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In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group (—OH) bonded directly to an aromatic hydrocarbon group. The simplest of the class is phenol, C
6
H
5
OH
. Phenolic compounds are classified as simple phenols or polyphenols based on the number of phenol units in the molecule.

Phenol – the simplest of the phenols.
Chemical structure of salicylic acid, the active metabolite of aspirin.

Phenols are synthesized industrially as well as naturally.[1]

Contents

PropertiesEdit

AcidityEdit

Phenols have distinct properties and are generally distinguished from other alcohols. They have higher acidities. The acidity of the hydroxyl group in phenols is commonly intermediate between that of aliphatic alcohols and carboxylic acids (their pKa is usually between 10 and 12). Loss of a hydrogen cation (H+) from the hydroxyl group of a phenol forms a corresponding negative phenolate ion or phenoxide ion, and the corresponding salts are called phenolates or phenoxides, although the term aryloxides is preferred according to the IUPAC Gold Book. Phenols can have two or more hydroxy groups bonded to the aromatic ring(s) in the same molecule. The simplest examples are the three benzenediols, each having two hydroxy groups on a benzene ring.

OxidationEdit

Phenols are reactive species toward oxidation. Oxidative cleavage, for instance cleavage of 1,2-dihydroxybenzene to the monomethylester of 2,4 hexadienedioic acid with oxygen, copper chloride in pyridine[2]

 

Electrophilic aromatic substitutionEdit

Phenols are highly susceptible to Electrophilic aromatic substitutions. Illustrative of a large-scale electrophilic aromatic substitution is the production of bisphenol A, which is produced on a scale 1 million tons. This compound is synthesized by the condensation of acetone.[5]

 

Other reactionsEdit

Phenols undergo esterfication. Phenol esters are active esters, being prone to hydrolysis.

Reaction of naphtols and hydrazines and sodium bisulfite in the Bucherer carbazole synthesis

SynthesisEdit

Several laboratory methods for the synthesis of phenols:

ClassificationEdit

There are various classification schemes.[8]:2 A commonly used scheme is based on the number of carbons and was devised by Jeffrey Harborne and Simmonds in 1964 and published in 1980:[8]:2[9]

 
Phenol-phenolate equilibrium, and resonance structures giving rise to phenol aromatic reactivity. See also the images at the wiki pages for phenols.
 
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 the parent compound, used as a disinfectant and for chemical synthesis
Bisphenol A and other bisphenols produced from ketones and phenol / cresol
BHT (butylated hydroxytoluene) - a fat-soluble antioxidant and food additive
4-Nonylphenol a breakdown product of detergents and nonoxynol-9
Orthophenyl phenol a fungicide used for waxing citrus fruits
Picric acid (trinitrophenol) - an explosive material
Phenolphthalein pH indicator
Xylenol used in antiseptics & disinfectants

Drugs, present and pastEdit

Diethylstilbestrol a synthetic estrogen with a stilbene structure; no longer marketed
L-DOPA a dopamine prodrug used to treat Parkinson's disease
Propofol a short-acting intravenous anesthetic agent
 
Phenol-phenolate equilibrium, and resonance structures giving rise to phenol aromatic reactivity.

The majority of these compounds are soluble molecules but the smaller molecules can be volatile.[citation needed]

Phenols chemically interact with many other substances.[citation needed] Stacking, a chemical property of molecules with aromaticity, is seen occurring between phenolic molecules.[citation needed] When studied in mass spectrometry, phenols easily form adduct ions with halogens.[citation needed] They can also interact with the food matrices or with different forms of silica (mesoporous silica, fumed silica[10] or silica-based sol gels[11]).

ReferencesEdit

  1. ^ Hättenschwiler, Stephan; Vitousek, Peter M. (2000). "The role of polyphenols in terrestrial ecosystem nutrient cycling". Trends in Ecology & Evolution. 15 (6): 238–243. doi:10.1016/S0169-5347(00)01861-9.
  2. ^ 2,4-Hexadienedioic acid, monomethyl ester, (Z,Z)- Organic Syntheses, Coll. Vol. 8, p.490 (1993); Vol. 66, p.180 (1988) Article
  3. ^ "2,5-Cyclohexadiene-1,4-dione, 2,3,5-trimethyl". Organic Syntheses, Coll. 6: 1010. 1988.; Vol. 52, p.83 (1972) Abstract.
  4. ^ Carreño, M. Carmen; González-López, Marcos; Urbano, Antonio (2006). "Oxidative De-aromatization of para-Alkyl Phenols into para-Peroxyquinols and para-Quinols Mediated by Oxone as a Source of Singlet Oxygen". Angewandte Chemie International Edition. 45 (17): 2737–2741. doi:10.1002/anie.200504605. PMID 16548026.
  5. ^ Fiege H; Voges H-W; Hamamoto T; Umemura S; Iwata T; Miki H; Fujita Y; Buysch H-J; Garbe D; Paulus W (2000). "Phenol Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_313. ISBN 978-3527306732.
  6. ^ Bracegirdle, Sonia; Anderson, Edward A. (2010). "Arylsilane oxidation—new routes to hydroxylated aromatics". Chem. Comm. 46 (20): 3454–6. doi:10.1039/b924135c. PMID 20582346.
  7. ^ mostami, Ros (2017). "Facile preparation of Ʌ-shaped building blocks: Hünlich-base derivatization". Synlett. 28: 1641–1645.
  8. ^ a b Wilfred Vermerris and Ralph Nicholson. Phenolic Compound Biochemistry Springer, 2008
  9. ^ Harborne, J. B. (1980). "Plant phenolics". In Bell, E. A.; Charlwood, B. V. (eds.). Encyclopedia of Plant Physiology, volume 8 Secondary Plant Products. Berlin Heidelberg New York: Springer-Verlag. pp. 329–395.
  10. ^ Kulik, T. V.; Lipkovska, N. A.; Barvinchenko, V. N.; Palyanytsya, B. B.; Kazakova, O. A.; Dovbiy, O. A.; Pogorelyi, V. K. (2009). "Interactions between bioactive ferulic acid and fumed silica by UV–vis spectroscopy, FT-IR, TPD MS investigation and quantum chemical methods". Journal of Colloid and Interface Science. 339 (1): 60–8. Bibcode:2009JCIS..339...60K. doi:10.1016/j.jcis.2009.07.055. PMID 19691966.
  11. ^ Lacatusu, Ioana; Badea, Nicoleta; Nita, Rodica; Murariu, Alina; Miculescu, Florin; Iosub, Ion; Meghea, Aurelia (2010). "Encapsulation of fluorescence vegetable extracts within a templated sol–gel matrix". Optical Materials. 32 (6): 711–718. Bibcode:2010OptMa..32..711L. doi:10.1016/j.optmat.2009.09.001.