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Phenols

Phenols are organic compounds which contain a hydroxyl (—OH) group attached to a carbon atom in a benzene ring.  Their chemical behavior is very distinct from that of alcohols, because they are not capable of undergoing the same oxidation reactions that alcohols participate it.  Also, unlike alcohols, phenols are weak acids, since the phenoxide anion generated by the loss of the hydroxyl proton is resonance-stabilized:

Phenols are named after the parent compound, phenol (hydroxybenzene).  When other substituents are present, the carbon bearing the OH group is carbon number 1, and the substituents around the ring are numbered accordingly.

Phenol

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Phenol, or hydroxybenzene, is the parent compound of the phenols, consisting of an OH group directly connected to a benzene ring.  Phenol is the active ingredient in some treatments for sore throats, and is found in several lozenges and throat sprays.  It is used in the manufacture of many other compounds, such as aspirin, and Bakelite.

Phenol was also the first compound to be used in antiseptic surgery.  Joseph Lister (1827-1912), an English surgeon at the Glasgow Royal Infirmary, used solutions of phenol dissolved in water, known as carbolic acid, to kill germs in wounds and to sterilize medical equipment, drastically cutting down the number of deaths caused by infections.  In the words of Isaac Asimov,

"Eventually chemical less irritating to tissue and even more effective in killing germs were discovered, but Lister and his carbolic acid had founded antiseptic surgery.  Overriding initial resistance to his findings by medical conservatives, he succeeded in converting hospitals into something more than elaborate pauses on the way to the grave." (1)

[Listerine, originally formulated by Joseph Lawrence and Jordan Lambert in 1879, as a surgical antiseptic, was named after Lister.]

 
   

2-Phenylphenol

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2-Phenylphenol, or ortho-phenylphenol, is an ingredient in Lysol, and is an agricultural fungicide.

 
   

2-Benzyl-4-chlorophenol

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2-Benzyl-4-chlorophenol is an ingredient in Lysol.

 
   

ortho-Cresol

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Cresol is a trivial name for the three isomeric methylphenols.  The structure shown above is ortho-cresol, or 2-methylphenol; the other isomers are shown below:

The cresols are commonly used as solvents, in disinfectants and deodorizers, and in the manufacture of other compounds (for example, the BHA and BHT shown below).

 
   

2,6-Xylenol

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Xylenol is a trivial name given to the six isomeric dimethylphenols.  The name is derived from xylene, which is the trivial name for the three isomeric dimethylbenzenes.  The structure shown above is 2,6-xylenol, or 2,6-dimethylphenol; the other isomers are shown below.

The xylenols are found in a number of pesticides, and are also used in the manufacture of many other compounds.

 
   

4-Chloro-3,5-dimethylphenol (Chloroxylenol)

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4-Chloro-3,5-dimethylphenol, or chloroxylenol, is a nonirritating topical antiseptic used in a number of antibacterial soaps.

 
   

BHA (butylated hydroxy anisole)

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Butylated hydroxy anisole (BHA), or 2-tert-butyl-4-methoxyphenol, is a commonly used antioxidant.  (Anisole itself is a benzene ring with a methoxy, OCH3, group.)  Together with the 3-tert-butyl isomer (not shown), this compound is commonly added to foods, gasoline, oils, rubber, and other products to react with and thereby neutralize the free radicals that would otherwise damage the products.

 
   

BHT (butylated hydroxy toluene)

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Butylated hydroxy toluene (BHT), or 2,6-di-tert-butyl-4-methylphenol, is another common antioxidant, used to prevent the spoiling of some foods, as well as to preserve cosmetics, drugs, gasoline, oils, rubber, and other products.

 
   

2,4,6-Trinitrophenol (Picric acid)

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2,4,6-Trinitrotoluene, or picric acid, is a yellow, crystalline substance that is explosive when it is dry.  It is fairly stable when wet, but the dry form is extremely sensitive to shock or friction.  It was extensively used in artillery shells in the late 1800's and early 1900's, but was eventually replaced by 2,4,6-trinitrotoluene (TNT, in which the OH in the structure above is replaced with a methyl group), which was more stable and less prone to forming dangerous metal salts when stored in a shell casing.

