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Ascorbic acid is an
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
with formula , originally called hexuronic acid. It is a white solid, but impure samples can appear yellowish. It dissolves freely in water to give mildly acidic solutions. It is a mild
reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...
. Ascorbic acid exists as two
enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...
s (mirror-image
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...
s), commonly denoted "" (for "levo") and "" (for "dextro"). The isomer is the one most often encountered: it occurs naturally in many foods, and is one form (" vitamer") of
vitamin C Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits, berries and vegetables. It is also a generic prescription medication and in some countries is sold as a non-prescription di ...
, an essential nutrient for humans and many animals. Deficiency of vitamin C causes
scurvy Scurvy is a deficiency disease (state of malnutrition) resulting from a lack of vitamin C (ascorbic acid). Early symptoms of deficiency include weakness, fatigue, and sore arms and legs. Without treatment, anemia, decreased red blood cells, gum d ...
, formerly a major disease of sailors in long sea voyages. It is used as a
food additive Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives, such as vinegar ( pickling), salt ( salting), smoke ( smoking) and sugar ( crystallization), have been used f ...
and a
dietary supplement A dietary supplement is a manufactured product intended to supplement a person's diet by taking a pill (pharmacy), pill, capsule (pharmacy), capsule, tablet (pharmacy), tablet, powder, or liquid. A supplement can provide nutrients eithe ...
for its
antioxidant Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...
properties. The "" form ( erythorbic acid) can be made by chemical synthesis, but has no significant biological role.


Etymology

The term ''ascorbic'' means antiscruvy and denotes the ability to fight off scurvy. It is related to combating Vitamin C deficiency.


History

The antiscorbutic properties of certain foods were demonstrated in the 18th century by James Lind. In 1907, Axel Holst and Theodor Frølich discovered that the antiscorbutic factor was a water-soluble chemical substance, distinct from the one that prevented
beriberi Thiamine deficiency is a medical condition of low levels of thiamine (vitamin B1). A severe and chronic form is known as beriberi. The name beriberi was possibly borrowed in the 18th century from the Sinhalese phrase (bæri bæri, “I canno ...
. Between 1928 and 1932,
Albert Szent-Györgyi Albert Imre Szent-Györgyi de Rapoltu Mare, Nagyrápolt (; September 16, 1893 – October 22, 1986) was a Hungarian biochemist who won the Nobel Prize in Physiology or Medicine in 1937. He is credited with first isolating vitamin C and disc ...
isolated a candidate for this substance, which he called "hexuronic acid", first from plants and later from animal adrenal glands. In 1932 Charles Glen King confirmed that it was indeed the antiscorbutic factor. In 1933, sugar chemist Walter Norman Haworth, working with samples of "hexuronic acid" that Szent-Györgyi had isolated from paprika and sent him in the previous year, deduced the correct structure and optical-isomeric nature of the compound, and in 1934 reported its first synthesis. In reference to the compound's antiscorbutic properties, Haworth and Szent-Györgyi proposed to rename it "a-scorbic acid" for the compound, and later specifically -ascorbic acid. Because of their work, in 1937 two
Nobel Prize The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...
s: in Chemistry and in Physiology or Medicine were awarded to Haworth and Szent-Györgyi, respectively.


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 reductones, and increases the acidity of one of the enol hydroxyl groups. The deprotonated conjugate base is the ascorbate anion, which is stabilized by electron delocalization that results from
resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
between two forms: : 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 salts, such as sodium ascorbate, calcium ascorbate, and potassium ascorbate.


Esters

Ascorbic acid can also react with organic acids as an
alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...
forming
ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
s such as ascorbyl palmitate and ascorbyl stearate.


