A phenol is one of a number of chemically active compounds which are found throughout nature, especially in plants. Their molecules each include a hydroxyl functional group (OH) bonded to the ring of an aromatic compound — a molecule that includes at least one ring of carbon atoms. Phenols exhibit a wide range of properties; some are heralded for their health benefits, while others are deadly poisons. Some have important industrial uses as drugs or food additives. The word phenol may also refer to carbolic acid (C6H5OH), the simplest of this group of chemicals.
The phenol category is chemically similar to the alcohols, but phenols form tighter hydrogen bonds with other chemical compounds. They are also set apart from alcohols by their higher acidity, solubility, and boiling points. Most are colorless, though some are brightly colored and play an important role in plant pigmentation. They are usually solid or liquid at room temperature.
There are hundreds of different kinds of phenols, and these contribute to the variety of plant life found on earth. Some, such as anthocyanins and flavonoids, provide coloration. Others such, as eugenol and ketol, provide aromas. Phenols also exhibit a wide variety of effects on the biological pathways of humans and other animals. These include everything from capsaicin, which makes hot peppers hot, to the cannabinoids, the active ingredient of marijuana. The anesthetic propofol, the antiseptic xylenol, and salicylic acid, a common anti-acne medication — each is a phenol.
Many that are important to human health are polyphenols, chemicals made of several phenol molecules chained together. This group includes the tannins, lignins, and flavonoids. Some polyphenols, such as tyrosol and oleuropein, are thought to have antioxidant properties. Others may reduce the likelihood of heart disease or cancer. At least one polyphenol, resveratrol, is believed to have potent anti-aging effects.
Polyphenols can be found in olive oil, fruit skins, leaves, berries, tea, coffee, chocolate, nuts, and a number of other plant sources. Many can be found in wine, especially red wine, where they contribute much to the taste and color. Some have been extracted and made into dietary supplements. Since so many health benefits have been associated with these compounds, some people are quick to attribute other, less well-established benefits to them. Not every claim has solid science behind it; let the buyer beware.
Some phenols are actually detrimental to health. Many plants secrete unpleasant or poisonous phenolic compounds to deter herbivores. One, urushiol, causes the rash associated with poison ivy and poison oak. Tannins give acorns their bitter taste, and are poisonous in high doses. Carbolic acid causes chemical burns, and may be carcinogenic. In short, since the category encompasses such a broad variety of chemicals, it also encompasses a broad variety of effects on human health.