Phenols

In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group (-O H) attached to an aromatic hydrocarbon group. The simplest of the class is phenol (C₆H₅OH).  

Although similar to alcohols, phenols have unique properties and are not classified as alcohols (since the hydroxyl group is not bonded to a saturated carbon atom). They have relatively higher acidities due to the aromatic ring's tight coupling with the oxygen and a relatively loose bond between the oxygen and hydrogen. The acidity of the hydroxyl group in phenols is commonly intermediate between that of aliphatic alcohols and carboxylic acids (their pK_a is usually comprised between 10 and 12). Loss of a positive hydrogen ion (H⁺) from the hydroxyl group of a phenol forms a negative phenolate ion.

Some phenols are germicidal and are used in formulating disinfectants. Others possess estrogenic or endocrine disrupting activity.

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Synthesis of phenols

Several laboratory methods for the synthesis of phenols:

• by an ester rearrangement in the Fries rearrangement
• by a rearrangement of N-phenylhydroxylamines in the Bamberger rearrangement
• by hydrolysis of phenolic esters or ethers
• by reduction of quinones
• by replacement of an aromatic amine by an hydroxyl group with water and sodium bisulfide in the Bucherer reaction
• by hydrolysis of diazonium salts
• by oligomerisation with formaldehyde + base catalysed reaction with epichlorohydrine to epoxi resin components
• by reaction with acetone/ketones to e.g. Bisphenol A, an important monomer for resins, e.g. polycarbonate(PC), epoxi resins
• by a rearrangement reaction of dienones ^[1] in the dienone phenol rearrangement ^[2]:

Reactions of phenols

Phenols react in a wide variety of ways.

• Esterfication reactions and ether formation
• Electrophilic aromatic substitutions as the hydroxyl group is activating, for example synthesis of calixarenes ^[3]
• Reaction of naphtols and hydrazines and sodium bisulfite in the Bucherer carbazole synthesis
• Oxidative cleavage, for instance cleavage of 1,2-dihydroxybenzene to the monomethylester of 2,4 hexadienedioic acid with oxygen, copper chloride in pyridine ^[4]
• Oxidative de-aromatization to quinones also known as the Teuber reaction. Oxidizing reagents are Fremy's salt ^[5] and oxone ^[6]. In reaction depicted below 3,4,5-trimethylphenol reacts with singlet oxygen generated from oxone/sodium carbonate in an acetonitrile/water mixture to a para-peroxyquinole. This hydroperoxide is oxidized to the quinole with sodium thiosulfate.

• Phenols are oxidized to benzenediols in the Elbs persulfate oxidation
• Phenolate anions (deriving from phenols by the loss of H⁺) can act as ligands towards metal cations

Phenolic compounds

For a full list, see Category:Phenols

Medicinals

References

1. ^ related to quinones, see for example the Zincke-Suhl reaction
2. ^ Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. page Jerry March ISBN 0-471-85472-7
3. ^ p-tert-butylcalix[8]arene, Organic Syntheses, CV 8, 80 Article
4. ^ 2,4-Hexadienedioic acid, monomethyl ester, (Z,Z)- Organic Syntheses, Coll. Vol. 8, p.490 (1993); Vol. 66, p.180 (1988) Article
5. ^ 2,5-Cyclohexadiene-1,4-dione, 2,3,5-trimethyl Organic Syntheses, Coll. Vol. 6, p.1010 (1988); Vol. 52, p.83 (1972) Abstract.
6. ^ Oxidative De-aromatization of para-Alkyl Phenols into para-Peroxyquinols and para-Quinols Mediated by Oxone as a Source of Singlet Oxygen M. Carmen Carreño, Marcos González-López, Antonio Urbano Angewandte Chemie International Edition Volume 45, Issue 17 , Pages 2737 - 2741 2006 Abstract