Phenols are aromatic compounds that have one or more hydroxyl groups attached to the carbon atoms of a benzene nucleus. According to the number of OH groups, mono-, di- and triatomic phenols are distinguished.
Monoatomic phenols are derivatives of benzene and its homologs, in the core of which one hydrogen atom is replaced by a hydroxyl group.
Isomerism and nomenclature. The simplest representative of phenols - carbolic acid (phenol) has no isomers; in its homologues there is an isomerism of the position of the hydroxyl group in the benzene nucleus (ortho, meta, para).
Three nomenclatures are used to name phenols - historical, rational, and IUPAC. According to the historical nomenclature, phenols are called trivially - carbolic acid (carbolic acid), cresols, etc.
The natural source for obtaining these substances is coal tar, stone seed oil, beech tar, etc. Coal tar is formed during the dry distillation of coal. Sources for phenols are medium (boiling at 170-230 ° C) and heavy (boiling at 230-270 ° C) oils. When they are treated with sodium hydroxide, sodium phenolate is obtained. The formula of the specified substance consists of a residue of phenol and sodium.
In laboratory conditions, most often, aromatic sulfosalts (sodium and potassium salts of sulfonic acids) are used to obtain phenols. In the process of chemical reactions, sodium or potassium phenolate is formed. Then these compounds are treated with mineral acids, resulting in free phenols.
The chemical properties of phenol are determined by the presence of an OH group in the benzene core. These substances can enter into reactions that are characteristic of alcohols (the formation of esters, phenolates, halogen derivatives) and arenes (the replacement of hydrogen atoms in a benzene nucleus with halogens, a nitro group, a sulfo group). Therefore, these substances easily interact with metals, forming sodium phenolate. It is under such conditions that the electronic structure of the molecules of alcohols and phenols is manifested.
Sodium phenolate (or phenoxide) is formed by the interaction of alkalis with phenols. The acidic properties of phenols are relatively poorly expressed. These substances do not stain litmus test. Sodium phenolate, unlike alcoholates, can exist in aqueous solutions of alkalis, while it does not decompose. Phenolates easily decompose upon interaction with acids (even the weakest, for example, coal).
Nevertheless, the acid properties of phenols are more pronounced than in aliphatic alcohols. The introduction of electron withdrawing substituents (nitro group, halogen, sulfo group, aldehyde group, etc.) into the phenol molecule increases the movement of hydrogen of the hydroxo group, therefore, the acid properties are enhanced.
The presence of a positive mesomeric effect in phenols determines their nucleophilic properties, which are less pronounced compared to alcohols. This property is used to obtain esters, but phenols themselves are not involved in the reactions, but phenolates and halogenated hydrocarbons.
The formation of esters occurs when phenols react with carboxylic acid chlorides or anhydrides. As with the formation of esters, reactions proceed more easily with potassium or sodium phenolates.
When halogens act on phenols, their halogen derivatives are formed. Phenol bromination is used in pharmaceutical analysis: 2,46-tribromophenol is poorly soluble in water and precipitates, which allows the use of this reaction to determine phenols in solutions.
Phenol nitration. Under the action of 20% nitric acid on phenol, a mixture of o-and p-nitrophenols is formed, which are separated by steam distillation (o-nitrophenol is distilled off, and p-nitrophenol remains in solution).