GALLIC ACID: STRUCTURE, PROPERTIES, OBTAINING, USES - CHEMISTRY - 2024

The gallic acid is an organic compound of molecular formula C ₆ H ₂ (OH) ₃ COOH belonging to the category of polyphenols. It is recognized as a crystalline powder with a white color close to pale yellow.

It is a trihydroxybenzoic acid that is formed by a benzene ring to which an acidic carboxylic group (-COOH) and 3 hydroxyl groups (-OH) located in positions 3, 4 and 5 of the ring are attached.

Molecular structure of gallic acid (3,4,5-trihydroxybenzoic acid). Vchorozopoulos. Source: Wikipedia Commons

In nature it is widely disseminated, as it is a product that is formed in significant quantities within plants and fungi. It exists free or attached to the tannins of most plant species, where walnuts, grapes, divi-divi plants, oak bark, pomegranate or its roots, sumac plants and tea stand out.

Oak bark. Rob Mitchell. Source: Wikipedia Commons

It is also found in honey, cocoa, various berries, mango, and other fruits and vegetables, and in some beverages such as wine and tea infusions.

In plant tissues it is in the form of an ester or gallate. The amount in which it is found depends on external stimuli, such as the amount of UV radiation that the plant has received, chemical stress and microbial infections.

In the case of grapes and wine, it depends on the variety of grape, processing and storage. In green tea the content of gallates is high, but cocoa contains more than green tea and red wine.

Chemically it behaves as a reducing agent. It is astringent and antioxidant. It has also been used in blue writing inks and is commonly used in the pharmaceutical industry.

It has wide potential in medical applications, since the multiple properties of gallic acid and its derivatives make it a promising therapeutic agent in preventive medicine.

Structure

Gallic acid crystallizes from absolute methanol or from chloroform in the form of white needles. It crystallizes in water in the form of silky needles from its monohydrate.

Nomenclature

- Gallic acid.

- 3,4,5-trihydroxybenzoic acid.

Properties

Physical state

Solid, crystalline needles.

Molecular weight

170.12 g / mol.

Melting point

It decomposes at 235-240 ºC, generating pyrogallol and CO ₂

Density

1.694 g / cm ³

Solubility

In water: moderately soluble.

- 1 g in 87 ml of water

- 1 g in 3 ml of boiling water

In ethanol: 1 g in 6 ml of alcohol.

In diethyl ether: 1 g in 100 ml of ether.

In glycerol: 1 g in 10 ml of glycerol.

In acetone: 1 g in 5 ml of acetone.

Practically insoluble in benzene, chloroform, and petroleum ether.

Dissociation constant

K ₁ 4.63 x 10 ^-3 (at 30 ° C).

K ₂ 1.41 x 10 ^-9

Chemical properties

Gallic acid solutions, particularly alkali metal salts, absorb oxygen and turn brown when exposed to air.

Gallic acid is a strong reducing agent that can reduce gold or silver salts to the metal. It is incompatible with chlorates, permanganate, ammonia, lead acetate, alkali hydroxides, alkali carbonates, silver salts and oxidizing agents in general.

With iron (II) salts, gallic acid forms a deep blue complex.

In gallic acid, the hydroxyl group (-OH) at the 4 position is the most chemically reactive.

Other properties

It must be protected from light because it photochemically degrades it.

It is a mild local irritant. Inhalation of dust can affect the nose and throat and contact with eyes and skin causes irritation.

Toxicity studies in mice indicate that ingested up to a level of 5000 mg / kg, gallic acid is not toxic to these animals. It is considered to be a low toxicity and confirms the safety of its use.

Obtaining

Gallic acid is obtained by alkaline or acid hydrolysis of the tannins of nuts or plant materials rich in these compounds.

Hydrolysis can also be carried out enzymatically using mold culture broths such as Penicillium glaucum or Aspergillus niger, which contain tannase, an enzyme that breaks or cleaves the tannin molecule.

Another way to obtain gallic acid is from p-hydroxybenzoic acid, by sulfonation and alkaline fusion, with which the addition of the other two -OH groups in the molecule is achieved.

