read/11807/chapter/12)
**Structure and synthesis.** Knowing that the elements of vitamin C resembled those of simple sugars, and using Szent-Györgyi's freely supplied pure extracts, [Norman Haworth](https://www.edgepedia.org/norman-haworth) of the [University of Birmingham](https://www.edgepedia.org/university-of-birmingham) deduced the correct chemical structure and then synthesized the vitamin.[12](https://www.nobelprize.org/prizes/chemistry/1937/speedread/) Haworth gave it the name ascorbic acid, referring to its antiscorbutic (scurvy-preventing) property.[13](https://www.nature.com/articles/140882b0) The 1937 [Nobel Prize in Chemistry](https://www.edgepedia.org/nobel-prize-in-chemistry) recognized Haworth for his investigations on carbohydrates and vitamin C, shared with Paul Karrer for work on carotenoids, flavins, and vitamins A and B2; Szent-Györgyi received the 1937 Prize in Physiology or Medicine for discoveries in biological combustion processes with special reference to vitamin C.[14](https://www.livescience.com/16384-nobel-prize-chemistry-list.html) • [15](https://www.livescience.com/16342-nobel-prize-medicine-history-list.html) The structural work made vitamin C available cheaply on a large scale for medicinal use.[12](https://www.nobelprize.org/prizes/chemistry/1937/speedread/)
## Stability and degradation
Ascorbic acid is chemically fragile, and its degradation involves complex oxidation and intermolecular rearrangement reactions that drive quality and color changes during food processing and storage.[16](https://www.researchgate.net/publication/357820220_Chemical_Stability_of_Ascorbic_Acid_Integrated_into_Commercial_Products_A_Review_on_Bioactivity_and_Delivery_Technology/fulltext/61eacc8ddafcdb25fd3fb49a/Chemical-Stability-of-Ascorbic-Acid-Integrated-into-Commercial-Products-A-Review-on-Bioactivity-and-Delivery-Technology.pdf) [Temperature](https://www.edgepedia.org/temperature) is the dominant factor determining stability in processed juices, directly affecting the degradation rate.[16](https://www.researchgate.net/publication/357820220_Chemical_Stability_of_Ascorbic_Acid_Integrated_into_Commercial_Products_A_Review_on_Bioactivity_and_Delivery_Technology/fulltext/61eacc8ddafcdb25fd3fb49a/Chemical-Stability-of-Ascorbic-Acid-Integrated-into-Commercial-Products-A-Review-on-Bioactivity-and-Delivery-Technology.pdf) Maximum stability is recorded in the pH range between 2 and 4, and both ascorbic acid and DHA are susceptible to degradation by natural and UV light.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8619176/) Oxygen, oxidative agents, and catalysts also matter: increasing water activity accelerates degradation, and Fe³⁺ ions accelerate both the oxidation and hydrolysis steps of the degradation path.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8619176/) In fruit juices treated with ultraviolet or pulsed light for pasteurization, photodegradation, aerobic oxidation, and anaerobic degradation are the major mechanisms governing ascorbic acid loss.[17](https://www.sciencedirect.com/science/article/pii/S2772753X23001557) One dietary consequence follows from these traits: since ascorbic acid dissolves in water and breaks down when heated, keeping food stored for a long time or cooking it can lower how much vitamin C it contains.[8](https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/)
## Biological chemistry
In the body, ascorbate's reducing power is put to work as an enzyme cofactor. It acts as an obligate cofactor for Fe²⁺-dependent dioxygenases, including the collagen hydroxylases that modify proline and lysine, and the prolyl hydroxylases that regulate HIF-1α in oxygen sensing.[2](https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2025.1633447/pdf) [Vitamin C](https://www.edgepedia.org/vitamin-c) is also required, along with iron, at two steps in the pathway of carnitine biosynthesis, and it regenerates other biological antioxidants such as glutathione and α-tocopherol back to their active state.[18](https://www.ncbi.nlm.nih.gov/books/NBK225483/pdf/Bookshelf_NBK225483.pdf) Humans lack the ability to synthesize vitamin C and depend on exogenous dietary sources.[19](https://www.accessdata.fda.gov/drugsatfda_docs/nda/2017/209112Orig1s000_OtherRev.pdf)