Nucleophilic attack

Nucleophilic attack of ascorbic acid on a proton results in a 1,3-diketone: :


Oxidation

The ascorbate ion is the predominant species at typical biological pH values. It is a mild
reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...
and
antioxidant Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...
, 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 Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
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 In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
s are commonly used as
antioxidant Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...
food additive Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives, such as vinegar ( pickling), salt ( salting), smoke ( smoking) and sugar ( crystallization), have been used f ...
s. These compounds are water-soluble, and thus cannot protect fats from oxidation: for this purpose, the fat- soluble
ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
s of ascorbic acid with long-chain
fatty acid In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...
s (ascorbyl palmitate and ascorbyl stearate) can be used as antioxidant food additives. Sodium-dependent active transport process enables absorption of ascorbic acid from the intestine. Ascorbate readily donates a hydrogen atom to free radicals, forming the radical anion semidehydroascorbate (also known as monodehydroascorbate), a resonance-stabilized semitrione: : 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: : \not\rightarrow 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 dehydroascorbate. In particular, semidehydroascorbate undergoes disproportionation to ascorbate and dehydroascorbate: : : Aqueous solutions of dehydroascorbate are unstable, undergoing hydrolysis with a half-life of 5–15 minutes at . Decomposition products include diketogulonic acid, xylonic acid,
threonic acid Threonic acid is a sugar acid derived from threose. The -isomer is a metabolite of ascorbic acid (vitamin C). One study suggested that because -threonate inhibits DKK1 expression ''in vitro'', it may have potential in treatment of androgenic alope ...
and oxalic acid.


Other reactions

It creates volatile compounds when mixed with
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
and
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s at 90 °C. It is a cofactor in tyrosine
oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
, though because a crude extract of animal liver is used, it is unclear which reaction catalyzed by which enzyme is being helped here. For known roles in enzymatic reactions, see . Because it reduces iron(III) and chelates iron ions, it enhances the oral absorption of non-heme iron. This property also applies to its enantiomer.


Conversion to oxalate

In 1958, it was discovered that ascorbic acid can be converted to oxalate, a key component of calcium oxalate kidney stones. 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 L-erythrulose and threosone. 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. 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.


Uses


Food additive

The main use of -ascorbic acid and its salts is as food additives, mostly to combat oxidation and prevent discoloration of the product during storage. It is approved for this purpose in the EU with
E number E numbers, short for Europe numbers, are codes for substances used as food additives, including those found naturally in many foods, such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...
E300,UK Food Standards Agency: the US,US Food and Drug Administration: Australia, and New Zealand.Australia New Zealand Food Standards Code The "" enantiomer ( erythorbic acid) shares all of the non-biological chemical properties with the more common enantiomer. As a result, it is an equally effective food antioxidant, and is also approved in processed foods.


Dietary supplement and biological relevance

Another major use of -ascorbic acid is as a
dietary supplement A dietary supplement is a manufactured product intended to supplement a person's diet by taking a pill (pharmacy), pill, capsule (pharmacy), capsule, tablet (pharmacy), tablet, powder, or liquid. A supplement can provide nutrients eithe ...
. It is on the World Health Organization's List of Essential Medicines. Its deficiency over a prolonged period of time could cause scurvy, which is characterized by fatigue, widespread weakness in connective tissues and capillary fragility. It affects multiple organ systems due to its role in the biochemical reactions of connective tissue synthesis. Ascorbic acid deficiency inhibits the body’s ability to synthesize collagen, which reslts in body deterioration such as producing tender joints, weakness, and ruptured blood vessels.


Niche, non-food uses

* Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a
preservative A preservative is a substance or a chemical that is added to products such as food products, beverages, pharmaceutical drugs, paints, biological samples, cosmetics, wood, and many other products to prevent decomposition by microbial growth or ...
. * In fluorescence microscopy and related fluorescence-based techniques, ascorbic acid can be used as an
antioxidant Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...
to increase fluorescent signal and chemically retard dye photobleaching. * It is also commonly used to remove dissolved metal stains, such as iron, from fiberglass swimming pool surfaces. * In plastic manufacturing, ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods. * Heroin users are known to use ascorbic acid as a means to convert heroin base to a water-soluble salt so that it can be injected. * As justified by its reaction with iodine, it is used to negate the effects of iodine tablets in water purification. It reacts with the sterilized water, removing the taste, color, and smell of the iodine. This is why it is often sold as a second set of tablets in most sporting goods stores as Potable Aqua-Neutralizing Tablets, along with the potassium iodide tablets. *
Intravenous Intravenous therapy (abbreviated as IV therapy) is a medical technique that administers fluids, medications and nutrients directly into a person's vein. The intravenous route of administration is commonly used for rehydration or to provide nutr ...
high-dose ascorbate is being used as a chemotherapeutic and biological response modifying agent. It is undergoing clinical trials. * It is sometimes used as a urinary acidifier to enhance the antiseptic effect of methenamine.


