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IUPAC name
Other names
Carbolic Acid, Benzenol, Phenylic Acid, Hydroxybenzene, Phenic acid
3D model (JSmol)
PubChem {{{value}}}
RTECS number SJ3325000
SMILES {{{value}}}
Molar mass 94.11 g·mol−1
Appearance transparent crystalline solid
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)
8.3 g/100 mL (20 °C)
Acidity (pKa) 9.95 (in water),

29.1 (in acetonitrile)[1]

λmax 270.75 nm[2]
1.7 D
EU classification Toxic (T)
Muta. Cat. 3
Corrosive (C)
NFPA 704

NFPA 704.svg

R-phrases R23/R24/R25-R34-R48/R20/R21/R22-R68
S-phrases (S1/2)-S24/S25-S26-S28-S36/S37/S39-S45
Flash point 79 °C
Related compounds
Related compounds Benzenethiol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
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Phenol, also known as carbolic acid and phenic acid, is an organic compound. It has the chemical formula C6H5OH. It is a white crystalline solid at room temperature. The molecule is made up of a phenyl (-C6H5). This phenyl is bonded to a hydroxyl (-OH) group. It is made on a large scale (about 7 billion kg every year).[3] It is only a little bit acidic but it must be handled carefully because it can burn human skin.

Phenol was first taken from coal tar. Phenol is mainly used in plastics or materials like it. Phenols are needed for building polycarbonates, epoxies, Bakelite, nylon, detergents and also many drugs, herbicides and pharmaceuticals.

Properties[change | change source]

Phenol solves in water. About 8.3 g dissolve in 100 mL (0.88 M). The sodium salt of phenol, sodium phenoxide, is far more water soluble.

Acidity[change | change source]

Phenol is slightly acidic. The phenol molecules have weak tendencies to lose the H+ ion from the hydroxyl group. This results in the highly water-soluble phenolate anion C6H5O (also called phenoxide).[4] 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+. 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 shared by the ortho and para carbon atoms.[5] In another explanation, increased acidity is the result of orbital overlap between the oxygen's lone pairs and the aromatic system.[6] In a third, the dominant effect is the induction from the sp2 hybridized 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 pKa of the enol of acetone, which is 19.0, in comparison to phenol with a pKa of 10.0.[7] However, this similarity of acidities of phenol and acetone enol is not observed in the gas phase. The difference is because the difference of solvation energies of the deprotonated acetone enol and phenoxide almost exactly offsets the experimentally observed gas phase acidity difference. It has recently been shown that only about 1/3 of the increased acidity of phenol is due to inductive effects, with resonance accounting for the rest.[8]

Phenoxide anion[change | change source]

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".[9]

Tautomerism[change | change source]

Phenol-cyclohexadienone tautomerism

Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone. 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.[10] 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.[11]

Reactions[change | change source]

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.

Production[change | change source]

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:[3]

C6H5CH(CH3)2 + O2 → C6H5OH + (CH3)2CO

Compared to most other processes, the cumene-hydroperoxide process uses relatively mild synthesis conditions, and relatively cheap 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 1900's, begins with the reaction of strong base with benzenesulfonate[12]:

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 recoverable byproduct of coal pyrolysis[13].

Uses[change | change source]

The major uses of phenol, consuming two thirds of its production, involve its conversion to plastics or related materials. 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. 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.[3]

Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also many herbicides and pharmaceuticals. Phenol is also used as an oral anesthetic/analgesic, commonly used to temporarily treat pharyngitis.

Niche uses[change | change source]

Phenol is so cheap that it has many small-scale uses. It once was widely used as an antiseptic, especially as Carbolic soap, from the early 1900s through 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.[14]

Phenol derivatives are also used in the preparation of cosmetics including sunscreens,[15] hair dyes, and skin lightening preparations.[16]

History[change | change source]

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

The antiseptic properties of phenol 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[17] covered in phenol, or carbolic acid as he called it. 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, an ineffective device marketed in London in the 19th century as protecting the user against influenza and other ailments, and the subject of a famous law case.

Second World War[change | change source]

Injections of phenol have occasionally been used as a means of execution. In particular, phenol and cyanide injections were used as a means of individual execution by the Nazis during the Second World War.[18] Originally used by the Nazis in 1939 as part of Action T4, phenol,[19] inexpensive, easy to make and quickly lethal, became the injectable toxin of choice as part of Nazi Germany's "euthanasia" program.[19][18][20] 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.[21] 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 adopted.[22] 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.[22]

Occurrence[change | change source]

Phenol is a measurable component in the aroma and taste of the distinctive Islay scotch whisky,[23] generally ~30, but up to 100[24] ppm.

Biodegradation[change | change source]

Cryptanaerobacter phenolicus is a bacterium species that produces benzoate from phenol via 4-hydroxybenzoate.[25] Rhodococcus phenolicus is a bacterium species able to degrade phenol as sole carbon sources.[26]

Toxicity[change | change source]

Phenol and its vapors are corrosive to the eyes, the skin, and the respiratory tract.[27] 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.[28] Inhalation of phenol vapor may cause lung edema.[27] The substance may cause harmful effects on the central nervous system and heart, resulting in dysrhythmia, seizures, and coma.[29] 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."[30] There is no evidence to believe that phenol causes cancer in humans.[31] Besides its hydrophobic effects, another mechanism for the toxicity of phenol may be the formation of phenoxyl radicals.[32]

Chemical burns from skin exposures can be decontaminated by washing with polyethylene glycol,[33] isopropyl alcohol,[34] or perhaps even copious amounts of water.[35] 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).

