The main purpose of this process is to produce metallurgical coke. By-products are liquid coking products and gas. By distillation of liquid coking products, along with benzene, toluene and naphthalene, phenol, thiophene, pyridine and their homologues, as well as more complex analogues with condensed nuclei, are obtained. The share of coal phenol, in comparison with the obtained cumene method, is insignificant.
2. Halogen substitution in aromatic compounds
The substitution of a halogen for a hydroxyl group proceeds under harsh conditions and is known as the "Dow" process (1928)
Previously, phenol (from chlorobenzene) was obtained in this way, but now its importance has decreased due to the development of more economical methods that do not involve the cost of chlorine and alkali and the formation of a large amount of wastewater.
In activated haloarenes (containing, along with a halogen, a nitro group in about- and P- provisions), the substitution of the halogen proceeds under milder conditions:
This can be explained by the electron-withdrawing effect of the nitro group, which pulls the electron density of the benzene ring towards itself and thus participates in the stabilization of the σ-complex:
3. Raschig method
This is a modified chlorine method: benzene undergoes oxidative chlorination by the action of hydrogen chloride and air, and then, without releasing the formed chlorobenzene, it is hydrolyzed with water vapor in the presence of copper salts. As a result, chlorine is not consumed at all, and the overall process is reduced to the oxidation of benzene to phenol:
4. Sulfonate method
Phenols can be obtained in good yield by fusing aromatic sulfonic acids Ar-SO 3 H with a mixture of sodium and potassium hydroxides (reaction alkali melting) at 300С, followed by neutralization of the resulting alcoholate by adding acid:
The method is still used in industry (to obtain phenol) and is used in laboratory practice.
5. Cumol method
The first large-scale production of phenol by the cumene method was carried out in 1949 in the Soviet Union. Currently, this is the main method for obtaining phenol and acetone.
The method includes two stages: the oxidation of isopropylbenzene (cumene) with atmospheric oxygen to hydroperoxide and its acidic decomposition:
The advantage of this method is the absence of by-products and the high demand for end products - phenol and acetone. The method was developed in our country by R.Yu. Udris, B.D. Krutalov and others in 1949
6. From diazonium salts
The method consists in heating diazonium salts in dilute sulfuric acid, which leads to hydrolysis - the replacement of a diazo group by a hydroxy group. The synthesis is very convenient for obtaining hydroxyarenes in the laboratory:
The structure of phenols
The structure and distribution of electron density in a phenol molecule can be represented by the following scheme:
The dipole moment of phenol is 1.55 D and is directed towards the benzene ring. The hydroxyl group in relation to the benzene ring exhibits the –I effect and +M effect. Since the mesomeric effect of the hydroxy group prevails over the inductive one, the conjugation of the lone electron pairs of the oxygen atom with the -orbitals of the benzene ring has an electron-donating effect on the aromatic system, which increases its reactivity in electrophilic substitution reactions.
There are one-, two-, three-atomic phenols depending on the number of OH groups in the molecule (Fig. 1)
Rice. one. SINGLE-, TWO- AND TRI-ATOMIC PHENOLS
In accordance with the number of fused aromatic cycles in the molecule, there are (Fig. 2) phenols themselves (one aromatic ring - benzene derivatives), naphthols (2 fused rings - naphthalene derivatives), anthranols (3 fused rings - anthracene derivatives) and phenantrols (Fig. 2).
Rice. 2. MONO- AND POLYNUCLEAR PHENOLS
Nomenclature of alcohols.
For phenols, trivial names that have developed historically are widely used. Prefixes are also used in the names of substituted mononuclear phenols ortho-,meta- and pair -, used in the nomenclature of aromatic compounds. For more complex compounds, the atoms that are part of the aromatic cycles are numbered and the position of the substituents is indicated using digital indices (Fig. 3).
Rice. 3. NOMENCLATURE OF PHENOLS. Substituent groups and corresponding numerical indices are highlighted in different colors for clarity.
Chemical properties of phenols.
The benzene nucleus and the OH group combined in the phenol molecule affect each other, significantly increasing the reactivity of each other. The phenyl group pulls the lone electron pair away from the oxygen atom in the OH group (Fig. 4). As a result, the partial positive charge on the H atom of this group increases (indicated by d+), the polarity of the O–H bond increases, which manifests itself in an increase in the acidic properties of this group. Thus, compared to alcohols, phenols are stronger acids. The partial negative charge (denoted by d–), passing to the phenyl group, is concentrated in the positions ortho- and pair-(with respect to the OH group). These reaction sites can be attacked by reagents that tend to electronegative centers, the so-called electrophilic ("electron loving") reagents.
