carbolic acid

Phenol
IUPAC name	Hydroxybenzene
Other names	Carbolic Acid Benzenol Phenylic Acid Hydroxybenzene Phenic acid Phenyl alcohol
Identifiers
CAS number	108-95-2 Y
RTECS number	SJ3325000
SMILES	Oc1ccccc1
InChI	1/C6H6O/c7-6-4-2-1-3-5-6/h1-5,7H
ChemSpider ID	971
Properties
Molecular formula	C6H5OH
Molar mass	94.11 g/mol
Appearance	White Crystalline Solid
Density	1.07 g/cm³
Melting point	40.5 °C, 314 K, 105 °F
Boiling point	181.7 °C, 455 K, 359 °F
Solubility in water	8.3 g/100 ml (20 °C)
Acidity (pKa)	9.95
Dipole moment	1.7 D
Hazards
EU classification	Toxic (T) Muta. Cat. 3 Corrosive (C)
NFPA 704	2 3 0 COR
Flash point	79 °C
Related compounds
Related compounds	Benzenethiol
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Phenol, also known as carbolic acid, is a toxic, white crystalline solid. Its chemical formula is C₆H₅OH and its structure is that of a hydroxyl group (-OH) bonded to a phenyl ring, making it an aromatic compound.

Phenols

The word phenol is also used to refer to any compound that contains a six-membered aromatic ring, bonded directly to a hydroxyl group (-OH). In effect, phenols are a class of organic compounds of which the phenol discussed in this article is the simplest member. Its chemical name is hydroxybenzene.

Properties

Phenol has a limited solubility in water (8.3 g/100 ml). It is slightly acidic: the phenol molecule has weak tendencies to lose the H⁺ ion from the hydroxyl group, resulting in the highly water-soluble phenolate anion C₆H₅O⁻, also called phenoxide anion^[1][2]. Compared to aliphatic alcohols, phenol shows much higher acidity; it even reacts with aqueous NaOH to lose H⁺, whereas aliphatic alcohols do not. However, many carboxylic acids are more acidic than phenol. One explanation for the increased acidity over alcohols is resonance stabilization of the phenoxide anion by the aromatic ring. In this way, the negative charge on oxygen is shared by the ortho and para carbon atoms.^[3] In another explanation, increased acidity is the result of orbital overlap between the oxygen's lone pairs and the aromatic system.^[4] In a third, the dominant effect is the induction from the sp² hybridised carbons; the comparatively more powerful inductive withdrawal of electron density that is provided by the sp² system compared to an sp³ system allows for great stabilization of the oxyanion. In making this conclusion, one can examine the pK_a of the enol of acetone, which is 10.9 in comparison to phenol with a pK_a of 10.0.^[5] However, this similarity of acidities of phenol and acetone enol is not observed in the gas phase, and is due to the fact that the difference of solvation energies of the deprotonated acetone enol and phenoxide almost exactly offsets the experimentally observed gas phase acidity difference. It has been recently shown that only about 1/3 of the increased acidity of phenol is due to inductive effects, with resonance accounting for the rest. ^[6]

Hydrothermal chemistry

Under laboratory conditions mimicking hydrothermal circulation (water, 200 °C, 1.9 GPa), phenol is found to form from sodium hydrogen carbonate and iron powder (1.8% chemical yield).^[7] This discovery made in 2007 may be relevant to the origin of life question as phenol is a fragment of the biomolecule tyrosine, though tyrosine is known to result directly from hydroxylation of phenylalanine in modern biological systems.^[8]

It is a measurable component (up to 30 ppm) in the aroma of Islay scotch whisky.^[9]

Production

Phenol can be made from the partial oxidation of benzene, the reduction of benzoic acid, by the cumene process, or by the Raschig-Hooker process. It can also be found as a product of coal oxidation.

Uses

Phenol has antiseptic properties, and was used by Sir Joseph Lister (1827–1912) in his pioneering technique of antiseptic surgery, though the skin irritation caused by continual exposure to phenol eventually led to the substitution of aseptic (germ-free) techniques in surgery. Lister decided that the wounds themselves had to be thoroughly cleaned. He then covered the wounds with a piece of "rag" or "lint"^[10] covered in carbolic acid. It is also the active ingredient in some oral analgesics such as Chloraseptic spray. Phenol was also the main ingredient of the Carbolic Smoke Ball, a device marketed in London in the 19th century as protecting the user against influenza and other ailments.

