Phenols are the common name for aromatic alcohols. By their properties, substances are weak acids. Of great practical importance are many homologues of hydroxybenzene C 6 H 5 0H (phenol formula) - the simplest representative of the class. Let us dwell on this in more detail.
Phenols. General formula and classification
The general formula for organic substances related to aromatic alcohols is R-OH. The molecules of phenols and cresols proper are formed by the radical - phenyl 65, to which one or several OH hydroxyl groups (hydroxy groups) are directly attached. According to their number in the molecule, phenols are classified into mono-, di- and polyatomic. Monatomic compounds of this type are phenol and cresol. The most common among polyhydric hydroxybenzenes are naphthols, which contain 2 condensed nuclei.
Phenol - a representative of aromatic alcohols
Phenol was already known to textile workers in the 18th century: weavers used it as a dye. During the distillation of coal tar in 1834, a chemist from Germany F. Runge isolated crystals of this substance with a characteristic sweetish smell. The Latin name for coal is carbo, so the compound was called carbolic acid ( carbolic acid ). The German researcher was unable to determine the composition of the substance. The molecular formula of phenol was established in 1842 by O. Laurent, who considered carbolic a derivative of benzene. For the new acid, the name "phenyl" was used. Charles Gerard determined that the substance is alcohol, and called it phenol. The initial fields of application of the compound are medicine, leather tanning, and the production of synthetic dyes. Characteristics of the substance in question:
- The rational chemical formula is C 6 H 5 OH.
- The molecular weight of the compound is 94.11 a. eat.
- The gross formula for the composition is C 6 H 6 O.
Electronic and spatial structure of the phenol molecule
The cyclic structural formula of benzene was proposed by the German organic chemist F. Kekule in 1865, and shortly before it, I. Loshmidt. Scientists imagined the molecule of organic matter in the form of a regular hexagon with alternating simple and double bonds. According to modern concepts, the aromatic core is a special type of ring structure, called the “conjugated bond”.
Six carbon atoms C undergo sp 2 hybridization of electronic orbitals. The p-electron clouds not participating in the formation of C – C bonds overlap above and below the plane of the nucleus of the molecule. There are two parts of a common electron cloud that spans the entire ring. The structural formula of phenol may look different, given the historical approach to the description of the structure of benzene. To emphasize the unsaturated nature of aromatic hydrocarbons, three of the six bonds, which alternate with three simple ones, are conventionally considered double.
Oxy group bond polarization
In the simplest aromatic hydrocarbon - C 6 H 6 benzene - the electron cloud is symmetrical. The phenol formula differs by one oxy group. The presence of hydroxyl breaks the symmetry, which is reflected in the properties of the substance. The bond between oxygen and hydrogen in the oxy group is polar covalent. The shift of the total pair of electrons to the oxygen atom leads to the appearance of a negative (partial) charge on it. Hydrogen loses an electron and acquires a partial “+” charge. In addition, oxygen in the O — H group possesses two unshared electron pairs. One of them is attracted by the electron cloud of the aromatic nucleus. For this reason, the bond becomes more polarized, the hydrogen atom is more easily replaced by metals. Models give an idea of the asymmetric nature of the phenol molecule.
Features of the mutual influence of atoms in phenol
A single electron cloud of the aromatic nucleus in the phenol molecule interacts with the hydroxyl group. A phenomenon called conjugation occurs, as a result of which the intrinsic pair of electrons of the oxygen atom of the oxy group is attracted to the benzene ring system. The decrease in the negative charge is compensated by an even greater polarization of the bond in the O – H group.
The aromatic nucleus also changes the electronic distribution system. It decreases on carbon, which is bound to oxygen, and increases on the atoms nearest to it located in the ortho positions (2 and 6). Pairing causes the accumulation of a charge “-” on them. A further "density shift is its movement from atoms in the meta-positions (3 and 5) to carbon in the para-position (4). The phenol formula for the convenience of studying conjugation and mutual influence usually contains the numbering of atoms of the benzene ring.
Explanation of the chemical properties of phenol based on its electronic structure
The conjugation processes of the aromatic nucleus and hydroxyl affect the properties of both particles and the whole substance. For example, the high electron density of atoms in the ortho and para positions (2, 4, 6) makes the C – H bonds of the aromatic phenol cycle more reactive. The negative charge of carbon atoms in the meta positions decreases (3 and 5). Electrophilic particles are attacked in chemical reactions by carbon in the ortho and para positions. In the benzene bromination reaction, changes occur with strong heating and the presence of a catalyst. A monohalogenated derivative is formed - bromobenzene. The phenol formula allows the substance to react with bromine almost instantly without heating the mixture.
The aromatic core affects the polarity of the bond in the oxy group, increasing it. The hydrogen atom becomes more mobile, compared to saturated alcohols. Phenol reacts with alkalis, forming salts - phenolates. Ethanol does not interact with alkalis, or rather, the reaction products - ethanolates - decompose. Chemically, phenols are stronger acids than alcohols.
Representatives of the class of aromatic alcohols
The gross formula of the phenol homolog — cresol (methylphenol, hydroxytoluene) —C 7 H 8 O. The substance in natural raw materials often accompanies phenol and also has antiseptic properties. Other phenol homologues:
- Pyrocatechol (1,2-hydroxybenzene). The chemical formula is C 6 H 4 (OH) 2 .
- Resorcinol (1,3-hydroxybenzene) - C 6 H 4 (OH) 2 .
- Pyrogallol (1,2,3-trihydroxybenzene) - C 6 H 3 (OH) 3 .
- Naftol. The gross formula of the substance is C 10 H 7 OH. It is used in the manufacture of dyes, medicines, fragrances.
- Thymol (2-isopropyl-5-methylphenol). The chemical formula is C 6 H 3 CH 3 (OH) (C 3 H 7 ). It is used in chemistry of organic synthesis, medicine.
- Vanillin, in addition to the phenolic radical, contains an ether group and an aldehyde residue. The gross formula of the compound is C 8 H 8 O 3 . Vanillin is widely used as an artificial fragrance.
Phenol recognition reagent formula
Qualitative determination of phenol can be carried out using bromine. As a result of the substitution reaction, a white precipitate of tribromphenol precipitates. Pyrocatechol (1,2-hydroxybenzene) turns green in the presence of dissolved ferric chloride. Phenol enters into a chemical reaction with the same reagent, and triphenolate is formed, which has a purple color. A qualitative reaction to resorcinol is the appearance of dark violet staining in the presence of ferric chloride. Gradually, the color of the solution turns black. The formula of the reagent, which is used to recognize phenol and some of its homologs, is FeCl 3 (iron (III) chloride).
Hydroxybenzene, naphthol, thymol are all phenols. The general formula and composition of substances makes it possible to determine whether these compounds belong to the aromatic series. All organic substances containing in their formula the phenyl radical C 6 H 5 with which hydroxy groups are directly bonded exhibit special properties. They differ from alcohols in a more pronounced acidic character. Compared with the substances of the homologous series of benzene, phenols are more active chemical compounds.