**Transport and concentration control.** Two sodium-dependent transporters are specific for ascorbic acid, while its oxidation product dehydroascorbic acid is transported by glucose transporters.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC4959991/) The body tightly controls tissue and plasma concentrations: approximately 70% to 90% of vitamin C is absorbed at moderate intakes of 30 to 180 mg/day, absorption falls to less than 50% at doses above 1 g/day, and absorbed, unmetabolized ascorbic acid is excreted in the urine.[8](https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/) The route of administration matters chemically as well. Oral doses, even very large ones, raise plasma concentrations to a maximum of only 220 micromoles/L, whereas intravenous administration can produce plasma concentrations as high as 26,000 micromoles/L; concentrations of this magnitude are selectively cytotoxic to tumor cells in vitro, and a high concentration of vitamin C may act as a pro-oxidant generating hydrogen peroxide with selective toxicity toward cancer cells.[8](https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/)
## References
1. [Progress in the design of ascorbic acid derivative-mediated drug delivery, RSC Advances (2025)](https://pubs.rsc.org/en/content/articlehtml/2025/ra/d5ra03825a)
2. [The dual role of vitamin C in cancer: from antioxidant prevention to prooxidant therapeutic applications, Frontiers in Medicine (2025)](https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2025.1633447/pdf)
3. [Ascorbic acid: Chemistry, biology and the treatment of cancer, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3608474/)
4. [L-Ascorbic Acid, CID 54670067, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/ascorbic_acid)
5. [Effect of Alternative Preservation Steps and Storage on Vitamin C Stability in Fruit and Vegetable Products, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8619176/)
6. [The Nobel Prize and the discovery of vitamins, NobelPrize.org](https://www.nobelprize.org/prizes/themes/the-nobel-prize-and-the-discovery-of-vitamins/)
7. [Charles Glen King, Biographical Memoirs: Volume 88, National Academy of Sciences (2006)](https://www.nationalacademies.org/read/11807/chapter/12)
8. [Vitamin C, Health Professional Fact Sheet, NIH Office of Dietary Supplements](https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/)
9. [Vitamin C physiology: the known and the unknown and Goldilocks, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4959991/)
10. [Chemical Stability of Ascorbic Acid Integrated into Commercial Products, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8773188)
11. [Vitamin C: antioxidant or pro-oxidant in vivo?, PubMed](https://pubmed.ncbi.nlm.nih.gov/8902542/)
12. [The Nobel Prize in Chemistry 1937, Speed read: The ABC of vitamins, NobelPrize.org](https://www.nobelprize.org/prizes/chemistry/1937/speedread/)
13. [Nobel Prize for Chemistry, Nature (1937)](https://www.nature.com/articles/140882b0)
14. [Nobel Prize in Chemistry: 1901–Present, Live Science](https://www.livescience.com/16384-nobel-prize-chemistry-list.html)
15. [Nobel Prize in Medicine: 1901–Present, Live Science](https://www.livescience.com/16342-nobel-prize-medicine-history-list.html)
16. [Chemical Stability of Ascorbic Acid Integrated into Commercial Products: A Review on Bioactivity and Delivery Technology](https://www.researchgate.net/publication/357820220_Chemical_Stability_of_Ascorbic_Acid_Integrated_into_Commercial_Products_A_Review_on_Bioactivity_and_Delivery_Technology/fulltext/61eacc8ddafcdb25fd3fb49a/Chemical-Stability-of-Ascorbic-Acid-Integrated-into-Commercial-Products-A-Review-on-Bioactivity-and-Delivery-Technology.pdf)
17. [Effect of ultraviolet and pulsed light treatments on ascorbic acid content in fruit juices, ScienceDirect](https://www.sciencedirect.com/science/article/pii/S2772753X23001557)
18. [Vitamin C, Dietary Reference Intakes, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK225483/pdf/Bookshelf_NBK225483.pdf)
19. [FDA NDA review, ascorbic acid (2017)](https://www.accessdata.fda.gov/drugsatfda_docs/nda/2017/209112Orig1s000_OtherRev.pdf)
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*Topic: Encyclopedia › Life and health › Human health and medicine › Nutrition and personal wellbeing › Nutrition science and human nutrition › Vitamins › Vitamin deficiency diseases › Scurvy and vitamin C deficiency*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license