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 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.

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 in the dienone phenol rearrangement :

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
• 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
• Oxidative de-aromatization to quinones also known as the Teuber reaction. Oxidizing reagents are Fremy's salt and oxone . 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 reduced 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 Phenols

Phenol	the parent compound, used as an disinfectant and for chemical synthesis
BHT	(butylated hydroxytoluene) - a fat-soluble antioxidant and food additive
Capsaicin	the pungent compound of chilli peppers
Bisphenol A	and other bisphenols produced from ketones and phenol / cresol
Cresol	found in coal tar and creosote
Estradiol	estrogen - hormones
Eugenol	the main constituent of the essential oil of clove
Gallic acid	found in galls
Guaiacol	(2-methoxyphenol) - has a smokey flavor, and is found in roasted coffee, whisky, and smoke
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
Polyphenol	e.g. flavonoids and tannins
Raspberry ketone	a compound with an intense raspberry smell
Serotonin / dopamine / adrenaline / noradrenaline	natural neurotransmitters
Thymol	(2-Isopropyl-5-methyl phenol) - an antiseptic that is used in mouthwashes
Tyrosine	an amino acid
Xylenol	-used in antiseptics & disinfecticides

Adverse health effects of some phenols

Some of the above substances are related to endocrine-disruptive chemicals.

Medicinals

Cannabinoids	the active constituents of cannabis
Diethylstilbestrol	a synthetic estrogen with a stilbene structure
L-DOPA	a dopamine prodrug
Methyl salicylate	the major constituent of the essential oil of wintergreen
Propofol	a short-acting intravenous anesthetic agent
Salicylic acid	a plant hormone used for its analgesic, antipyretic, and anti-inflammatory properties, also a precursor compound to Aspirin

See also

• Phenolic compounds in wine

References

External links

• Special Issue on Phenolics (flavonoids and non-flavonoids)