Ascorbic acid is an natural compound with method C6H8O6, initially known as hexuronic acid. It’s a white stable, however impure samples can seem yellowish. It dissolves effectively in water to provide mildly acidic options. It’s a delicate lowering agent.
Ascorbic acid exists as two enantiomers (mirror-image isomers), generally denoted “l” (for “levo”) and “d” (for “dextro”). The l isomer is the one most frequently encountered: it happens naturally in lots of meals, and is one kind (“vitamer”) of vitamin C, a necessary nutrient for people and lots of animals. Deficiency of vitamin C causes scurvy, previously a serious illness of sailors in lengthy sea voyages. It’s used as a meals additive and a dietary complement for its antioxidant properties. The “d” kind could be made through chemical synthesis however has no important organic position.
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Historical past[edit]
The antiscorbutic properties of sure meals had been demonstrated within the 18th century by James Lind. In 1907, Axel Holst and Theodor Frølich found that the antiscorbutic issue was a water-soluble chemical substance, distinct from the one which prevented beriberi. Between 1928 and 1932, Albert Szent-Györgyi remoted a candidate for this substance, which he known as it “hexuronic acid”, first from vegetation and later from animal adrenal glands. In 1932 Charles Glen King confirmed that it was certainly the antiscorbutic issue.
In 1933, sugar chemist Walter Norman Haworth, working with samples of “hexuronic acid” that Szent-Györgyi had remoted from paprika and despatched him within the earlier yr, deduced the proper construction and optical-isomeric nature of the compound, and in 1934 reported its first synthesis.[2][3] In reference to the compound’s antiscorbutic properties, Haworth and Szent-Györgyi proposed to rename it “a-scorbic acid” for the compound, and later particularly l-ascorbic acid.[4] Due to their work, in 1937 the Nobel Prizes for chemistry and medication had been awarded to Haworth and Szent-Györgyi, respectively.
Chemical properties[edit]
Acidity[edit]
Ascorbic acid is a vinylogous acid and kinds the ascorbate anion when deprotonated on one of many hydroxyls. This property is attribute of reductones: enediols with a carbonyl group adjoining to the enediol group, specifically with the group –C(OH)=C(OH)–C(=O)–. The ascorbate anion is stabilized by electron delocalization that outcomes from resonance between two kinds:
Because of this, ascorbic acid is way more acidic than can be anticipated if the compound contained solely remoted hydroxyl teams.
Salts[edit]
The ascorbate anion kinds salts, corresponding to sodium ascorbate, calcium ascorbate, and potassium ascorbate.
Esters[edit]
Ascorbic acid can even react with natural acids as an alcohol forming esters corresponding to ascorbyl palmitate and ascorbyl stearate.
Nucleophilic assault[edit]
Nucleophilic assault of ascorbic acid on a proton ends in a 1,3-diketone:
Oxidation[edit]
The ascorbate ion is the predominant species at typical organic pH values. It’s a delicate lowering agent and antioxidant. It’s oxidized with lack of one electron to kind a radical cation after which with lack of a second electron to kind dehydroascorbic acid. It sometimes reacts with oxidants of the reactive oxygen species, such because the hydroxyl radical.
Ascorbic acid is particular as a result of it will probably switch a single electron, owing to the resonance-stabilized nature of its personal radical ion, known as semidehydroascorbate. The web response is:
On publicity to oxygen, ascorbic acid will endure additional oxidative decomposition to varied merchandise together with diketogulonic acid, xylonic acid, threonic acid and oxalic acid.[6]
Reactive oxygen species are damaging to animals and vegetation on the molecular degree resulting from their doable interplay with nucleic acids, proteins, and lipids. Typically these radicals provoke chain reactions. Ascorbate can terminate these chain radical reactions by electron switch. The oxidized types of ascorbate are comparatively unreactive and don’t trigger mobile injury.
Nonetheless, being a very good electron donor, extra ascorbate within the presence of free steel ions can’t solely promote but in addition provoke free radical reactions, thus making it a doubtlessly harmful pro-oxidative compound in sure metabolic contexts.