Rice. 4. ELECTRON DENSITY DISTRIBUTION IN PHENOL
As a result, two types of transformations are possible for phenols: the substitution of a hydrogen atom in the OH group and the substitution of the H-atomobenzene nucleus. A pair of electrons of the O atom, drawn to the benzene ring, increases the strength of the C–O bond, so reactions that occur with the breaking of this bond, which are characteristic of alcohols, are not typical for phenols.
1. Substitution reactions of the hydrogen atom in the OH group. When phenols are treated with alkalis, phenolates are formed (Fig. 5A), the catalytic reaction with alcohols leads to ethers (Fig. 5B), and as a result of the reaction with anhydrides or acid chlorides of carboxylic acids, esters are formed (Fig. 5C). When interacting with ammonia (elevated temperature and pressure), the OH group is replaced by NH 2, aniline is formed (Fig. 5D), reducing reagents convert phenol to benzene (Fig. 5E)
2. Substitution reactions of hydrogen atoms in the benzene ring.
During halogenation, nitration, sulfonation and alkylation of phenol, centers with increased electron density are attacked (Fig. 4), i.e. substitution takes place mainly in ortho- and pair- positions (fig.6).
With a deeper reaction, two and three hydrogen atoms are replaced in the benzene ring.
Of particular importance are the condensation reactions of phenols with aldehydes and ketones, in essence, this is alkylation, which takes place easily and under mild conditions (at 40–50 ° C, an aqueous medium in the presence of catalysts), while the carbon atom is in the form of a methylene group CH 2 or substituted methylene group (CHR or CR 2) is inserted between two phenol molecules. Such condensation often leads to the formation of polymeric products (Fig. 7).
Dihydric phenol (trade name bisphenol A, Fig. 7) is used as a component in the production of epoxy resins. The condensation of phenol with formaldehyde underlies the production of widely used phenol-formaldehyde resins (phenolic plastics).
Methods for obtaining phenols.
Phenols are isolated from coal tar, as well as from pyrolysis products of brown coal and wood (tar). The industrial method for obtaining C 6 H 5 OH phenol itself is based on the oxidation of the aromatic hydrocarbon cumene (isopropylbenzene) with atmospheric oxygen, followed by decomposition of the resulting hydroperoxide diluted with H 2 SO 4 (Fig. 8A). The reaction proceeds with a high yield and is attractive in that it allows one to obtain two technically valuable products at once - phenol and acetone. Another method is the catalytic hydrolysis of halogenated benzenes (Fig. 8B).
Rice. eight. METHODS FOR OBTAINING PHENOL
The use of phenols.
A solution of phenol is used as a disinfectant (carbolic acid). Diatomic phenols - pyrocatechol, resorcinol (Fig. 3), as well as hydroquinone ( pair- dihydroxybenzene) is used as antiseptics (antibacterial disinfectants), introduced into tanning agents for leather and fur, as stabilizers for lubricating oils and rubber, as well as for processing photographic materials and as reagents in analytical chemistry.
In the form of individual compounds, phenols are used to a limited extent, but their various derivatives are widely used. Phenols serve as starting compounds for the production of various polymeric products, such as phenol-aldehyde resins (Fig. 7), polyamides, and polyepoxides. Based on phenols, numerous drugs are obtained, for example, aspirin, salol, phenolphthalein, in addition, dyes, perfumes, plasticizers for polymers and plant protection products.
Mikhail Levitsky
DEFINITION
Phenols- derivatives of aromatic hydrocarbons, in the molecules of which the hydroxyl groups are directly bonded to the carbon atoms of the benzene ring. The functional group, like alcohols, is OH.
Phenol is a solid colorless crystalline substance, low melting point, very hygroscopic, with a characteristic odor. In air, phenol oxidizes, so its crystals initially acquire a pinkish tint (Fig. 1), and darken and become more red during long-term storage. It is slightly soluble in water at room temperature, but dissolves quickly and well at 60 - 70 o C. Phenol is fusible, its melting point is 43 o C. Poisonous.
Rice. 1. Phenol. Appearance.
Getting phenol
On an industrial scale, phenol is obtained from coal tar. Among the laboratory methods most often used are the following:
– hydrolysis of chlorobenzene
C 6 H 5 Cl + NaOH→C 6 H 5 OH + NaCl (kat = Cu, t 0).
