Phenol

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This article is about the molecule. For the class of chemicals containing a phenol group, see Phenols.
Phenol
IUPAC name

Phenol

Other names

Carbolic acid, benzenol, phenylic acid, hydroxybenzene, phenic acid

Identifiers
CAS number 108-95-2 YesY
PubChem 996
ChemSpider 971 YesY
UNII 339NCG44TV YesY
DrugBank DB03255
KEGG D06536 YesY
ChEBI CHEBI:15882 YesY
ChEMBL CHEMBL14060 YesY
RTECS number SJ3325000
ATC code C05BB05,D08AE03, N01BX03, R02AA19
Jmol-3D images Image 1
Properties
Molecular formula C6H6O
Molar mass 94.11 g mol−1
Appearance Transparent crystalline solid
Odor Sweet and tarry
Density 1.07 g/cm3
Melting point 40.5 °C; 104.9 °F; 313.6 K
Boiling point 181.7 °C; 359.1 °F; 454.8 K
Solubility in water 8.3 g/100 mL (20 °C)
Acidity (pKa) 9.95 (in water),

29.1 (in acetonitrile)[1]

λmax 270.75 nm[2]
Dipole moment 1.7 D
Hazards
GHS pictograms [3]
GHS hazard statements H301, H311, H314, H331, H341, H373[3]
GHS precautionary statements P261, P280, P301+310, P305+351+338, P310[3]
EU classification Toxic (T)
Muta. Cat. 3
Corrosive (C)
R-phrases R23/R24/R25-R34- R48/R20/R21/R22-R68
S-phrases (S1/2)-S24/S25-S26-S28- S36/S37/S39-S45
NFPA 704
2
3
0
COR
Flash point 79 °C; 174 °F; 352 K
Related compounds
Related compounds Thiophenol
Sodium phenoxide
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Phenol — also known as carbolic acid — is an aromatic organic compound with the molecular formula C6H5OH. It is a white crystalline solid that is volatile. The molecule consists of a phenyl group (-C6H5) bonded to a hydroxyl group (-OH). It is mildly acidic, but requires careful handling due to its propensity to cause burns.

Phenol was first extracted from coal tar, but today is produced on a large scale (about 7 billion kg/year) from petroleum. It is an important industrial commodity as a precursor to many materials and useful compounds.[4] Its major uses involve its conversion to plastics or related materials. Phenol and its chemical derivatives are key for building polycarbonates, epoxies, Bakelite, nylon, detergents, herbicides such as phenoxy herbicides, and numerous pharmaceutical drugs.

Although similar to alcohols, phenols have unique distinguishing properties. Unlike in alcohols where the hydroxyl group is bound to a saturated carbon atom,[5] in phenols the hydroxyl group is attached to an unsaturated ring such as benzene or other arene ring.[6] Consequently, phenols have greater acidity than alcohols due to stabilization of the conjugate base through resonance in the aromatic ring.

Properties

Phenol is appreciably soluble in water, with about 8.42 g dissolving in 100 mL (0.88 M). Homogeneous mixtures of phenol and water at phenol to water mass ratios of ~2.6 and higher are also possible. The sodium salt of phenol, sodium phenoxide, is far more water soluble.

Acidity

Phenol is weakly acidic and at high pHs gives the phenolate anion C6H5O− (also called phenoxide):[7]

PhOH ⇌ PhO− + H+          (K = 10−10)

Compared to aliphatic alcohols, phenol is about 1 million times more acidic, although it is still considered a weak acid. It reacts completely with aqueous NaOH to lose H+, whereas most alcohols react only partially. Phenols are less acidic than carboxylic acids, and even carbonic acid.

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 delocalized on to the ortho and para carbon atoms.[8] In another explanation, increased acidity is the result of orbital overlap between the oxygen's lone pairs and the aromatic system.[9] 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.

The pKa of the enol of acetone is 10.9, comparable to that for phenol.[10] The acidities of phenol and acetone enol diverge in the gas phase owing to the effects of solvation. About 1/3 of the increased acidity of phenol is attributable to inductive effects, with resonance accounting for the remaining difference.[11]

Phenoxide anion

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 more stable toward hydrolysis than acid anhydrides and acyl halides but are sufficiently reactive under mild conditions to facilitate the formation of amide bonds.

