Phenol

Phenol
IUPAC name Hydroxybenzene
Other names Carbolic Acid, Benzenol, Phenylic Acid, Hydroxybenzene, Phenic acid, Phenyl alcohol
Identifiers
CAS number	108-95-2 Y
ChemSpider	971
RTECS number	SJ3325000
SMILES Oc1ccccc1
InChI InChI=1/C6H6O/c7-6-4-2-1-3-5-6/h1-5,7H
Properties
Molecular formula	C6H6O
Molar mass	94.11 g mol−1
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 an organic compound with the chemical formula C₆H₅OH. It is a white, crystalline solid. This functional group consists of a phenyl, bonded to a hydroxyl (-OH). It is produced on a large scale (about 7 billion kg/year) as a precursor to many materials and useful compounds.^[1] It is a mildly acidic compound that requires careful handling.

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.

Properties

Phenol is appreciably soluble in water, with about 8.3 g dissolving in 100 ml (0.88 M). The sodium salt of phenol, sodium phenoxide, is far more water soluble. It is a reactive molecule.

Acidity

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⁻, called phenoxide anion.^[2] Compared to aliphatic alcohols, phenol shows much higher acidity (about 1 million times more acidic). It reacts completely with aqueous NaOH to lose H⁺, whereas most alcohols react only partially. Phenols are less acidic than carboxylic acids.

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 19.0 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]

Phenoxide anion

Phenol can be deprotonated with moderate base such as triethylamine, forming the nucleophilic phenoxide anion or phenolate anion, which is highly water-soluble.

Resonance structures of the phenoxide anion

The phenoxide anion has a similar nucleophilicity to free amines, with the further advantage that its conjugate acid (neutral phenol) does not become entirely deactivated as a nucleophile even in moderately acidic conditions. Phenols are sometimes used in peptide synthesis to "activate" carboxylic acids or esters to form activated esters. Phenolate esters are far more stable than acid anhydrides or acyl halides but are sufficiently reactive under mild conditions to facilitate the formation of amide bonds.

Phenoxides are enolates stabilised by aromaticity. Under normal circumstances, phenoxide is more reactive at the oxygen position, but the oxygen position is a "hard" nucleophile whereas the alpha-carbon positions tend to be "soft".^[7]

Phenol-cyclohexadienone tautomerism

Tautomerism

Phenol exhibits tautomerism with the unstable tautomer cyclohexadienone.

Reactions

Phenol is highly reactive toward electrophilic aromatic substitution as the oxygen atom's pi electrons donate electron density into the ring. By this general approach, many groups can be appended to the ring, via halogenation, acylation, sulfonation, and other processes.

Production

Phenol can be made from the partial oxidation of benzene, by the cumene process, or by the Raschig-Hooker process. It can also be found as a product of coal oxidation. The dominant method starts from cumene (isopropylbenzene):^[1]

C₆H₅CH(CH₃)₂ + O₂ → C₆H₅OH + (CH₃)₂CO

Uses

The major uses of phenol involve its conversion to plastics or related materials. Condensation with acetone gives bisphenol-A, a key building block for polycarbonates. Condensation with formaldehyde gives phenolic resins, the most famous of which is Bakelite. Hydrogenation of phenol gives cyclohexanone, an intermediate en route to nylon. Nonionic detergents are produced by alkylation of phenol to give the alkylphenols, which are then subjected to ethoxylation.^[1]

Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also many herbicides and pharmaceuticals.

Niche uses

Phenol is the preferred chemical for embalming bodies for 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,^[8] hair dyes, and skin lightening preparations.^[9]

In cosmetic surgery, phenol serves as an exfoliant. 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 from transmitting impulses in some intractable cases of chronic neuropathic pain.

History

Phenol was discovered in 1834, when it was extracted from coal tar, which remained the primary source until the development of the petrochemical industry.

Phenol's antiseptic properties were used by Sir Joseph Lister (1827–1912) in his pioneering technique of antiseptic surgery, although 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 as well as Carmex. 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.

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, inexpensive 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. 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.^[11] One of the best known inmates to be executed with a phenol 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.^[11]

Occurrence

Phenol is a measurable component in the aroma and taste of the distinctive Islay scotch whisky^[12]; generally ~30, but up to 100^[13] ppm.

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] There is no evidence to believe that phenol causes cancer in humans.^[18] Besides its hydrophobic effects, another mechanism for the toxicity of phenol may be the formation of phenoxyl radicals.^[19]

Chemical burns from skin exposures can be decontaminated by washing with polyethylene glycol,^[20] isopropyl alcohol,^[21] or perhaps even copious amounts of water.^[22] 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).

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

References