— alkaline melting of salts of arenesulfonic acids
C 6 H 5 SO 3 Na + 2NaOH → C 6 H 5 OH + Na 2 SO 3 + H 2 O (t 0).
– cumene method (oxidation of isopropylbenzene)
C 6 H 5 -C (CH 3) H-CH 3 + O 2 → C 6 H 5 OH + CH 3 -C (O) -CH 3 (H +, t 0).
Chemical properties of phenol
Chemical transformations of phenol proceed mainly with splitting:
1) O-N connections
- interaction with metals
2C 6 H 5 OH + 2Na→ 2C 6 H 5 ONa + H 2 .
- interaction with alkalis
C 6 H 5 OH + NaOH → C 6 H 5 ONa + H 2 O.
— interaction with anhydrides of carboxylic acids
C 6 H 5 -OH + Cl-C (O) -O-C (O) -CH 3 → C 6 H 5 -O-C (O) -CH 3 + CH 3 COOH (t 0).
- interaction with carboxylic acid halides
C 6 H 5 -OH + Cl-C (O) -CH 3 → C 6 H 5 -O-C (O) -CH 3 + HCl (t 0).
- interaction with FeCl 3 (qualitative reaction to phenol - the appearance of a purple color that disappears when acid is added)
6C 6 H 5 OH + FeCl 3 → (C 6 H 5 OH) 3 + 3Cl -.
2) connections C sp 2 -H predominantly in about- and n-provisions
- bromination
C 6 H 5 -OH + 3Br 2 (aq) →Br 3 -C 6 H 2 -OH ↓ + 3HBr.
- nitration (formation of picric acid)
C 6 H 5 -OH + 3HONO 2 (conc) → (NO 2) 3 -C 6 H 2 -OH + 3H 2 O (H +).
3) a single 6π-electron cloud of the benzene ring
– hydrogenation
C 6 H 5 OH + 3H 2 → C 6 H 11 -OH (kat \u003d Ni, t 0 \u003d 130 - 150, p \u003d 5 - 20 atm).
Application of phenol
Phenol is used in large quantities for the production of dyes, phenol-formaldehyde plastics, and medicinal substances.
Of the diatomic phenols, resorcinol is used in medicine as an antiseptic and a substance for some clinical tests, and hydroquinone and other diatomic phenols are used as developers in the processing of photographic materials.
In medicine, lysol, which includes various phenols, is used to disinfect rooms and furniture.
Some phenols are used as antioxidants - substances that prevent food spoilage during long-term storage (fats, oils, food concentrates).
Examples of problem solving
EXAMPLE 1
| Exercise | An aqueous solution containing 32.9 g of phenol was treated with an excess of bromine. Calculate the mass of the resulting bromo derivative. |
| Decision | Let us write the reaction equation for the interaction of phenol with bromine: C 6 H 5 OH + 3Br 2 → C 6 H 2 Br 3 OH + 3HBr. As a result of this interaction, 2,4,6-tribromophenol is formed. Calculate the amount of phenol substance (molar mass is 94 g / mol): n (C 6 H 5 OH) \u003d m (C 6 H 5 OH) / M (C 6 H 5 OH); n (C 6 H 5 OH) \u003d 32.9 / 94 \u003d 0.35 mol. According to the reaction equation n(C 6 H 5 OH) :n(C 6 H 2 Br 3 OH) = 1:1, i.e. n (C 6 H 2 Br 3 OH) \u003d n (C 6 H 5 OH) \u003d 0.35 mol. Then the mass of 2,4,6-tribromophenol will be equal to (molar mass -331 g / mol): m (C 6 H 2 Br 3 OH) \u003d 0.35 × 331 \u003d 115.81 g. |
| Answer | The mass of the resulting bromo derivative is 115.81 g. |
EXAMPLE 2
| Exercise | How to get phenol from iodobenzene? Calculate the mass of phenol that can be obtained from 45.9 g of iodobenzene. |
| Decision | We write the reaction equation for the production of phenol from iodobenzene: C 6 H 5 I + NaOH → C 6 H 5 OH + NaI (kat = Cu, t 0). |
Phenol is a colorless crystalline substance with a very specific odor. This substance is widely used in the production of various dyes, plastics, various synthetic fibers (mainly nylon). Before the development of the petrochemical industry, phenol production was carried out exclusively from coal tars. Of course, this method was not able to cover all the needs of the rapidly developing industry in phenol, which has now become an important component of almost all the objects around us.