Tautomerism

Phenol-cyclohexadienone tautomerism

Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone, but only a tiny fraction of phenol exists as the keto form. The equilibrium constant for enolisation is approximately 10−13, meaning that only one in every ten trillion molecules is in the keto form at any moment.[12] The small amount of stabilisation gained by exchanging a C=C bond for a C=O bond is more than offset by the large destabilisation resulting from the loss of aromaticity. Phenol therefore exists entirely in the enol form.[13]

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".[14]

Reactions

Neutral phenol substructure "shape". An image of a computed electrostatic surface of neutral phenol, showing neutral regions in green, electronegative areas in orange-red, and the electropositive phenolic proton in blue.

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. However, phenol's ring is so strongly activated — second only to aniline - that bromination or chlorination of phenol leads to substitution on all carbons ortho and para to the hydroxy group, not only on one carbon.

Aqueous solution of phenol is weakly acidic and turns blue litmus slightly to red. Phenol is easily neutralized by sodium hydroxide forming sodium phenate or phenolate, but it being weaker than carbonic acid cannot be neutralized by sodium bicarbonate or sodium carbonate to liberate carbon dioxide

C6H5OH + NaOH → C6H5ONa + H2O

When a mixture of phenol and benzoyl chloride when shaken in presence of dilute sodium hydroxide solution, phenyl benzoate is formed. This is an example of Schotten-Baumann reaction:

C6H5OH + C6H5COCl → C6H5OCOC6H5 + HCl

Phenol is reduced to benzene when it is distilled with zinc dust or its vapour is passed over granules of zinc at 400 °C:[15]

C6H5OH + Zn → C6H6 + ZnO

When phenol is reacted with diazomethane in the presence of boron trifluoride (BF3), anisole is obtained as the main product and nitrogen gas is released:

C6H5OH + CH2N2 → C6H5OCH3 + N2

Production

Because of phenol's commercial importance, many methods have been developed for its production. The dominant current route, accounting for 95% of production (2003), involves the partial oxidation of cumene (isopropylbenzene) via the Hock rearrangement:[4]

C6H5CH(CH3)2 + O2 → C6H5OH + (CH3)2CO

Compared to most other processes, the cumene-hydroperoxide process uses relatively mild synthesis conditions, and relatively inexpensive raw materials. However, to operate economically, there must be demand for both phenol, and the acetone by-product.

An early commercial route, developed by Bayer and Monsanto in the early 1900s, begins with the reaction of a strong base with benzenesulfonate:[16]

C6H5SO3H + 2 NaOH → C6H5OH + Na2SO3 + H2O

Other methods under consideration involve:

C6H5Cl + H2O → C6H5OH + HCl
C6H6 + N2O → C6H5OH + N2
  • oxidation of toluene, as developed by Dow Chemical:
C6H5CH3 + 2 O2 → C6H5OH + CO2 + H2O

In the Lummus Process, the oxidation of toluene to benzoic acid is conducted separately.

Phenol is also a recoverable byproduct of coal pyrolysis.[17]

Uses

The major uses of phenol, consuming two thirds of its production, involve its conversion to precursors to plastics. Condensation with acetone gives bisphenol-A, a key precursor to polycarbonates and epoxide resins. Condensation of phenol, alkylphenols, or diphenols with formaldehyde gives phenolic resins, a famous example of which is Bakelite. Partial hydrogenation of phenol gives cyclohexanone, a precursor to nylon. Nonionic detergents are produced by alkylation of phenol to give the alkylphenols, e.g., nonylphenol, which are then subjected to ethoxylation.[4]

Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also many herbicides and pharmaceutical drugs. Phenol is also used as an oral anesthetic/analgesic in products such as Chloraseptic or other brand name and generic equivalents, commonly used to temporarily treat pharyngitis.

Niche uses

Phenol is so inexpensive that it attracts many small-scale uses. It once was widely used as an antiseptic, especially as carbolic soap, from the early 1900s to the 1970s. It is a component of industrial paint strippers used in the aviation industry for the removal of epoxy, polyurethane and other chemically resistant coatings.[18]

Phenol derivatives are also used in the preparation of cosmetics including sunscreens,[19] hair colorings, and skin lightening preparations.[20]

Concentrated phenol liquids are commonly used in the surgical treatment of ingrown toenails to prevent a section of the toenail from growing back. This process is called phenolization.

