Phenol is an organic compound appreciably soluble in water, with about 84.2 g dissolving in 1000 ml (0.895 M). Homogeneous mixtures of phenol and water at phenol to water mass ratios of ~2.6 and higher are possible. The sodium salt of phenol, sodium phenoxide, is far more water-soluble. It is a combustible solid (NFPA rating = 2). When heated, phenol produces flammable vapors that are explosive at concentrations of 3 to 10% in air. Carbon dioxide or dry chemical extinguishers should be used to fight phenol fires.[5]
Acidity
Phenol is a weak acid (pH 6.6). In aqueous solution in the pH range ca. 8 - 12 it is in equilibrium with the phenolateanionC6H5O− (also called phenoxide or carbolate):[10]
C6H5OH ⇌ C6H5O− + H+
Phenol is more acidic than aliphatic alcohols. Its enhanced acidity is attributed to resonance stabilization of phenolate anion. In this way, the negative charge on oxygen is delocalized on to the ortho and para carbon atoms through the pi system.[11] An alternative explanation involves the sigma framework, postulating that the dominant effect is the induction from the more electronegative 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 support of the second explanation, the pKa of the enol of acetone in water is 10.9, making it only slightly less acidic than phenol (pKa 10.0).[5] Thus, the greater number of resonance structures available to phenoxide compared to acetone enolate seems to contribute little to its stabilization. However, the situation changes when solvation effects are excluded.
Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone, but the effect is nearly negligible. The equilibrium constant for enolisation is approximately 10−13, which means only one in every ten trillion molecules is in the keto form at any moment.[15] 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 essentially entirely in the enol form.[16] 4, 4' Substituted cyclohexadienone can undergo a dienone–phenol rearrangement in acid conditions and form stable 3,4‐disubstituted phenol.[17]
For substituted phenols, several factors can favor the keto tautomer: (a) additional hydroxy groups (see resorcinol) (b) annulation as in the formation of naphthols, and (c) deprotonation to give the phenolate.[18]
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".[19]
Phenol is so strongly activated that bromination and chlorination lead readily to polysubstitution.[20] The reaction affords 2- and 4-substituted derivatives. The regiochemistry of halogenation changes in strongly acidic solutions where PhOH2]+ predominates. Phenol reacts with dilute nitric acid at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol while with concentrated nitric acid, additional nitro groups are introduced, e.g. to give 2,4,6-trinitrophenol. Friedel Crafts alkylations of phenol and its derivatives often proceed without catalysts. Alkylating agents include alkyl halides, alkenes, and ketones. Thus, adamantyl-1-bromide, dicyclopentadiene), and cyclohexanones give respectively 4-adamantylphenol, a bis(2-hydroxyphenyl) derivative, and a 4-cyclohexylphenols. Alcohols and hydroperoxides alkylate phenols in the presence of solid acidcatalysts (e.g. certain zeolite). Cresols and cumyl phenols can be produced in that way.[21]
Phenol is reduced to benzene when it is distilled with zinc dust or when its vapour is passed over granules of zinc at 400 °C:[22]
C6H5OH + Zn → C6H6 + ZnO
When phenol is treated with diazomethane in the presence of boron trifluoride (BF3), anisole is obtained as the main product and nitrogen gas as a byproduct.
C6H5OH + CH2N2 → C6H5OCH3 + N2
Phenol and its derivatives react with iron(III) chloride to give intensely colored solutions containing phenoxide complexes.
Production
Because of phenol's commercial importance, many methods have been developed for its production, but the cumene process is the dominant technology.
Cumene process
Accounting for 95% of production (2003) is the cumene process, also called Hock process. It involves the partial oxidation of cumene (isopropylbenzene) via the Hock rearrangement:[8] Compared to most other processes, the cumene process uses mild conditions and inexpensive raw materials. For the process to be economical, both phenol and the acetone by-product must be in demand.[23][24] In 2010, worldwide demand for acetone was approximately 6.7 million tonnes, 83 percent of which was satisfied with acetone produced by the cumene process.
The direct oxidation of benzene (C6H6) to phenol is possible, but it has not been commercialized:
C6H6 + O → C6H5OH
Nitrous oxide is a potentially "green" oxidant that is a more potent oxidant than O2. Routes for the generation of nitrous oxide however remain uncompetitive.[26][23][25]
Early methods relied on extraction of phenol from coal derivatives or the hydrolysis of benzene derivatives.
