Phenol is an organic compound appreciablysoluble in water, with about 84.2 g dissolving in 1000 ml (0.895M). 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]
Phenol is a weak acid, with a pH range of 5 to 6. In aqueous solution in the pH range ca. 8–12 it is in equilibrium with thephenolateanionC6H5O− (also calledphenoxide orcarbolate):[11]
Phenol is more acidic thanaliphatic alcohols. Its enhanced acidity is attributed toresonance stabilization ofphenolate anion. In this way, the negative charge on oxygen is delocalized on to theortho and para carbon atoms through the pi system.[12] An alternative explanation involves the sigma framework, postulating that the dominant effect is theinduction from the more electronegativesp2 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, thepKa of theenol ofacetone 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 exhibitsketo-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.[16] 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.[17] 4,4' Substituted cyclohexadienone can undergo adienone–phenol rearrangement in acid conditions and form stable 3,4‐disubstituted phenol.[18]
For substituted phenols, several factors can favor the keto tautomer: (a) additional hydroxy groups (seeresorcinol) (b) annulation as in the formation ofnaphthols, and (c) deprotonation to give the phenolate.[19]
Phenoxides areenolates stabilised byaromaticity. 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".[20]
The Hock process leading to phenol viaautoxidation of cumene.Polar surface area of a "neutral" phenol substructure "shape". An image of a computed electrostatic surface of neutral phenol molecule, showing neutral regions in green, electronegative areas in orange-red, and the electropositive phenolic proton in blue.Phenol water phase diagram: Certain combinations of phenol and water can make two solutions in one bottle.
Phenol is so strongly activated that bromination and chlorination lead readily to polysubstitution.[21] The reaction affords 2- and 4-substituted derivatives. The regiochemistry of halogenation changes in strongly acidic solutions wherePhOH2]+ 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 give2,4,6-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), andcyclohexanones give respectively 4-adamantylphenol, a bis(2-hydroxyphenyl) derivative, and a 4-cyclohexylphenols.Alcohols andhydroperoxides alkylate phenols in the presence ofsolid acidcatalysts (e.g. certainzeolite).Cresols and cumyl phenols can be produced in that way.[22]
Phenol is reduced tobenzene when it is distilled withzinc dust or when its vapour is passed over granules of zinc at 400 °C:[23]
C6H5OH + Zn → C6H6 + ZnO
When phenol is treated withdiazomethane in the presence ofboron 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.
Accounting for 95% of production (2003) is thecumene process, also calledHock process. It involves the partialoxidation ofcumene (isopropylbenzene) via theHock rearrangement:[9] 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.[24][25] 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 ofbenzene (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.[27][24][26]
The original commercial route was developed byBayer andMonsanto in the early 1900s, based on discoveries byWurtz andKekulé. The method involves the reaction of a strong base withbenzenesulfonic acid, proceeded by the reaction of hydroxide withsodium benzenesulfonate to give sodium phenoxide. Acidification of the latter gives phenol. The net conversion is:[29]
Phenol, which is produced and hence transported in large volumes, is shipped in a molten state below 70 °C (158 °F). The melting point is lowered and corrosive nature enhanced in the presence of small amounts of water. Typically, stainless steel containers and nitrogen-blanketing are required to prevent discoloration.[9]
Exposure to phenol may occur in people living nearlandfills, hazardous waste sites or factories manufacturing it.[8][33] Low levels of phenol exposure may occur in consumer products, such as toothpastes andthroat lozenges, skin or pain treatments, cigarette smoke, and in some foods or water.[33]
Exposure to phenol through any form of ingestion or contact can produce systemic poisoning, with possible symptoms including transient brain stimulation, followed bycoma andseizures over minutes to hours following exposure.[8] Other symptoms may includehemolytic anemia, profuse sweating, hypotension,arrhythmia,pulmonary edema, nausea, vomiting, and diarrhea.[8][34] Chronic exposure to phenol or its vapor may causekidney toxicity, skin lesions, orgastrointestinal disease.[34][35] Phenol is metabolized in the liver, and excreted by the kidneys.[34]