History

Phenol was discovered in 1834 by Friedlieb Ferdinand Runge who extracted it (in impure form) from coal tar.[21] Runge called phenol "Karbolsäure" (coal-oil-acid, carbolic acid). Coal tar remained the primary source until the development of the petrochemical industry. In 1841, the French chemist Auguste Laurent obtained phenol in pure form.[22]

In 1836, Auguste Laurent coined the name "phène" for benzene;[23] this is the root of the word "phenol" and "phenyl". In 1843, French chemist Charles Gerhardt coined the name "phénol".[24]

The antiseptic properties of phenol were used by Sir Joseph Lister (1827–1912) in his pioneering technique of antiseptic surgery. Lister decided that the wounds themselves had to be thoroughly cleaned. He then covered the wounds with a piece of rag or lint[25] covered in phenol, or carbolic acid as he called it. The skin irritation caused by continual exposure to phenol eventually led to the substitution of aseptic (germ-free) techniques in surgery.

Phenol is the active ingredient in some oral analgesics such as Chloraseptic spray and Carmex.

Phenol was the main ingredient of the Carbolic Smoke Ball, an ineffective device marketed in London in the 19th century as protecting against influenza and other ailments, and the subject of the famous law case Carlill v Carbolic Smoke Ball Company.

Second World War

Injections of phenol were used as a means of individual execution by the Nazis during the Second World War.[26] It was originally used by the Nazis in 1939 as part of Action T4.[27] Although Zyklon-B pellets 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 with phenol. Phenol injections were given to thousands of people, especially at Auschwitz-Birkenau. Approximately one gram is sufficient to cause death.[28][29] 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 two weeks of starvation and dehydration in the place of another inmate.[29]

Natural occurrences

Temporal glands secretion examination showed the presence of phenol and 4-methylphenol during musth in male elephants.[30][31]

It is also one of the chemical compounds found in castoreum. This compound is gathered from the beaver plant food.[32]

Occurrence in whisky

Phenol is a measurable component in the aroma and taste of the distinctive Islay scotch whisky,[33] generally ~30, but can be over 150[34] ppm in the malted barley used to produce whisky.

Biodegradation

Cryptanaerobacter phenolicus is a bacterium species that produces benzoate from phenol via 4-hydroxybenzoate.[35] Rhodococcus phenolicus is a bacterium species able to degrade phenol as sole carbon sources.[36]

Toxicity

Phenol and its vapors are corrosive to the eyes, the skin, and the respiratory tract.[37] Repeated or prolonged skin contact with phenol may cause dermatitis, or even second and third-degree burns.[38] Inhalation of phenol vapor may cause lung edema.[37] The substance may cause harmful effects on the central nervous system and heart, resulting in dysrhythmia, seizures, and coma.[39] The kidneys may be affected as well. Long-term or repeated exposure of the substance may have harmful effects on the liver and kidneys.[40] There is no evidence that phenol causes cancer in humans.[41] Besides its hydrophobic effects, another mechanism for the toxicity of phenol may be the formation of phenoxyl radicals.[42]

Chemical burns from skin exposures can be decontaminated by washing with polyethylene glycol,[43] isopropyl alcohol,[44] or perhaps even copious amounts of water.[45] 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 and RNA purification).

Phenols

Main article: 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). Thus, phenols are a class of organic compounds of which the phenol discussed in this article is the simplest member.