Ascorbic acid and its sodium, potassium, and calcium salts are generally used as antioxidant meals components. These compounds are water-soluble and, thus, can not shield fat from oxidation: For this function, the fat-soluble esters of ascorbic acid with long-chain fatty acids (ascorbyl palmitate or ascorbyl stearate) can be utilized as meals antioxidants.
Different reactions[edit]
It creates unstable compounds when blended with glucose and amino acids in 90 °C.[7]
It’s a cofactor in tyrosine oxidation.[8]
Makes use of[edit]
Meals additive[edit]
The principle use of l-ascorbic acid and its salts is as meals components, principally to fight oxidation. It’s authorised for this function within the EU with E quantity E300,[9] USA,[10] Australia, and New Zealand)[11]
Dietary complement[edit]
One other main use of l-ascorbic acid is as dietary complement.
Area of interest, non-food makes use of[edit]
Synthesis[edit] – “vitamin c chemical formula”
Pure biosynthesis of vitamin C happens in lots of vegetation, and animals, by quite a lot of processes.
Industrial preparation[edit]
Eighty p.c of the world’s provide of ascorbic acid is produced in China.[17]
Ascorbic acid is ready in trade from glucose in a way based mostly on the historic Reichstein course of. Within the first of a five-step course of, glucose is catalytically hydrogenated to sorbitol, which is then oxidized by the microorganism Acetobacter suboxydans to sorbose. Solely one of many six hydroxy teams is oxidized by this enzymatic response. From this level, two routes can be found. Therapy of the product with acetone within the presence of an acid catalyst converts 4 of the remaining hydroxyl teams to acetals. The unprotected hydroxyl group is oxidized to the carboxylic acid by response with the catalytic oxidant TEMPO (regenerated by sodium hypochlorite — bleaching answer). Traditionally, industrial preparation through the Reichstein course of used potassium permanganate because the bleaching answer. Acid-catalyzed hydrolysis of this product performs the twin perform of eradicating the 2 acetal teams and ring-closing lactonization. This step yields ascorbic acid. Every of the 5 steps has a yield bigger than 90%.[18]
A extra biotechnological course of, first developed in China within the Sixties, however additional developed within the Nineteen Nineties, bypasses the usage of acetone-protecting teams. A second genetically modified microbe species, corresponding to mutant Erwinia, amongst others, oxidises sorbose into 2-ketogluconic acid (2-KGA), which may then endure ring-closing lactonization through dehydration. This technique is used within the predominant course of utilized by the ascorbic acid trade in China, which provides 80% of world’s ascorbic acid.[19] American and Chinese language researchers are competing to engineer a mutant that may perform a one-pot fermentation immediately from glucose to 2-KGA, bypassing each the necessity for a second fermentation and the necessity to cut back glucose to sorbitol.[20]
There exists a d-ascorbic acid, which doesn’t happen in nature however could be synthesized artificially. To be particular, l-ascorbate is thought to take part in lots of particular enzyme reactions that require the proper enantiomer (l-ascorbate and never d-ascorbate).[21] l-Ascorbic acid has a selected rotation of [α]20D = +23°.[22]
Dedication[edit]
The normal solution to analyze the ascorbic acid content material is the method of titration with an oxidizing agent, and several other procedures have been developed.
The favored iodometry strategy makes use of iodine within the presence of a starch indicator. Iodine is decreased by ascorbic acid, and, when all of the ascorbic acid has reacted, the iodine is then in extra, forming a blue-black complicated with the starch indicator. This means the end-point of the titration.
In its place, ascorbic acid could be handled with iodine in extra, adopted by again titration with sodium thiosulfate utilizing starch as an indicator.[23]
This iodometric technique has been revised to take advantage of response of ascorbic acid with iodate and iodide in acid answer. Electrolyzing the answer of potassium iodide produces iodine, which reacts with ascorbic acid. The top of course of is set by potentiometric titration in a fashion much like Karl Fischer titration. The quantity of ascorbic acid could be calculated by Faraday’s regulation.
One other various makes use of N-bromosuccinimide (NBS) because the oxidizing agent, within 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 kinds the blue-black complicated with starch.
See additionally[edit]
Notes and references[edit]