Phenol
Phenol is an aromatic organic compound with the molecular formula. It is a white crystalline solid that is volatile and can catch fire.
The molecule consists of a phenyl group bonded to a hydroxy group. Mildly acidic, it requires careful handling because it can cause chemical burns. It is acutely toxic and is considered a health hazard.
Phenol was first extracted from coal tar, but today is produced on a large scale from petroleum-derived feedstocks. It is an important industrial commodity as a precursor to many materials and useful compounds, and is a liquid when manufactured. It is primarily used to synthesize plastics and related materials. Phenol and its chemical derivatives are essential for production of polycarbonates, epoxies, explosives such as picric acid, Bakelite, nylon, detergents, herbicides such as phenoxy herbicides, and numerous pharmaceutical drugs.
Properties
Phenol is an organic compound appreciably soluble in water, with about 84.2 g dissolving in 1000 ml. 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. Phenol is a combustible solid. 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.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 phenolate anion :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. 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. 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 IR frequency shifts accompanying adduct formation have been compiled. Phenol is classified as a hard acid.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. 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. 4,4' Substituted cyclohexadienone can undergo a dienone–phenol rearrangement in acid conditions and form stable 3,4‐disubstituted phenol.For substituted phenols, several factors can favor the keto tautomer: additional hydroxy groups annulation as in the formation of naphthols, and deprotonation to give the phenolate.
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".
Reactions
Phenol is highly reactive toward electrophilic aromatic substitution. The enhanced nucleophilicity is attributed to donation pi electron density from O into the ring. Many groups can be attached to the ring, via halogenation, acylation, sulfonation, and related processes.Phenol is so strongly activated that bromination and chlorination lead readily to polysubstitution. The reaction affords 2- and 4-substituted derivatives. The regiochemistry of halogenation changes in strongly acidic solutions where 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 derivative, and a 4-cyclohexylphenols. Alcohols and hydroperoxides alkylate phenols in the presence of solid acid catalysts. Cresols and cumyl phenols can be produced in that way.
Aqueous solutions of phenol are weakly acidic and turn blue litmus slightly to red. Phenol is neutralized by sodium hydroxide forming sodium phenate or phenolate, but being weaker than carbonic acid, it cannot be neutralized by sodium bicarbonate or sodium carbonate to liberate carbon dioxide.
When a mixture of phenol and benzoyl chloride are shaken in presence of dilute sodium hydroxide solution, phenyl benzoate is formed. This is an example of the Schotten–Baumann reaction:
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:
When phenol is treated with diazomethane in the presence of boron trifluoride, anisole is obtained as the main product and nitrogen gas as a byproduct.
Phenol and its derivatives react with neutral iron chloride to give intensely violet colored solutions containing phenoxide complexes.
Phenol gives a positive result with the Ceric Ammonium Nitrate test, and produces a dark brownish precipitate, contrary to standard aliphatic alcohols which typically give a pink or red color.
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 is the cumene process, also called Hock process. It involves the partial oxidation of cumene via the Hock rearrangement: 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. 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.A route analogous to the cumene process begins with cyclohexylbenzene. It is oxidized to a hydroperoxide, akin to the production of cumene hydroperoxide. Via the Hock rearrangement, cyclohexylbenzene hydroperoxide cleaves to give phenol and cyclohexanone. Cyclohexanone is an important precursor to some nylons.
Oxidation of benzene, toluene, cyclohexylbenzene
The direct oxidation of benzene to phenol is possible, but it has not been commercialized: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.
An electrosynthesis employing alternating current gives phenol from benzene.
The oxidation of toluene, as developed by Dow Chemical, involves copper-catalyzed reaction of molten sodium benzoate with air:
The reaction is proposed to proceed via formation of benzyoylsalicylate.
Autoxidation of cyclohexylbenzene gives the hydroperoxide. Decomposition of this hydroperoxide affords cyclohexanone and phenol.
Older methods
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:Hydrolysis of chlorobenzene
can be hydrolyzed to phenol using a base or steam :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. In the Lummus process, the oxidation of toluene to benzoic acid is conducted separately.Miscellaneous methods
salts hydrolyze to phenol. The method is of no commercial interest since the precursor is expensive.Salicylic acid decarboxylates to phenol.
Shipping
Phenol, which is produced and hence transported in large volumes, is shipped in a molten state below. 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.Exposure and toxicity
Exposure to phenol may occur in people living near landfills, hazardous waste sites or factories manufacturing it. 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.Exposure to phenol through any form of ingestion or contact can produce systemic poisoning, with possible symptoms including, followed by coma and seizures over minutes to hours following exposure. Other symptoms may include hemolytic anemia, profuse sweating, hypotension, arrhythmia, pulmonary edema, nausea, vomiting, and diarrhea. Chronic exposure to phenol or its vapor may cause kidney toxicity, skin lesions, or gastrointestinal disease. Phenol is metabolized in the liver, and excreted by the kidneys.
If inhaled, ingested or by skin contact, phenol can enter the blood, possibly causing breathing problems, headaches, or sore eyes. High amounts of phenol contacting the skin may cause liver disease, irregular heartbeat, seizures, coma, and, rarely, death. Repeated or prolonged skin contact with phenol may cause dermatitis, or even second and third-degree burns. Its corrosive effect on skin and mucous membranes is due to a protein-degenerating effect. Chemical burns from skin exposures can be decontaminated by washing with polyethylene glycol, isopropyl alcohol, or with copious amounts of water.
Safety concerns have caused phenol to be banned from use in cosmetic products in the European Union and Canada.
Besides its hydrophobic effects, another possible mechanism for the toxicity of phenol is the formation of phenoxyl radicals.