See also

References

  1. ↑ Kütt, Agnes; Movchun, Valeria; Rodima, Toomas; Dansauer, Timo; Rusanov, Eduard B.; Leito, Ivo; Kaljurand, Ivari; Koppel, Juta; Pihl, Viljar; Koppel, Ivar; Ovsjannikov, Gea; Toom, Lauri; Mishima, Masaaki; Medebielle, Maurice; Lork, Enno; Röschenthaler, Gerd-Volker; Koppel, Ilmar A.; Kolomeitsev, Alexander A. (2008). "Pentakis(trifluoromethyl)phenyl, a Sterically Crowded and Electron-withdrawing Group:  Synthesis and Acidity of Pentakis(trifluoromethyl)benzene, -toluene, -phenol, and -aniline". The Journal of Organic Chemistry 73 (7): 2607–20. doi:10.1021/jo702513w. PMID 18324831. 
  2. ↑ http://omlc.ogi.edu/spectra/PhotochemCAD/html/phenol.html
  3. ↑ 3.0 3.1 3.2 Sigma-Aldrich Co., Phenol. Retrieved on 2013-07-20.
  4. ↑ 4.0 4.1 4.2 Weber, Manfred; Weber, Markus; Kleine-Boymann, Michael (2004). "Phenol". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a19_299.pub2. ISBN 3527306730. 
  5. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Alcohols".
  6. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Phenols".
  7. ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7 
  8. ↑ Organic Chemistry 2nd Ed. John McMurry ISBN 0-534-07968-7
  9. ↑ "The Acidity of Phenol". ChemGuide. Jim Clark. Retrieved 2007-08-05. 
  10. ↑ http://isites.harvard.edu/fs/docs/icb.topic93502.files/Lectures_and_Handouts/20-Acidity_Concepts.pdf
  11. ↑ Pedro J. Silva (2009). "Inductive and Resonance Effects on the Acidities of Phenol, Enols, and Carbonyl α-Hydrogens". J. Org. Chem. 74 (2): 914–916. doi:10.1021/jo8018736. PMID 19053615. (Solvation effects on the relative acidities of acetaldehyde enol and phenol described in the Supporting Information)
  12. ↑ Capponi, Marco; Gut, Ivo G.; Hellrung, Bruno; Persy, Gaby; Wirz, Jakob (1999). "Ketonization equilibria of phenol in aqueous solution". Can. J. Chem. 77 (5–6): 605–613. doi:10.1139/cjc-77-5-6-605. 
  13. ↑ Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. p. 531. ISBN 978-0-19-850346-0. 
  14. ↑ David Y. Curtin and Allan R. Stein (1966). "2,6,6-Trimethyl-2,4-Cyclohexadione". Organic Syntheses 46: 115. 
  15. ↑ Roscoe, Henry (1891). A treatise on chemistry, Volume 3, Part 3. London: Macmillan & Co. p. 23. 
  16. ↑ Wittcoff, H.A., Reuben, B.G. Industrial Organic Chemicals in Perspective. Part One: Raw Materials and Manufacture. Wiley-Interscience, New York. 1980.
  17. ↑ 17.0 17.1 Franck, H.-G., Stadelhofer, J.W. Industrial Aromatic Chemistry. Springer-Verlag, New York. 1988. pp. 148-155.
  18. ↑ "CH207 Aircraft paintstripper, phenolic, acid". Callington. 14 October 2009. Retrieved 27 August 2011. 
  19. ↑ A. Svobodová*, J. Psotová, and D. Walterová (2003). "Natural Phenolics in the Prevention of UV-Induced Skin Damage. A Review". Biomed. Papers 147 (2): 137–145. doi:10.5507/bp.2003.019. 
  20. ↑ DeSelms, R. H.; UV-Active Phenol Ester Compounds; Enigen Science Publishing: Washington, DC, 2008.
  21. ↑ F. F. Runge (1834) "Ueber einige Produkte der Steinkohlendestillation" (On some products of coal distillation), Annalen der Physik und Chemie, 31 : 65-78. On page 69 of volume 31, Runge names phenol "Karbolsäure" (coal-oil-acid, carbolic acid). Runge characterizes phenol in: F. F. Runge (1834) "Ueber einige Produkte der Steinkohlendestillation," Annalen der Physik und Chemie, 31 : 308-328.
  22. ↑ Auguste Laurent (1841) "Mémoire sur le phényle et ses dérivés" (Memoir on benzene and its derivatives), Annales de Chimie et de Physique, series 3, 3 : 195-228. On page 198, Laurent names phenol "hydrate de phényle" and "l'acide phénique".
  23. ↑ Auguste Laurent (1836) "Sur la chlorophénise et les acides chlorophénisique et chlorophénèsique," Annales de Chemie et de Physique, vol. 63, pp. 27–45, see p. 44: Je donne le nom de phène au radical fondamental des acides précédens (φαινω, j'éclaire), puisque la benzine se trouve dans le gaz de l'éclairage. (I give the name of "phène" (φαινω, I illuminate) to the fundamental radical of the preceding acid, because benzene is found in illuminating gas.)