Picric acid was a component in the Halifax Explosion, which occurred when a French cargo ship, the Mont-Blanc, carrying nitrocellulose, picric acid, and TNT, collided with a Norwegian ship, the Imo, which was carrying relief supplies to Belgium, in the harbor of Halifax, Nova Scotia, Canada on December 6, 1917.  The Mont-Blanc exploded shortly after the collision, leveling all of the structure within two square kilometers of the harbor, and causing a tidal wave in the harbor; approximately 2,000 people were killed and over 9,000 injured.  The explosion was the largest man-made chemical explosion ever recorded.

 
   
   

Catechol

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Catechol, or 1,2-benzenediol, or 2-hydroxyphenol, contains two OH groups ortho to each other on a benzene ring.  The catechol structure is found in the urushiols, the skin irritants in poison ivy (see below), and is also found in many important neurotransmitters (see page on neurotransmitters for examples).

 
   

Urushiols

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The urushiols consists of a cathechol ring to which is attached a long hydrocarbon chain, consisting of 15 to 17 carbons, which may be saturated, unsaturated, or polyunsaturated.  These viscous oils cause skin rashes (urushiol-induced contact dermatitis), and are found in the plants of the genus Toxicodendron, such as poison ivy (Toxicodendron radicans), poison oak (Toxicodendron diversilobum, Toxicodendron pubescens), and poison sumac (Toxicodendron vernix); they are also found in the shell of the cashew (Anacardium occidentale) and the skin of the fruit of the mango tree (Mangifera indica).

The 3D structure shown above is that of the first R group listed in the 2D structure.
   
   

Resorcinol

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Resorcinol, or 1,3-benzenediol, or 3-hydroxyphenol, contains two OH groups meta to each other on a benzene ring.  It if found in many resins; it is used as an antiseptic and disinfectant, in the treatment of psoriasis, eczema, and other skin conditions, and is found in some shampoos as a treatment for dandruff.

 
   

4-Hexylresorcinol

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4-Hexylresorcinol is used in some mouthwashes, throat lozenges, and some skin medications.

 
   

Hydroquinone

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Hydroquinone, or 1,4-benzenediol, or 4-hydroxyphenol, contains two OH groups para to each other on a benzene ring.  Hydroquinone is easily oxidized by mild oxidizing agents to produce para-benzoquinone (which is often just called quinone), which itself is easily reduced by mild reducing agents back to the hydroquinone form:

This reversible redox reaction involving hydroquinone and quinone is exploited by nature in enzyme-catalyzed electron transport reactions in the form of the ubiquonones (see entry below).

Hydroquinone is also used in photograph developing solutions (in non-digital photography, of course), where it helps to reduce silver halides to metallic silver.

Hydroquinone is also part of the chemical defensive system employed by the bombardier beetle (family Carabidae).  Hydroquinone is mixed with hydrogen peroxide from a separate reservoir in the beetle's body, which oxidizes the hydroquinone into quinone, generating enough heat to boil the solution, producing a hot, corrosive, defensive spray.

para-benzoquinone


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Ubiquinones

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The structure shown above is the ubiquinone form with n=6

The ubiquinones, also known as the coenzymes Q, are quinone derivatives (see above) which have isoprene-derived hydrocarbon chains.  These moleculess are found in the mitrochondria of all living cells (hence the name:  ubiquitous quinones), as well as in other organelles such as the endoplasmic reticulum, peroxisomes, lysosomes, and vesicles; in humans the isoprene chain is 10 units long, and is known as Coenzyme Q10.  The ubiquinones carry electrons through the electron transport chain in mitochondria, and assist the mitochondria to produce energy.  The ubiquinones also act as antioxidants, protecting organelle and cell membranes from free radicals.

 
   

1-Napthol

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1-Napthol consists of two fused benzene rings (naphthalene) with a OH group attached to position 1.  There is one other structural isomer, not shown, in which the OH is attached to position 2.

 
   

Calixarenes

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4-tert-Butylcalix[4]arene


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The calixarenes are macrocycles (large molecular rings) formed by the reaction of phenols with aldehydes.  These molecules form three-dimensional shapes that resemble a bowl or a cup, from which the name "calixarene" is derived (Latin, calix, for "cup" or "chalice" + arene for the aromatic ring portion of the molecule).  The four oxygen atoms pointing toward the center of the "cup" can form complexes with Lewis acids, and calixarenes of various sizes can be synthesized that are capable of forming coordination complexes with a wide variety of metal ions, such as sodium (where it is used in probes to measure sodium levels in blood), lead, cadmium, mercury, cesium, potassium, calcium, lithium, and silver.

