Ascorbic acid

This article will describe the chemistry of Ascorbic Acid. For the function and effect of Ascorbic Acid on plant and animal species, its use by humans and the higher primates, please see Vitamin C.

• Ascorbic acid is an organic acid with antioxidant properties. Its appearance is white crystals or powder (from light yellow to brown if oxidized). It is water soluble. The L-enantiomer of ascorbic acid is commonly known as vitamin C, CASNo = 50-81-7.
• The name is derived from the alpha privative a- (meaning no) and scorbuticus (scurvy), the disease caused by a deficiency of vitamin C. In 1937 the Nobel Prize for chemistry was awarded to Walter Haworth for his work in determining the structure of ascorbic acid (shared with Paul Karrer, who received his award for work on vitamins), and the prize for Physiology or Medicine that year went to Albert Szent-Györgyi for his studies of the biological functions of L-ascorbic acid. At the time of its discovery in the 1920s it was called hexuronic acid by some researchers.

Chemistry

Acidity

Ascorbic acid behaves as a vinylogous carboxylic acid, where the double bond ("vinyl") transmits electron pairs between the hydroxyl and the carbonyl. There are two resonance structures for the deprotonated form, differing in the position of the double bond.

Another way to look at ascorbic acid is to consider it as an enol. The deprotonated form is an enolate, which are usually strongly basic. However, the adjacent double bond stabilizes the deprotonated form.

Tautomerism

Ascorbic acid also rapidly interconverts into two unstable diketone tautomers by proton transfer, although it is the most stable in the enol form. The proton of the enol is lost, and reacquired by electrons from the double bond, to produce a diketone. This is an enol reaction. There are two possible forms, 1,2-diketone and 1,3-diketone.

Determination of concentrations

The concentration of a solution of ascorbic acid can be determined in many ways, the most common ways involving titration with an oxidizing agent.

DCPIP

A commonly used titration method involve oxidising agent is the dye 2,6-dichlorophenol-indophenol, or DCPIP for short. The blue dye is run into the ascorbic acid solution until a faint pink colour persists for 15 seconds.

Iodine

Another method involves using iodine and a starch indicator where iodine reacts with ascorbic acid and when all the ascorbic acid has reacted the iodine is then in excess, forming a blue-black complex with the starch indicator. This indicates the end point of the titration. Alternatively, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as an indicator.

Iodate and iodine

The above method involving iodine requires making up and standardising the iodine solution. One way round this is to generate the iodine in the presence of the ascorbic acid by the reaction of iodate and iodide ion in acid solution.

N-Bromosuccinimide

A much less common oxidising agent is N-bromosuccinimide, (NBS). In this titration the NBS oxidises the ascorbic acid (in the presence of potassium iodide and starch). When the NBS is in excess (i.e. the reaction is complete) the NBS liberates the iodine from the potassium iodide which then forms the blue/black complex with starch, indicating the end point of the titration.

Uses

Top: ascorbic acid
(reduced form of Vitamin C)
Bottom: dehydroascorbic acid
(oxidized form of Vitamin C)

Industry

Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a preservative.

In plastic manufacturing ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.

Food industry

The L-enantiomer of ascorbic acid is also known as vitamin C. The name "ascorbic" comes from its property of preventing and curing scurvy. Primates, including humans, and a few other species in all divisions of the animal kingdom, notably the guinea pig, have lost the ability to synthesize ascorbic acid and must obtain it in their food.

Ascorbic acid and its sodium, potassium, and calcium salts are commonly used as antioxidant food additives. These compounds are water soluble and thus cannot protect fats from oxidation: For this purpose, the fat-soluble esters of ascorbic acid with long-chain fatty acids (ascorbyl palmitate or ascorbyl stearate) can be used as food antioxidants. Eighty percent of the world's supply of ascorbic acid is produced in China.

The relevant European food additive E numbers are:

1. E300 ascorbic acid,
2. E301 sodium ascorbate,
3. E302 calcium ascorbate,
4. E303 potassium ascorbate,
5. E304 fatty acid esters of ascorbic acid (i) ascorbyl palmitate (ii) ascorbyl stearate.

It can be added to water that has been treated with iodine to make it potable, neutralizing the unpleasant iodine taste.

Pharmacology

In pharmacology ascorbic acid used as vitamin (pure substance and mixtures) or in complex preparations, as Ferroplex (the mixture of ascorbic acid and Fe²⁺).

Analytical chemistry

Exposure to oxygen, metals, light and heat destroy ascorbic acid, so it must be stored in dark and cold and not in a metal container.

The oxidized form of ascorbic acid is known as dehydroascorbic acid.

Antioxidant mechanism

Ascorbate acts as an antioxidant by being itself available for energetically favourable oxidation. Many oxidants (typically, reactive oxygen species) such as the hydroxyl radical (formed from hydrogen peroxide), contain an unpaired electron and thus are highly reactive and damaging to humans and plants at the molecular level. This is due to their interaction with nucleic acid, proteins and lipids. Reactive oxygen species oxidize (take electrons from) ascorbate first to monodehydroascorbate and then dehydroascorbate. The reactive oxygen species are reduced to water while the oxidized forms of ascorbate are relatively stable and unreactive, and do not cause cellular damage.

Ascorbic acid synthesis in non-primates

Ascorbic acid is found in plants, animals, and single cell organisms. All living animals either make it, eat it, or die from scurvy due to lack of it. Reptiles and older orders of birds make ascorbic acid in their kidneys. Recent orders of birds and most mammals make ascorbic acid in their livers where the enzyme L-gulonolactone oxidase is required to convert glucose to ascorbic acid. Humans, guinea pigs, and some other primates are not able to make L-gulonolactone oxidase because of a genetic defect and are therefore unable to make ascorbic acid in their livers. This genetic mutation occurred about 63 million years ago This would have had lethal consequences for the mutated primate were it not for the fact that it occurred to an arboreal animal living in a tropical environment where plenty of foodstuffs containing ascorbic acid were available throughout the year. Although ascorbic acid is a vital food nutrient for humans and is therefore termed a vitamin, it is a natural liver metabolite in most other animals.

Methods of manufacture (chemical synthesis)

Vitamin C is produced from glucose by two main routes. The Reichstein process developed in the 1930s uses a single pre-fermentation followed by a purely chemical route. The more modern two-step fermentation process was originally developed in China in the 1960s, uses additional fermentation to replace part of the later chemical stages. Both processes yield approximately 60% vitamin C from the glucose feed.

Research is underway at the Scottish Crop Research Institute to create yeast micro organisms to synthesise ascorbic acid in a single fermentation step, a technology which is expected to reduce manufacturing costs considerably.

World production of synthesised vitamin C is currently estimated at approximately 110,000 tonnes annually. Main producers today are BASF/Takeda, DSM, Merck and the China Pharmaceutical Group Ltd. of the People's Republic of China. China is slowly becoming the major world supplier as its prices undercut those of the US and European manufacturers.