Hydrolysis of benzenesulfonic acid
The original commercial route was developed by Bayer and Monsanto in the early 1900s, based on discoveries by Wurtz and Kekule. The method involves the reaction of strong base with benzenesulfonic acid, proceeding by the reaction of hydroxide with sodium benzenesulfonate to give sodium phenoxide. Acidification of the latter gives phenol. The net conversion is:[28]
Phenol is so inexpensive that it also attracts many small-scale uses. It is a component of industrial paint strippers used in the aviation industry for the removal of epoxy, polyurethane and other chemically resistant coatings.[33] Due to safety concerns, phenol is banned from use in cosmetic products in the European Union[34][35] and Canada.[36][37]
Medical
Phenol was widely used as an antiseptic, and it is used in the production of carbolic soap. Concentrated phenol liquids are used for permanent treatment of ingrown toe and finger nails, a procedure known as a chemical matrixectomy. The procedure was first described by Otto Boll in 1945. Since that time phenol has become the chemical of choice for chemical matrixectomies performed by podiatrists.
Concentrated liquid phenol can be used topically as a local anesthetic for otology procedures, such as myringotomy and tympanotomy tube placement, as an alternative to general anesthesia or other local anesthetics. It also has hemostatic and antiseptic qualities that make it ideal for this use. Phenol spray, usually at 1.4% phenol as an active ingredient, is used medically to treat sore throat.[38] It is the active ingredient in some oral analgesics such as Chloraseptic spray, TCP and Carmex.[39]
History
Phenol was discovered in 1834 by Friedlieb Ferdinand Runge, who extracted it (in impure form) from coal tar.[41] Runge called phenol "Karbolsäure" (coal-oil-acid, carbolic acid). Coal tar remained the primary source until the development of the petrochemical industry. French chemist Auguste Laurent extracted phenol in its pure form, as a derivative of benzene, in 1841.[42] In 1836, Auguste Laurent coined the name "phène" for benzene;[43] this is the root of the word "phenol" and "phenyl". In 1843, French chemist Charles Gerhardt coined the name "phénol".[44]
The antiseptic properties of phenol were used by Sir Joseph Lister in his pioneering technique of antiseptic surgery. Lister decided that the wounds had to be thoroughly cleaned. He then covered the wounds with a piece of rag or lint[45] covered in phenol. The skin irritation caused by continual exposure to phenol eventually led to the introduction of aseptic (germ-free) techniques in surgery. Lister's work was inspired by the works and experiments of his contemporary Louis Pasteur in sterilizing various biological media. He theorized that if germs could be killed or prevented, no infection would occur. Lister reasoned that a chemical could be used to destroy the micro-organisms that cause infection.[46]
Meanwhile, in Carlisle, England, officials were experimenting with sewage treatment using carbolic acid to reduce the smell of sewage cesspools. Having heard of these developments, and having previously experimented with other chemicals for antiseptic purposes without much success, Lister decided to try carbolic acid as a wound antiseptic. He had his first chance on August 12, 1865, when he received a patient: an eleven-year-old boy with a tibia bone fracture which pierced the skin of his lower leg. Ordinarily, amputation would be the only solution. However, Lister decided to try carbolic acid. After setting the bone and supporting the leg with splints, he soaked clean cotton towels in undiluted carbolic acid and applied them to the wound, covered with a layer of tin foil, leaving them for four days. When he checked the wound, Lister was pleasantly surprised to find no signs of infection, just redness near the edges of the wound from mild burning by the carbolic acid. Reapplying fresh bandages with diluted carbolic acid, the boy was able to walk home after about six weeks of treatment.[47]
By 16 March 1867, when the first results of Lister's work were published in the Lancet, he had treated a total of eleven patients using his new antiseptic method. Of those, only one had died, and that was through a complication that was nothing to do with Lister's wound-dressing technique. Now, for the first time, patients with compound fractures were likely to leave the hospital with all their limbs intact
— Richard Hollingham, Blood and Guts: A History of Surgery, p. 62[47]
Before antiseptic operations were introduced at the hospital, there were sixteen deaths in thirty-five surgical cases. Almost one in every two patients died. After antiseptic surgery was introduced in the summer of 1865, there were only six deaths in forty cases. The mortality rate had dropped from almost 50 per cent to around 15 per cent. It was a remarkable achievement
— Richard Hollingham, Blood and Guts: A History of Surgery, p. 63[48]
Phenol was the main ingredient of the "carbolic smoke ball," an ineffective device marketed in London in the 19th century as protection against influenza and other ailments, and the subject of the famous law case Carlill v Carbolic Smoke Ball Company. In the tort law case of Roe v Minister of Health, phenol was used to sterilize anaesthetic packed in ampoules, in which it contaminated the anaesthetic through invisible micro-cracks and caused paraplegia to the plaintiffs.