 
   

Anthocyanidins

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The anthocyanidins are members of a class of compounds called the flavonoids (see page on Ketones); they are pigments found in a wide range of plants, and are usually either red, blue, or purple, depending on the pH of their environment.  Because their colors change with pH, they are often used in acid-base indicators.  The positively charged oxygen atom, known as a benzopyrilium cation, is usually associated with a chloride anion.  Most anthocyanidins are combined with sugar molecules to make glycosides called anthocyanins (Greek, "blue flower").

The anthocyanidins are antioxidants, protecting plants cells from damage caused by high-energy sunlight.  Their bright colors help the plants to attract insect pollinators, as well as animals that eat the fruit of the plants and thereby distribute its seeds.

The fall colors of many trees are partly caused by the production of anthocyanidins as chlorophyll in the leaves breaks down; the carotenes in the leaves also contribute to the fall colors.

Some examples of the anthocyanidins are listed below:

The 3D structure shown above is that of pelargonidin:

 

  R1 R2 R3 R4 R5 R6 R7 Occurrence
Apigeninidin H OH H H OH H OH an anti-microbial antioxidant fount in the leaf sheath of genus Sorghum
Aurantinidin H OH H OH OH OH OH found in the flowers of the genus Impatiens (family Balsaminaceae)
Capensinidin OCH3 OH OCH3 OH OCH3 H OH  
Cyanidin OH OH H OH OH H OH found in a wide variety of berries, such as bilberries, blackberries, blueberries, cherries, cranberries, elderberries, hawthorn, loganberries, plums,  raspberries, and strawberries; also found in apple skins, red cabbage, and rhubarb
Delphinidin OH OH OH OH OH H OH causes the blue color of violets (genus Viola) and delphiniums (genus Delphinium); also found in cranberries, Concord grapes, and the Cabernet Sauvignon grape
Europinidin OCH3 OH OH OH OCH3 H OH  
Hirsutidin OCH3 OH OCH3 OH OH H OCH3  
Luteolinidin OH OH H H OH H OH found in the fern Blechnum novae-zelandiae (family Blechnaceae)
Malvidin OCH3 OH OCH3 OH OH H OH

found in the European grapevine (Vitis vinifera), and responsible for the color of red wine; also responsible for the blue color of some flowers of the genus Primula (family Primulaceae)

Pelargonidin H OH H OH OH H OH responsible for the red color of the geranium (genus Geranium) and pelargoniums (genus Pelargonium); also found in raspberries and strawberries
Peonidin OCH3 OH H OH OH H OH found in the peony (genus Paeonia), roses, morning glories (family Convolvulaceae); also found in cherries, grapes, and cranberries
Petunidin OH OH OCH3 OH OH H OH found in the flowers of the genus Petunia (family Solanaceae)
Pulchellidin OH OH OH OH OCH3 H OH  
Rosinidin OCH3 OH H OH OH H OCH3  
Tricetinidin OH OH OH H OH H OH  

 

 

References

(1) Isaac Asimov, Isaac Asimov's Biographical Encyclopedia of Science and Technology.  New York:  Avon Books, 1976, p. 388.

P. W. Atkins, Molecules, 2nd ed.  Cambridge: Cambridge University Press, 2003, p. 174-176.

Jin-Ming Kong, Lian-Sai Chia, Ngoh-Khang Goh, Tet-Fatt Chia, and R. Brouillard, "Analysis and biological activities of anthocyanins" (Phytochemistry, 2003, 64, 923)

Richard J. Lewis, Sr., Hawley's Condensed Chemical Dictionary, 13th ed.  New York:  Van Nostrand Reinhold, 1997.

D. W. A. Sharp, The Penguin Dictionary of Chemistry, 2nd ed.  London:  Penguin Books, 1990.

Graham Solomons and Craig Fryhle, Organic Chemistry, 7th ed.  New York:  John Wiley & Sons, Inc., 2000, p. 1014-1017, 1030-1031.

Jeffrey B. Harborne and Christine A. Williams, "Anthocyanins and other flavonoids."  (Natural Products Reports, 1998, 15, 631-652).

Jeffrey B. Harborne and Christine A. Williams, "Anthocyanins and other flavonoids."  (Natural Products Reports, 2001, 18, 310-333).

Christine A. Williams and Renée J. Grayer, "Anthocyanins and other flavonoids."  (Natural Products Reports, 2004, 21, 539-573).

Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.