  24. ↑ Gerhardt, Charles (1843) "Recherches sur la salicine," Annales de Chimie et de Physique, series 3, 7 : 215-229. Gerhardt coins the name "phénol" on page 221.
  25. ↑ Lister, Joseph (1867). "Antiseptic Principle Of The Practice Of Surgery". 
  26. ↑ The Experiments by Peter Tyson. NOVA
  27. ↑ The Nazi Doctors, Chapter 14, Killing with Syringes: Phenol Injections. By Dr. Robert Jay Lifton
  28. ↑ "Phenol: Hazards and Precautions". University of Connecticut, USA. Retrieved 2011-12-02. 
  29. ↑ 29.0 29.1 "Killing through phenol injection". Auschwitz — FINAL STATION EXTERMINATION. Johannes Kepler University, Linz, Austria. Retrieved 2006-09-29. 
  30. ↑ Rasmussen, L.E.L; Perrin, Thomas E (1999). "Physiological Correlates of Musth". Physiology & Behavior 67 (4): 539. doi:10.1016/S0031-9384(99)00114-6. 
  31. ↑ Musth in elephants. Deepa Ananth, Zoo's print journal, 15(5), pages 259-262 (article)
  32. ↑ The Beaver: Its Life and Impact. Dietland Muller-Schwarze, 2003, page 43 (book at google books)
  33. ↑ "Peat, Phenol and PPM, by Dr P. Brossard" (PDF). Retrieved 2008-05-27. 
  34. ↑ "Bruichladdich Octomore 4.2 "Comus" Islay Single Malt Whisky". 
  35. ↑ Juteau, P.; Côté, V; Duckett, MF; Beaudet, R; Lépine, F; Villemur, R; Bisaillon, JG (2005). "Cryptanaerobacter phenolicus gen. nov., sp. nov., an anaerobe that transforms phenol into benzoate via 4-hydroxybenzoate". International Journal of Systematic and Evolutionary Microbiology 55 (Pt 1): 245–50. doi:10.1099/ijs.0.02914-0. PMID 15653882. 
  36. ↑ Rehfuss, Marc; Urban, James (2005). "Rhodococcus phenolicus sp. nov., a novel bioprocessor isolated actinomycete with the ability to degrade chlorobenzene, dichlorobenzene and phenol as sole carbon sources". Systematic and Applied Microbiology 28 (8): 695–701. doi:10.1016/j.syapm.2005.05.011. PMID 16261859. 
  37. ↑ 37.0 37.1 Budavari, S, ed. (1996). The Merck Index: An Encyclopedia of Chemical, Drugs, and Biologicals. Whitehouse Station, NJ: Merck. 
  38. ↑ Lin TM, Lee SS, Lai CS, Lin SD (June 2006). "Phenol burn". Burns: Journal of the International Society for Burn Injuries 32 (4): 517–21. doi:10.1016/j.burns.2005.12.016. PMID 16621299. 
  39. ↑ Warner, MA; Harper, JV (1985). "Cardiac dysrhythmias associated with chemical peeling with phenol". Anesthesiology 62 (3): 366–7. doi:10.1097/00000542-198503000-00030. PMID 2579602. 
  40. ↑ World Health Organization/International Labour Organization: International Chemical Safety Cards, http://www.inchem.org/documents/icsc/icsc/eics0070.htm
  41. ↑ U.S. Department of Health and Human Services. "How can phenol affect my health?". Toxicological Profile for Phenol: 24. 
  42. ↑ Hanscha, Corwin; McKarns, Susan C; Smith, Carr J; Doolittle, David J (June 15, 2000). "Comparative QSAR evidence for a free-radical mechanism of phenol-induced toxicity". Chemico-Biological Interactions 127 (1): 61–72. doi:10.1016/S0009-2797(00)00171-X. PMID 10903419. 
  43. ↑ Brown, VKH; Box, VL; Simpson, BJ (1975). "Decontamination procedures for skin exposed to phenolic substances". Archives of Environmental Health 30 (1): 1–6. doi:10.1080/00039896.1975.10666623. PMID 1109265. 
  44. ↑ Hunter, DM; Timerding, BL; Leonard, RB; McCalmont, TH; Schwartz, E (1992). "Effects of isopropyl alcohol, ethanol, and polyethylene glycol/industrial methylated spirits in the treatment of acute phenol burns". Annals of Emergency Medicine 21 (11): 1303–7. doi:10.1016/S0196-0644(05)81891-8. 
  45. ↑ Pullin, TG; Pinkerton, MN; Johnston, RV; Kilian, DJ (1978). "Decontamination of the skin of swine following phenol exposure: a comparison of the relative efficacy of water versus polyethylene glycol/industrial methylated spirits". Toxicol Appl Pharmacol 43 (1): 199–206. doi:10.1016/S0041-008X(78)80044-1. PMID 625760. 

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