Chemistry of ascorbic acid

Ascorbic acid is an organic compound with the molecular formula C6 hours 8O.Formerly known as hexuronic acid.It is a white solid, but impure samples can be pale yellow.It dissolves well in water, forming a slightly acidic solution. It is a mild reducing agent.Ascorbic acid exists in two enantiomeric (mirror image isomers) forms, usually denoted "L" ("left") and "D" ("right").The L isomer is the most commonly encountered one:it occurs naturally in many foods and is a form ("vitamer") of vitamin C, an essential nutrient for humans and many animals.A lack of vitamin C leads to scurvy, formerly a major disease of seafaring sailors.It is used as a food additive and dietary supplement due to its antioxidant properties. Form "D" can be produced by chemical synthesis, but has no important biological role.

History 

The antiscorbutic properties of certain foods were demonstrated by James Lind in the 18th century.In 1907, Axel Holst and Theodor Frølich discovered that antiscorbate factor was a water-soluble chemical different from that used to prevent beriberi.Between 1928 and 1932, Albert Szent-Györgyi isolated candidates for this substance, which he called "hexuronic acid", first from plants and later from the adrenal glands of animals.In 1932 Charles Glen King confirmed that it was indeed the antiscorbutic factor.In 1933, sugar chemistWalter Norman Haworth deduced that the compound was correct structure and optical isomeric properties, and reported its first synthesis in 1934.Regarding the ascorbic properties of the compound, Haworth and Szent-Györgyi proposed renaming the compound "a-ascorbic acid" and later specifically L-ascorbic acid.For their work, the 1937 Nobel Prizes in Chemistry and Medicine were awarded to Haworthand Szent-Györgyi respectively.

Chemical properties

Acidity

Ascorbic acid is a furan-based 2-ketogluconolactone.It contains adjacent enediols adjacent to the carbonyl group.This -C(OH)=C(OH)-C(=O)- structural pattern is characteristic of reductones and adds the acidity of an enol hydroxyl group.The deprotonated conjugate base is the ascorbate anion,which is stabilized by electron delocalization generated by resonance between the two forms:For this reason, ascorbic acid is much more acidic than expected if the compound contains only isolated hydroxyl groups.

Salt 

The ascorbate anion forms salts such as sodium ascorbate, calcium ascorbate and potassium ascorbate.

Esters

Ascorbic acid can also react as an alcohol with organic acids to form esters,such as ascorbyl palmitate and ascorbyl stearate.

Nucleophile attack

The nucleophilic attack of ascorbic acid on the proton produces a 1,3-diketone:

Oxidation

Ascorbate ion is the major species at typical biological pH.It is a mild reducing agent and antioxidant.It is oxidized, losing one electron to form a radical cation, and then losing a second electron to form dehydroascorbic acid.It usually reacts with active oxygen oxidizing agents such as hydroxyl radicals.Ascorbic acid is special because it can transfer a single electron, thanks to the resonance-stabilizing properties of its own radical ion, known as half-dehydroascorbate.net response is:

RO^• + C₆H₇O⁻₆ → RO⁻ + C₆H₇O^•₆ → ROH + C₆H₆O_6.

Upon exposure to oxygen, ascorbic acid undergoes further oxidative breakdown to various products, including diketogulonic acid, xylonic acid, threonic acid, and oxalic acid.Since reactive oxygen species may interact with nucleic acids, proteins, and lipids, they cause damage to plants and animals at the molecular level.Sometimes these free radicals start a chain reaction. Ascorbic acid can terminate these chain free radical reactions by electron transfer.The oxidized form of ascorbic acid is relatively inactive and does not cause cellular damage.However, as a good electron donor, excess ascorbic acid 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 settings.Ascorbic acid and its sodium, potassium and calcium salts are commonly used as antioxidant food additives.These compounds are water-soluble and therefore do not protect fats from oxidation: for this reason, fat-soluble esters of ascorbic acid with long-chain fatty acids (ascorbyl palmitate or ascorbyl stearate) are used as food antioxidants.