If inhaled, ingested or by skin contact, phenol can enter the blood, possibly causing breathing problems, headaches, or sore eyes.[8][33][35] High amounts of phenol contacting the skin may causeliver disease, irregular heartbeat,seizures,coma, and, rarely, death.[8][33][36] Repeated or prolonged skin contact with phenol may causedermatitis, or even second and third-degree burns.[37] Its corrosive effect on skin and mucous membranes is due to a protein-degenerating effect.[38]Chemical burns from skin exposures can be decontaminated by washing withpolyethylene glycol,[39]isopropyl alcohol,[40] or with copious amounts of water.[41]
Phenol is so inexpensive that it also attracts many small-scale uses. It is a component of industrialpaint strippers used in the aviation industry for the removal of epoxy, polyurethane and other chemically resistant coatings.[50]
Concentrated liquid phenol can be usedtopically as a localanesthetic forotology procedures, such asmyringotomy andtympanotomy tube placement, as an alternative to general anesthesia or other local anesthetics.[51] Phenol spray, with phenol as the active ingredient, is used medically to treat sore throat.[51] It is the active ingredient in some oralanalgesics.[51]
Concentrated phenol liquids are used for permanent treatment of ingrown toe and finger nails, a procedure known as a chemicalmatrixectomy. The procedure was first described by Otto Boll in 1945.[52]
Phenol is used as a chemicaldenervation agent inanalgesia treatment, such as forspasticity, arthritis, or cancer pain.[34] Its effect on the nerve is todenature protein, diminish nerve fat andmyelin content, and interrupt sensory transmission to the brain.[34] If successful, pain relief may last for weeks to two years.[34] Complications may include pain on injection, bleeding, or infection.[34]
Clinical studies from 2023-25 reported that local injection of phenol (1.5–3 ml of 6% phenol in sterile water) at three to fivesensory knee nerves was effective as aneurolytic treatment to relieve pain associated with chronicosteoarthritis.[53][54] The phenol method may be used for people who did not experience pain relief fromradiofrequency ablation of knee nerves.[53][54]
Phenol was discovered in 1834 byFriedlieb Ferdinand Runge, who extracted it (in impure form) fromcoal tar.[56] Runge called phenol "Karbolsäure" (coal-oil-acid, carbolic acid). Coal tar remained the primary source until the development of thepetrochemical industry. French chemistAuguste Laurent extracted phenol in its pure form, as a derivative of benzene, in 1841.[57] In 1836, Auguste Laurent coined the name "phène" for benzene;[58] this is the root of the word "phenol" and "phenyl". In 1843, French chemistCharles Gerhardt coined the name "phénol".[59]
Theantiseptic properties of phenol were used by SirJoseph 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[60] 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 contemporaryLouis 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.[61]
Meanwhile, inCarlisle, England, officials were experimenting withsewage treatment using carbolic acid to reduce the smell of sewagecesspools. 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.[62]
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[62]
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[63]
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 caseCarlill v Carbolic Smoke Ball Company. In the tort law case ofRoe v Minister of Health, phenol was used to sterilizeanaesthetic packed inampoules, in which it contaminated the anaesthetic through invisible micro-cracks and causedparaplegia to the plaintiffs.
Thetoxic 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.[38] Injections of phenol were used as a means of individual execution byNazi Germany during theSecond World War.[64] It was originally used by the Nazis in 1939 as part of the mass-murder of disabled people underAktion T4.[65] 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 inAuschwitz when he volunteered to die in place ofa stranger. Approximately one gram is sufficient to cause death.[66]
Phenol is a normal metabolic product, excreted in quantities up to 40 mg/L in human urine.[38] Thetemporal gland secretion of maleelephants showed the presence of phenol and4-methylphenol duringmusth.[67] It is also one of the chemical compounds found incastoreum. This compound is ingested from the plants the beaver eats.[68]
Phenol is a measurable component in the aroma and taste of the distinctiveIslay scotch whisky,[69] generally ~30 ppm, but it can be over 160 ppm in the maltedbarley used to producewhisky.[70] This amount is different from and presumably higher than the amount in the distillate.[69]
The wordphenol is also used to refer to any compound that contains a six-memberedaromatic ring, bonded directly to ahydroxyl group (-OH). Thus, phenols are a class oforganic compounds of which the phenol discussed in this article is the simplest member.
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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, seep. 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.
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