Applications

- In inks and colorants

Gallic acid is used in the manufacture of anthragalol and pyrogallol, which are intermediaries in the production of colorants, such as gallocyanin and galoflavin. In turn, it is a raw material in the synthesis of oxazine derivatives, which are also used as colorants.

Because it forms a blue complex with iron, gallic acid is very important in the production of writing inks. These inks mainly contain a mixture of gallic acid, ferrous sulfate (FeSO ₄) and gum arabic.

Iron gallic acid inks have been indispensable materials for writing documents, drawing plans, and preparing written materials.

Blue ink pen. Butterflylunch. Source: Wikipedia Commons

- In medical applications

It is used as an intestinal astringent and a bleeding stop agent (styptic). Gallic acid is the raw material in obtaining rufigalol, which is an antimalarial agent.

Due to its chemical reducing capacity, gallic acid is used in the manufacture of pharmaceutical products.

- In potential medical applications

Against cancer

Gallic acid has been identified as the main responsible for the anticancer properties of various plant extracts.

Contrary to its particular antioxidant action, it has been found that it can present pro-oxidant characteristics in the induction of apoptosis of cancer cells. Apoptosis is the orderly destruction of damaged cells caused by the same organism.

Apoptosis of a cell. Ltumanovskaya V. Nagibin. Source: Wikipedia Commons

There is evidence that gallic acid and gallates induce selective apoptosis in rapidly growing tumor cells, leaving healthy cells intact. In addition, it has been reported that it slows angiogenesis, and consequently, cancer invasion and metastasis.

Gallic acid anticancer activity has been found in leukemia, prostate, lung, stomach, pancreas and colon cancer, breast, cervical and esophageal cancer.

Against various pathologies

In several studies it has been shown that it has antifungal, antibacterial, antiviral, antiallergic, anti-inflammatory, antimutagenic, anti-cholesterol, antiobesity and immunomodulatory activity.

Gallic acid is a good candidate for controlling periodontal disease (gum disease).

It also exhibits neuroprotective, cardioprotective, hepatoprotective, and nephroprotective potential. For example, several studies of heart tissues in rats have confirmed that gallic acid exerts a protective effect on the myocardium against oxidative stress.

As a cellular anti-aging agent

The gallic acid provides effective protection against oxidative damage caused by reactive species often found in biological systems, such as hydroxyl radicals (OH .), Superoxide (O ₂ .) And peroxyl (ROO .).

It has been found to be absorbed faster by the digestive tract than most polyphenols. And it is one of those with the highest antioxidant capacity.

Additionally, some researchers claim that gallic acid can be transported by niosomes to increase its anti-aging activity. The niosome is a molecular system for the controlled release of drugs in the body that requires it.

These characteristics give it a high potential against cell aging.

- In veterinary uses

It has been used as an intestinal astringent in animals.

- In various applications

Gallic acid is used in the manufacture of its esters, such as methyl gallate, propyl gallate, octyl gallate, and lauryl gallate.

These derivatives are widely used as antioxidants and preservatives in processed foods, in food packaging materials, to prevent rancidity and oxidative deterioration. The derivatives mentioned are also used in cosmetics.

Gallic acid is used as a photographic developer and in the manufacture of paper. In addition, it is used extensively in the stabilization of collagen in the leather tanning process.

As an analytical reagent, gallic acid is ideal as a standard for determining the phenolic content of plant extracts, and the results are expressed as Gallic Acid Equivalents.

It is also used in tests for the determination of free mineral acids, dihydroxyacetone and alkaloids.

- Usefulness in natural aquatic environments

Gallic acid, naturally present in aquifers in plant matter, is one of those responsible for the nutritional availability of Fe (II) necessary for the growth of aquatic species.

This is because it is capable of maintaining high levels of dissolved iron (II) concentration under aerobic conditions. This is because it forms a complex with Fe (II) resistant to oxidation.

Aquatic species: Trout. Image by John French. Source: Pixabay

References

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