Phenols

As phenols compounds are referred to in the chemistry of an aromatic ring ( Arene ) exist and one or more hydroxy groups bonded thereto. After chemical nomenclature phenols are called hydroxy by appending the suffix- ol or prepending the prefix.

Properties

Phenols are similar due to the hydroxyl group of the alcohols but react in water as weak acids and result in slightly acidic solutions. Reason for the acidic nature of the hydroxyl group is the stabilization of the forming phenolate anion by resonance. With bases phenols therefore form salts phenolates. Despite the differences go phenols also many features typical of alcohols reactions, such as esterification with acids or the formation of phenol ethers.

Phenols possess a keto -enol tautomerism, which is strong but by the energetic favored by the formation of the aromatic system (see Hückel rule ) in contrast to the aliphatic ketones on the enol side.

Representative

Simple phenols

Phenol is the simplest benzene-derived phenol with only one hydroxyl group and the molecular formula C6H5OH.

More phenols having a hydroxyl group, so-called monovalent phenols, the toluene derivatives: the cresols (o-, m-and p- cresol), and naphthols ( α - and β -naphthol ), and thymol.

The best known dihydroxybenzenes ( diphenols or divalent phenols), phenols having two hydroxyl groups are catechol, resorcinol, hydroquinone and 1,4- naphthohydroquinone.

An important triphenol ( trivalent phenol) is phloroglucinol (1,3,5- trihydroxybenzene ), which is used in hydrochloric acid solution as a detection reagent for the pulp lignin. Pyrogallol ( 1,2,3- trihydroxy benzene ) is used in chemical analysis for the detection and absorption of oxygen. The third representative is hydroxyhydroquinone (1,2,4- trihydroxy ).

Substituted phenols

Picric acid is an example of a substituted phenol derivative. Another chemically significant phenol derivatives include salicylic acid, o-and p -nitro phenol, gallic acid, eugenol, hexachlorophene, chloroxylenol, adrenaline and noradrenaline. Among the phenols include tannins and many flavoring substances which determine the aroma and flavor of the wine. Other phenols are known, for example, the vanillin, the main flavor component of the vanilla and the vegetable dyes from the groups of flavonoids.

Phenolates

Phenolates are salts of metal cations and Phenolatanionen. The formula of the simplest phenolate is ( C6H5O ) nMe (Me = metal ion, n is the value of this metal ion ). Phenolate produced by deprotonation of phenols. Phenolate ions are weaker bases than alkoxide.

Occurrence and representation

Some phenols are obtained by distillation from the tar of bituminous and brown coal, others can be isolated from natural products. Most phenols are today - as phenol itself - after the cumene hydroperoxide process synthesized. Phenols are the basis for the production of synthetic resins ( phenolic ). They are also used in the production of plastics, dyes, pharmaceuticals and pesticides. Hydroquinone ( 1,4-dihydroxybenzene ) is a mild reducing agent, and is therefore used as a photographic developer.

Proof

By iron ( III ) chloride, most phenols are green - blue. Specific detection is possible for many phenols via reaction with benzoyl chloride to the corresponding, readily crystallizing esters.

Flavor carrier

Among the phenols include tannins and other flavors that are, for example, for the taste and smell of wine is important. The tannins are mainly found in the skins and seeds of red grapes. A derivative of phenol, 2,4,6 - Trichloranisol believed to be responsible for the " cork " flavor of wines, with even the tiniest traces of this substance suffice.

Phenols are also important in the whiskey flavor carrier; among other things, this includes 4- ethylphenol, 4- Ethylguajacol, guaiacol, eugenol, syringaldehyde and vanillin, of which especially the three former are responsible for the smoky taste.