Synthesis

Natural biosynthesis of vitamin C occurs through various processes in many plants and animals.


Industrial preparation

Seventy percent of the world's supply of ascorbic acid is produced in China. Ascorbic acid is prepared in industry from
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
in a method based on the historical Reichstein process. In the first of a five-step process, glucose is catalytically hydrogenated to
sorbitol Sorbitol (), less commonly known as glucitol (), is a sugar alcohol with a sweet taste which the human body metabolizes slowly. It can be obtained by reduction of glucose, which changes the converted aldehyde group (−CHO) to a primary alco ...
, which is then oxidized by the
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
'' Acetobacter suboxydans'' to sorbose. Only one of the six hydroxy groups is oxidized by this enzymatic reaction. From this point, two routes are available. Treatment of the product with
acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...
in the presence of an acid catalyst converts four of the remaining hydroxyl groups to
acetal In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments n ...
s. The unprotected hydroxyl group is oxidized to the carboxylic acid by reaction with the catalytic oxidant
TEMPO In musical terminology, tempo (Italian for 'time'; plural 'tempos', or from the Italian plural), measured in beats per minute, is the speed or pace of a given musical composition, composition, and is often also an indication of the composition ...
(regenerated by sodium hypochlorite bleaching solution). Historically, industrial preparation via the Reichstein process used potassium permanganate as the bleaching solution. Acid-catalyzed hydrolysis of this product performs the dual function of removing the two acetal groups and ring-closing lactonization. This step yields ascorbic acid. Each of the five steps has a yield larger than 90%. A biotechnological process, first developed in China in the 1960s but further developed in the 1990s, bypasses acetone-protecting groups. A second genetically modified microbe species, such as mutant ''
Erwinia ''Erwinia'' is a genus of Enterobacterales bacteria containing mostly plant pathogenic species which was named for the famous plant pathologist, Erwin Frink Smith. It contains Gram-negative bacteria related to ''Escherichia coli'', '' Shigella ...
'', among others, oxidises sorbose into 2-ketogluconic acid (2-KGA), which can then undergo ring-closing lactonization via dehydration. This method is used in the predominant process used by the ascorbic acid industry in China, which supplies 70% of the world's ascorbic acid. Researchers are exploring means for one-step fermentation.


Determination

The traditional way to analyze the ascorbic acid content is by
titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of Quantitative research, quantitative Analytical chemistry, chemical analysis to determine the concentration of an identified analyte (a substance to be ...
with an oxidizing agent, and several procedures have been developed. The popular iodometry approach uses iodine in the presence of a starch indicator. Iodine is reduced by ascorbic acid, and when all the ascorbic acid has reacted, the iodine is 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 treated with iodine in excess, followed by back titration with sodium thiosulfate using starch as an indicator. This iodometric method has been revised to exploit the reaction of ascorbic acid with iodate and iodide in
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
solution. Electrolyzing the potassium iodide solution produces iodine, which reacts with ascorbic acid. The end of the process is determined by potentiometric titration like Karl Fischer titration. The amount of ascorbic acid can be calculated by Faraday's law. Another alternative uses ''N''-bromosuccinimide (NBS) as the oxidizing agent in the presence of potassium iodide and starch. The NBS first oxidizes the ascorbic acid; when the latter is exhausted, the NBS liberates the iodine from the potassium iodide, which then forms the blue-black complex with starch.


See also

* Colour retention agent * Erythorbic acid: a diastereomer of ascorbic acid. * Mineral ascorbates: salts of ascorbic acid * Acids in wine


References


Further reading

* * * * *


External links

* *
IPCS Poisons Information Monograph (PIM) 046
with details on the x-ray structure {{DEFAULTSORT:Ascorbic Acid Organic acids Antioxidants Dietary antioxidants Coenzymes Corrosion inhibitors Furanones Vitamers Vitamin C Biomolecules 3-Hydroxypropenals