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.<ref name=pubchem/>
Acidity
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 the phenolateanionTemplate:Chem2 (also called phenoxide or carbolate):<ref>Template:March6th</ref>
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.<ref>Organic Chemistry 2nd Ed. John McMurry Template:ISBN</ref> 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).<ref name=pubchem/> 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.
Hydrogen bonding
In carbon tetrachloride and in alkane solvents, phenol hydrogen bonds with a wide range of Lewis bases such as pyridine, diethyl ether, and diethyl sulfide. The enthalpies of adduct formation and the Template:Chem2 IR frequency shifts accompanying adduct formation have been compiled.<ref>Drago, R S. Physical Methods For Chemists, (Saunders College Publishing 1992), ISBN 0-03-075176-4</ref> Phenol is classified as a hard acid.<ref>Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) pp 50-51 ISBN 978-0-470-74957-9</ref><ref>Template:Cite journal The plots shown in this paper used older parameters. Improved E&C parameters are listed in ECW model.</ref>
Tautomerism
Phenol-cyclohexadienone tautomerism
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.<ref>Template:Cite journal</ref> 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.<ref>Template:Clayden</ref> 4,4' Substituted cyclohexadienone can undergo a dienone–phenol rearrangement in acid conditions and form stable 3,4‐disubstituted phenol.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite book</ref>
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".<ref>Template:Cite journal</ref>
Reactions
The Hock process leading to phenol via autoxidation 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.<ref>Template:Ullmann</ref> The reaction affords 2- and 4-substituted derivatives. The regiochemistry of halogenation changes in strongly acidic solutions where Template:Chem2 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.<ref name=reddy>Template:Cite book</ref>
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:<ref>Template:Cite book</ref>
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:<ref name="Ullmann"/> 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.<ref name="essential chemical">Template:Cite web</ref><ref name="chemistry.org"/> 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.
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.<ref name=CatalysisToday2005>Template:Cite journal</ref><ref name="essential chemical"/><ref name="acs phenol"/>
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 Kekulé. The method involves the reaction of a strong base with benzenesulfonic acid, proceeded by the reaction of hydroxide with sodium benzenesulfonate to give sodium phenoxide. Acidification of the latter gives phenol. The net conversion is:<ref name="Wittcoff">Wittcoff, H.A., Reuben, B.G. Industrial Organic Chemicals in Perspective. Part One: Raw Materials and Manufacture. Wiley-Interscience, New York. 1980.</ref>
These methods suffer from the cost of the chlorobenzene and the need to dispose of the chloride byproduct.
Coal pyrolysis
Phenol is also a recoverable byproduct of coal pyrolysis.<ref name="Franck">Franck, H.-G., Stadelhofer, J.W. Industrial Aromatic Chemistry. Springer-Verlag, New York. 1988. pp. 148-155.</ref> In the Lummus process, the oxidation of toluene to benzoic acid is conducted separately.
Miscellaneous methods
Amine to phenol<ref name=":0"/>
Phenyldiazonium salts hydrolyze to phenol. The method is of no commercial interest since the precursor is expensive.<ref name=":0">Template:Cite journal</ref>
Phenol, which is produced and hence transported in large volumes, is shipped in a molten state below Template:Convert. 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.<ref name="Ullmann"/>
Exposure and potential toxicity
Exposure to phenol may occur in people living near landfills, hazardous waste sites or factories manufacturing it.<ref name=cdc/><ref name="tox">Template:Cite web</ref> Low levels of phenol exposure may occur in consumer products, such as toothpastes and throat lozenges, skin or pain treatments, cigarette smoke, and in some foods or water.<ref name=tox/>
Exposure to phenol through any form of ingestion or contact can produce systemic poisoning, with possible symptoms including transient brain stimulation, followed by coma and seizures over minutes to hours following exposure.<ref name=cdc/> Other symptoms may include hemolytic anemia, profuse sweating, hypotension, arrhythmia, pulmonary edema, nausea, vomiting, and diarrhea.<ref name=cdc/><ref name=stat/> Chronic exposure to phenol or its vapor may cause kidney toxicity, skin lesions, or gastrointestinal disease.<ref name="stat">Template:Cite web</ref><ref name="merck"/> Phenol is metabolized in the liver, and excreted by the kidneys.<ref name=stat/>