Second World War
The toxic effect of phenol on the central nervous system causes sudden collapse and loss of consciousness in both humans and animals; a state of cramping precedes these symptoms because of the motor activity controlled by the central nervous system.[49] Injections of phenol were used as a means of individual execution by Nazi Germany during the Second World War.[50] It was originally used by the Nazis in 1939 as part of the mass-murder of disabled people under Aktion T4.[51] The Germans learned that extermination of smaller groups was more economical by injection of each victim with phenol. Phenol injections were given to thousands of people. Maximilian Kolbe was also murdered with a phenol injection after surviving two weeks of dehydration and starvation in Auschwitz when he volunteered to die in place of a stranger. Approximately one gram is sufficient to cause death.[52]
Occurrences
Phenol is a normal metabolic product, excreted in quantities up to 40 mg/L in human urine.[49] The temporal gland secretion of male elephants showed the presence of phenol and 4-methylphenol during musth.[53][54] It is also one of the chemical compounds found in castoreum. This compound is ingested from the plants the beaver eats.[55]
Phenol is a measurable component in the aroma and taste of the distinctive Islay scotch whisky,[56] generally ~30 ppm, but it can be over 160ppm in the malted barley used to produce whisky.[57] This amount is different from and presumably higher than the amount in the distillate.[56]
Phenol and its vapors are corrosive to the eyes, the skin, and the respiratory tract.[60] Its corrosive effect on skin and mucous membranes is due to a protein-degenerating effect.[49] Repeated or prolonged skin contact with phenol may cause dermatitis, or even second and third-degree burns.[61] Inhalation of phenol vapor may cause lung edema.[60] The substance may cause harmful effects on the central nervous system and heart, resulting in dysrhythmia, seizures, and coma.[62] The kidneys may be affected as well. Long-term or repeated exposure of the substance may have harmful effects on the liver and kidneys.[63] There is no evidence that phenol causes cancer in humans.[64] Besides its hydrophobic effects, another mechanism for the toxicity of phenol may be the formation of phenoxylradicals.[65]
Since phenol is absorbed through the skin relatively quickly, systemic poisoning can occur in addition to the local caustic burns.[49] Resorptive poisoning by a large quantity of phenol can occur even with only a small area of skin, rapidly leading to paralysis of the central nervous system and a severe drop in body temperature. The LD50 for oral toxicity is less than 500 mg/kg for dogs, rabbits, or mice; the minimum lethal human dose was cited as 140 mg/kg.[49] The Agency for Toxic Substances and Disease Registry (ATSDR), U.S. Department of Health and Human Services states the fatal dose for ingestion of phenol is from 1 to 32 g.[66]
Chemical burns from skin exposures can be decontaminated by washing with polyethylene glycol,[67]isopropyl alcohol,[68] or perhaps even copious amounts of water.[69] 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).[citation needed] Phenol is also a reproductive toxin causing increased risk of miscarriage and low birth weight indicating retarded development in utero.[5]
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.
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^Arnold, Richard T.; Buckley, Jay S. (1 May 1949). "The Dienone-Phenol Rearrangement. II. Rearrangement of 1-Keto-4-methyl-4-phenyl-1,4-dihydronaphthalene". J. Am. Chem. Soc. 71 (5): 1781. doi:10.1021/ja01173a071.
^Sergei M. Lukyanov, Alla V. Koblik (2003). "Tautomeric Equilibria and Rearrangements Involving Phenols". In Zvi Rappoport (ed.). The Chemistry of Phenols. PATAI'S Chemistry of Functional Groups. John Wiley & Sons. pp. 713–838. doi:10.1002/0470857277.ch11. ISBN0471497371.
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^Lee, Byungik; Naito, Hiroto; Nagao, Masahiro; Hibino, Takashi (9 July 2012). "Alternating-Current Electrolysis for the Production of Phenol from Benzene". Angewandte Chemie International Edition. 51 (28): 6961–6965. doi:10.1002/anie.201202159. PMID22684819.
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^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".
^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.)
^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.
^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. Bibcode:2005SyApM..28..695R. doi:10.1016/j.syapm.2005.05.011. PMID16261859.
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^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. PMID1109265.
^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. PMID1416322.
^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. Bibcode:1978ToxAP..43..199P. doi:10.1016/S0041-008X(78)80044-1. PMID625760.
External links
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