Ascorbic Acid

Ascorbic acid, commonly known as vitamin C and represented by the formula C₆H₈O₆, is a natural, water-soluble vitamin. Ascorbic acid is predominantly found in citrus fruits and vegetables. Since humans cannot produce or store it, it must be obtained through the diet.

Ascorbic acid, previously called hexuronic acid, is a powerful antioxidant and a mild reducing agent. Its pure form appears as a white solid, but impure samples may look slightly yellow. It dissolves easily in water, creating mildly acidic solutions.

Ascorbic acid exists in two forms called enantiomers, which are mirror images of each other: Levo ("l") and Dextro ("d"). The most common form is the Levo isomer ("l"), the natural vitamin C essential for humans and many animals. A deficiency in vitamin C causes scurvy, a condition historically common among sailors on long voyages.

Because of its antioxidant properties, ascorbic acid is widely used as a dietary supplement and food additive. The "d" form, on the other hand, can only be made synthetically.

1.0Structure 

Ascorbic acid's composition includes the following functional groups:

• -OH group (Hydroxyl group): Contributes to its solubility in water and its antioxidant properties.
• -C=O group (Carbonyl group): Found in the structure, playing a role in its reactivity.
• -O- group (Ether group): Part of its molecular framework, influencing its chemical behaviour.

2.0Physical Properties of Ascorbic Acid

• Chemical Formula: C₆H₈O₆
• Molecular Weight/Molar Mass: 176.12 g/mol
• Density: 1.694 g/cm³
• Boiling Point: 553°C
• Melting Point: 190°C
• Solubility: Soluble in water

3.0Chemical Properties of Ascorbic Acid

• Formation of Ascorbate Anion:
• When one of the hydroxyl groups of ascorbic acid is deprotonated, it forms the ascorbate anion, which behaves as a vinylogous carboxylic acid.
• Unique Reductone Structure:
• The ascorbate anion contains a reductone structure, specifically the group -C(OH)=C(OH)-C(=O)-, where the carbonyl group is adjacent to an enediol group. This structure contributes to its strong reducing properties.
• Electron Delocalization and Resonance Stability:
• The ascorbate anion is stabilised by electron delocalisation through resonance, making it more acidic than isolated hydroxyl groups.
• Salt Formation:
• The ascorbate anion readily forms salts, such as sodium ascorbate and calcium ascorbate, commonly used in supplements and pharmaceuticals.

Reaction with Organic Acids:

• • Ascorbic acid can react with organic acids or alcohols to form esters, such as ascorbyl palmitate and ascorbyl stearate, fat-soluble derivatives in food and cosmetics.

4.0Production of Ascorbic Acid (Vitamin C)

In industrial settings, glucose is converted into ascorbic acid using a process based on the classic Reichstein technique. This five-step method is summarised as follows:

• Hydrogenation
• Glucose is catalytically hydrogenated to produce sorbitol.
• Oxidation
• Acetobacter suboxydans oxidises sorbitol to sorbose, targeting only one of the six hydroxyl groups in an enzymatic reaction.
• Acetal Formation
• The sorbose is treated with acetone and an acid catalyst, converting four hydroxyl groups into acetals and protecting them from further reactions.
• Oxidation to Carboxylic Acid
• The exposed hydroxyl group is oxidised to a carboxylic acid using the catalytic oxidant TEMPO (2,2,6,6-tetramethylpiperidin-1-yl oxidanyl), which is regenerated using sodium hypochlorite (commonly known as bleach).
• Lactonization and Hydrolysis
• Acid-catalyzed hydrolysis removes the acetal groups, and the molecule undergoes lactonisation (ring closure) to produce ascorbic acid.

The overall process yields more than 90% for each stage. Historically, potassium permanganate was used as the oxidising agent in the Reichstein method, but it has been replaced by TEMPO and sodium hypochlorite for greater efficiency and environmental safety. 

5.0Uses of Ascorbic Acid (Vitamin C)

• Vitamin C is essential for the hydroxylation of proline and lysine in collagen fibres, supporting the health of connective tissues, bones, teeth, and fibrous tissues.
• It enhances iron absorption by maintaining iron in its ferrous (Fe²⁺) state, making it easier for the body to utilise.
• Vitamin C deficiency leads to scurvy, bruising, swollen gums, and poor dentine formation.
• It boosts the immune system and aids in combating viral and bacterial infections, such as pneumonia and the common cold.
• Vitamin C is a powerful antioxidant, helping neutralise harmful free radicals and cleanse the body.
• Due to its healing and antimicrobial properties, it treats skin conditions like acne, pimples, and gum infections.
• Vitamin C helps treat gastric and peptic ulcers caused by Helicobacter pylori bacteria, which infects the gastrointestinal tract.
• Regular Vitamin C intake, physical activity, and lower cholesterol levels can help prevent gallbladder diseases, including gallstones.