If inhaled, ingested or by skin contact, phenol can enter the blood, possibly causing breathing problems, headaches, or sore eyes.<ref name=cdc/><ref name=tox/><ref name="merck">Template:Cite book</ref> High amounts of phenol contacting the skin may cause liver disease, irregular heartbeat, seizures, coma, and, rarely, death.<ref name=cdc/><ref name=tox/><ref>Template:Cite journal</ref> Repeated or prolonged skin contact with phenol may cause dermatitis, or even second and third-degree burns.<ref>Template:Cite journal</ref> Its corrosive effect on skin and mucous membranes is due to a protein-degenerating effect.<ref name=U/> Chemical burns from skin exposures can be decontaminated by washing with polyethylene glycol,<ref>Template:Cite journal</ref> isopropyl alcohol,<ref>Template:Cite journal</ref> or with copious amounts of water.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite web</ref>
Topical anesthetic
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.<ref name="drugs">Template:Cite web</ref> Phenol spray, with phenol as the active ingredient, is used medically to treat sore throat.<ref name=drugs/> It is the active ingredient in some oral analgesics.<ref name=drugs/>
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.<ref>Template:Cite journal</ref>
Nerve block
Template:See also
Phenol is used as a chemical denervation agent in analgesia treatment, such as for spasticity, arthritis, or cancer pain.<ref name=stat/> Its effect on the nerve is to denature protein, diminish nerve fat and myelin content, and interrupt sensory transmission to the brain.<ref name=stat/> If successful, pain relief may last for weeks to two years.<ref name=stat/> Complications may include pain on injection, bleeding, or infection.<ref name=stat/>
Clinical studies from 2023-25 reported that local injection of phenol (1.5–3 ml of 6% phenol in sterile water) at three to five sensory knee nerves was effective as a neurolytic treatment to relieve pain associated with chronic osteoarthritis.<ref name="tay">Template:Cite journal</ref><ref name="wilk">Template:Cite journal</ref> The phenol method may be used for people who did not experience pain relief from radiofrequency ablation of knee nerves.<ref name=tay/><ref name=wilk/>
Phenol was discovered in 1834 by Friedlieb Ferdinand Runge, who extracted it (in impure form) from coal tar.<ref>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.</ref> 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.<ref>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".</ref> In 1836, Auguste Laurent coined the name "phène" for benzene;<ref>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.)</ref> this is the root of the word "phenol" and "phenyl". In 1843, French chemist Charles Gerhardt coined the name "phénol".<ref>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.</ref>
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<ref>Template:Cite web</ref> 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.<ref>Template:Cite book</ref>
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.<ref name="BBC Books - Randomhouse">Template:Cite book</ref>
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<ref name="BBC Books - Randomhouse"/>
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<ref>Template:Cite book</ref>
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.<ref name=U/> Injections of phenol were used as a means of individual execution by Nazi Germany during the Second World War.<ref name=TysonNOVA>The Experiments by Peter Tyson. NOVA</ref> It was originally used by the Nazis in 1939 as part of the mass-murder of disabled people under Aktion T4.<ref name=NaziDrs>The Nazi DoctorsTemplate:Webarchive, Chapter 14, Killing with Syringes: Phenol Injections. By Dr. Robert Jay Lifton</ref> 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.<ref name="carbolic">Template:Cite web</ref>
Occurrences
Phenol is a normal metabolic product, excreted in quantities up to 40 mg/L in human urine.<ref name=U>Template:Cite encyclopedia</ref> The temporal gland secretion of male elephants showed the presence of phenol and 4-methylphenol during musth.<ref>Template:Cite journal</ref> It is also one of the chemical compounds found in castoreum. This compound is ingested from the plants the beaver eats.<ref>The Beaver: Its Life and Impact. Dietland Muller-Schwarze, 2003, page 43 (book at google books)</ref>
Phenol is a measurable component in the aroma and taste of the distinctive Islay scotch whisky,<ref name="auto">Template:Cite web</ref> generally ~30 ppm, but it can be over 160 ppm in the malted barley used to produce whisky.<ref>Template:Cite web</ref> This amount is different from and presumably higher than the amount in the distillate.<ref name="auto"/>
Template:Main
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.