Phenols[change | change source]

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.

References[change | change source]

  1. Kütt, A.; Movchun, V.; Rodima, T.; Dansauer, T.; Rusanov, E. B.; Leito, I.; Kaljurand, I.; Koppel, J.; Pihl, V.; Koppel, I.; Ovsjannikov, G.; Toom, L.; Mishima, M.; Medebielle, M.; Lork, E.; Röschenthaler, G.-V.; Koppel, I. A.; Kolomeitsev, A. A. Pentakis(trifluoromethyl)phenyl, a Sterically Crowded and Electron-withdrawing Group: Synthesis and Acidity of Pentakis(trifluoromethyl)benzene, -toluene, -phenol, and -aniline. J. Org. Chem. 2008, 73, 2607-2620. DOI: 10.1021/jo702513w
  3. 3.0 3.1 3.2 Manfred Weber, Markus Weber, Michael Kleine-Boymann "Phenol" in Ullmann's Encyclopedia of Industrial Chemistry 2004, Wiley-VCH. doi:10.1002/14356007.a19_299.pub2.
  4. Template:March6th
  5. Organic Chemistry 2nd Ed. John McMurry ISBN 0534079687
  6. "The Acidity of Phenol". ChemGuide. Jim Clark. Retrieved 2007-08-05.
  7. For further reading on the fine points of this topic, see David A. Evans's explanation.
  8. 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)
  9. David Y. Curtin and Allan R. Stein (1966). "2,6,6-Trimethyl-2,4-Cyclohexadione.". Organic Syntheses 46: 115. 
  10. Capponi, Marco; Gut, Ivo G.; Hellrung, Bruno; Persy, Gaby; Wirz, Jakob (1999). "Ketonization equilibria of phenol in aqueous solution". Can. J. Chem. 77: 605–613. doi:10.1139/cjc-77-5-6-605. 
  11. Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. p. 531. ISBN 978-0-19-850346-0.
  12. Wittcoff, H.A., Reuben, B.G. Industrial Organic Chemicals in Perspective. Part One: Raw Materials and Manufacture. Wiley-Interscience, New York. 1980.
  13. 13.0 13.1 Franck, H.-G., Stadelhofer, J.W. Industrial Aromatic Chemistry. Springer-Verlag, New York. 1988. pp. 148-155.
  14. "CH207 Aircraft paintstripper, phenolic, acid" (PDF). Callington. 14 October 2009. Retrieved 27 August 2011.
  15. 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. 
  16. DeSelms, R. H.; UV-Active Phenol Ester Compounds; Enigen Science Publishing: Washington, DC, 2008.
  17. Lister, Joseph (1867). "Antiseptic Principle Of The Practice Of Surgery".
  18. 18.0 18.1 The Experiments by Peter Tyson. NOVA
  19. 19.0 19.1 The Nazi Doctors, Chapter 14, Killing with Syringes: Phenol Injections. By Dr. Robert Jay Lifton
  20. Euthanasia Program: Holocaust Encyclopedia
  21. "Phenol: Hazards and Precautions" (PDF). University of Connecticut, USA. Retrieved 2011-12-02.
  22. 22.0 22.1 "Killing through phenol injection". Auschwitz - FINAL STATION EXTERMINATION. Johannes Kepler University, Linz, Austria. Retrieved 2006-09-29.
  23. "Peat, Phenol and PPM, by Dr P. Brossard" (PDF). Retrieved 2008-05-27.
  24. "Ardbeg "Supernova" Islay Single Malt Whisky".
  25. Cryptanaerobacter phenolicus gen. nov., sp. nov., an anaerobe that transforms phenol into benzoate via 4-hydroxybenzoate. Pierre Juteau, Valérie Côté, Marie-France Duckett, Réjean Beaudet, François Lépine, Richard Villemur and Jean-Guy Bisaillon, IJSEM, January 2005, vol. 55, no. 1, pages 245-250, doi:10.1099/ijs.0.02914-0
  26. Rhodococcus phenolicus sp. nov., a novel bioprocessor isolated actinomycete with the ability to degrade chlorobenzene, dichlorobenzene and phenol as sole carbon sources. Rehfuss M and Urban J, Syst. Appl. Microbiol. (2005), 28, pages 695-701 (Erratum: Syst. Appl. Microbiol. (2006) 29, page 182), PubMed, doi:10.1016/j.syapm.2005.05.011
  27. 27.0 27.1 Budavari, S, ed. (1996). The Merck Index: An Encyclopedia of Chemical, Drugs, and Biologicals. Whitehouse Station, NJ: Merck. 
  28. 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. 
  29. 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. 
  30. World Health Organization/International Labour Organization: International Chemical Safety Cards,
  31. U.S. Department of Health and Human Services. "How can phenol affect my health?". Toxicological Profile for Phenol: 24. 
  32. Hanscha, Corwin; McKarnsb, 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. 
  33. Brown, VKH; Box, VL; Simpson, BJ (1975). "Decontamination procedures for skin exposed to phenolic substances". Archives of Environmental Health 30 (1): 1–6. PMID 1109265. 
  34. 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. 
  35. Pullin, TG; Pinkerton, MN; Johnson, 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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