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Gallic Acid Chemistry

Posted by on July 12, 2010 · 3 Comments 

Gallic acid is a plant chemical found in most plants but especially abundant in tea, grapes and oak. It is in a group of chemicals called phenolics. Phenolics are chemicals based on the structure of phenol which is a molecule with a ring structure containing 6 carbons with a –OH (hydroxyl group) attached to one of the carbons. A molecule with more than one phenol ring is called a polyphenol. These phenols and polyphenols are known as powerful antioxidants and found in many plants.


Interestingly, this 6 member ring structure also referred to as an aromatic nucleus is only synthesized by plants and microorganisms, not animals or humans. Generally speaking phenolics are acidic meaning their pH is low (from 1-7).

Gallic acid has 3 –OH groups and one –COOH (carboxylic acid) group attached to the ring. Note that in these abbreviated structures shown that each of the 6 corners represents a carbon atom, so there are 6 carbon atoms in phenol.

Gallic acid can be found either alone or as part of plant tannins. Tannins are a family of high molecular weight, water soluble plant molecules. Another definition of tannin is a natural product containing phenolic structures that can precipitate proteins. They have an astringent taste (think tea) and have the ability to tan leather. When gallic acid becomes linked to a sugar such as glucose it can form a polygalloyl ester which is a simple type of tannin.

Gallic acid itself is an antifungal agent, antiviral, astringent and antioxidant. Some studies have indicated that it has anticancer activity and that it can relax blood vessels. Gallic acid isjust one of many polyphenols found in tea and grape seed that provide health benefits. Gallic acid is soluble in water and has a boiling point of 251 degrees C. This is a low enough temperature that it can be found in the distilled products of herbs.
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More about the author:  Cindy Jones is a formulator and microbiologist. After receiving a Ph.D. in biochemistry from the University of Cincinnati she worked in cancer research, later as a health/medical writer and now in cosmetic science. Read more from this author

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  • Elaine Koronich

    Hi Cindy,

    I believe you have listed the melting point and not the boiling point of gallic acid at 251°C. Also I would think that 251°C is a pretty high temperature for distillation of a herb. What is the herbal material and conditions for this distillation?

    Thanks,
    Elaine Koronich
    Exquisite Soaps

  • Cindy

    Elaine, thanks for catching that. Yes, I listed the boiling point. The temperature at which it vaporizes would be higher, although some vaporization would occur at lower temperatures. This is not referring to any specific distillation as gallic acid is a component of many herbs including witch hazel. The purpose of the article is more to understand various way of extracting chemicals of interest in herbs. Temperatures in a still will vary at different parts of the still and various chemicals of interest can be isolated by controlling the temperatures carefully; a process called fractional distillation.