It is also used in the production of drugs (it is the starting material in the industrial production of aspirin), herbicides, and synthetic resins (Bakelite, one of the first synthetic resins to be manufactured, is a polymer of phenol with formaldehyde).

Phenol is the preferred chemical in use of embalming bodies for anatomical use and study because of its ability to preserve tissues for extended periods of time. However, formaldehyde is usually preferred over phenol for embalming with intent of public viewing because of phenol's tendency to turn tissues an unpleasant bleach-white color.

Phenol is also used in the preparation of cosmetics including sunscreens,^[11] hair dyes, and skin lightening preparations.^[12] Compounds containing phenol moieties can be used to prevent ultraviolet light-induced damage to hair and skin due to the UV-absorbing properties of the aromatic ring of the phenol.

It is also used in cosmetic surgery as an exfoliant, to remove layers of dead skin. It is also used in phenolization, a surgical procedure used to treat an ingrown nail, in which it is applied to the nail bed to prevent regrowth of nails. 5% Phenol is sometimes injected near a sensory nerve in order to temporarily (up to a year) stop it transmitting impulses in some intractable cases of chronic neuropathic pain.

Second World War

Injections of phenol have occasionally been used as a means of rapid execution. In particular, phenol was used as a means of extermination by the Nazis before and during the Second World War. Originally used by the Nazis in the 1930s as part of its euthanasia program, phenol, cheap and easy to make and quickly effective, became the injectable toxin of choice through the last days of the War. Although Zyklon-B pellets, invented by Gerhard Lenz were used in the gas chambers to exterminate large groups of people, the Nazis learned that extermination of smaller groups was more economical via injection of each victim one at a time with phenol instead. Phenol injections were given to thousands of people in concentration camps, especially at Auschwitz-Birkenau. Approximately one gram is enough to cause death. Injections were administered by medical doctors, their assistants, or sometimes prisoner doctors; such injections were originally given intravenously, more commonly in the arm, but injection directly into the heart, so as to induce nearly instant death, was later preferred.^[13] One of the best known inmates to be executed with carbolic acid injection in Auschwitz was St. Maximilian Kolbe, a Catholic priest who volunteered to undergo three weeks of starvation and dehydration in the place of another inmate. ^[13]

Toxicity

Phenol and its vapor are corrosive to the eyes, the skin and the respiratory tract.^[14] Repeated or prolonged skin contact with phenol may cause dermatitis, or even second and third-degree burns due to phenol's caustic and defatting properties.^[15] Inhalation of phenol vapor may cause lung edema.^[14] The substance may cause harmful effects on the central nervous system and heart, resulting in dysrhythmia, seizures, and coma.^[16] The kidneys may be affected as well. Exposure may result in death and the effects may be delayed. Long-term or repeated exposure of the substance may have harmful effects on the liver and kidneys."^[17] The substance is a suspected carcinogen. Besides its hydrophobic effects, another mechanism for the toxicity of phenol may be the formation of phenoxyl radicals.^[18]

Chemical burns from skin exposures can be decontaminated by washing with polyethylene glycol,^[19] isopropyl alcohol,^[20] or perhaps even copious amounts of water.^[21] Removal of contaminated clothing is required, as well as immediate hospital treatment for large splashes. This is particularly important if the phenol is mixed with chloroform (a commonly-used mixture in molecular biology for DNA & RNA purification from proteins).

References

See also

• Bamberger rearrangement
• Claisen rearrangement
• Cresols
• Fries rearrangement
• Phenolic compounds in wine
• Polyphenol antioxidant
• Resveratrol

External links

• International Chemical Safety Card 0070
• Phenol Material Safety Data Sheet
• National Pollutant Inventory: Phenol Fact Sheet
• NIOSH Pocket Guide to Chemical Hazards
• IARC Monograph: "Phenol"
• Arcane Radio Trivia outlines competing uses for Phenol circa 1915