Phenol, the production of which has become an urgent need due to the emergence of an extremely wide range of new materials and substances, of which it is an integral ingredient, is used in the synthesis process. And it, in turn, is an important component of phenolics. Also, a large amount of phenol is processed into cyclohexanol, which is necessary for manufacturing on an industrial scale.
Another important area is the production of a mixture of creosols, which is synthesized into a creosol formaldehyde resin used to make many medicines, antiseptics and antioxidants. Therefore, today the production of phenol in large quantities is an important task of petrochemistry. Many methods have already been developed to produce this substance in sufficient volumes. Let's dwell on the main ones.
The oldest and most proven method is the alkaline melting method, which is characterized by a large consumption of sulfuric acid for and caustic, followed by their fusion into benzenesulfonatrium salt, from which this substance is directly separated. Obtaining phenol by the method of benzene chlorination followed by saponification of chlorobenzene is profitable only if there is a large amount of cheap electricity needed for the production of caustic and chlorine. The main disadvantages of this technique are the need to create high pressure (at least three hundred atmospheres) and an extremely significant degree of equipment corrosion.
A more modern method is to obtain phenol by decomposition of isopropylbenzene hydroperoxide. True, the scheme for isolating the required substance here is rather complicated, since it involves the preliminary production of hydroperoxide by the method of benzene alkylation with a propylene solution. Further, the technology provides for the oxidation of the resulting isopropylbenzene with an air mixture to form hydroperoxide. As a positive factor of this technique, one can note the production of another important substance, acetone, in parallel with phenol.
There is also a method for isolating phenol from coke and semi-coke tars of solid fuel materials. Such a procedure is necessary not only to obtain valuable phenol, but also to improve the quality of various hydrocarbon products. One of the properties of phenol is rapid oxidation, which leads to accelerated aging of the oil and the formation of viscous resinous fractions in it.
But the most modern method and the latest achievement of the petrochemical industry is to obtain phenol from benzene directly by oxidizing it. The whole process is carried out in a special adiabatic reactor, which contains a zeolite-containing catalyst. The original nitrous oxide is obtained by oxidizing ammonia with air or by separating it from, more precisely, from its by-products formed during the synthesis. This technology is capable of producing high-purity phenol with a minimum total content of impurities.
Based on benzene. Under normal conditions, they are solid toxic substances with a specific aroma. In modern industry, these chemical compounds play an important role. In terms of use, phenol and its derivatives are among the twenty most demanded chemical compounds in the world. They are widely used in the chemical and light industries, pharmaceuticals and energy. Therefore, the production of phenol on an industrial scale is one of the main tasks of the chemical industry.
Phenol designations
The original name of phenol is carbolic acid. Later, this compound learned the name "phenol". The formula of this substance is shown in the figure:
The phenol atoms are numbered starting from the carbon atom that is connected to the OH hydroxo group. The sequence continues in such order that the other substituted atoms receive the lowest numbers. Phenol derivatives exist as three elements whose characteristics are explained by the difference in their structural isomers. Various ortho-, meta-, paracresols are only a modification of the basic structure of the compound of the benzene ring and the hydroxyl group, the basic combination of which is phenol. The formula of this substance in chemical notation looks like C 6 H 5 OH.
Physical properties of phenol
Visually, phenol is a solid colorless crystals. In the open air, they oxidize, giving the substance a characteristic pink tint. Under normal conditions, phenol is rather poorly soluble in water, but with an increase in temperature to 70 ° C, this figure increases sharply. In alkaline solutions, this substance is soluble in any quantities and at any temperature.
These properties are also preserved in other compounds, the main component of which is phenols.
Chemical properties
The unique properties of phenol are explained by its internal structure. In the molecule of this chemical, the p-orbital of oxygen forms a single p-system with the benzene ring. This tight interaction increases the electron density of the aromatic ring and lowers that of the oxygen atom. In this case, the polarity of the bonds of the hydroxo group increases significantly, and the hydrogen in its composition is easily replaced by any alkali metal. This is how various phenolates are formed. These compounds do not decompose with water, like alcoholates, but their solutions are very similar to salts of strong bases and weak acids, so they have a fairly pronounced alkaline reaction. Phenolates interact with various acids, as a result of the reaction, phenols are reduced. The chemical properties of this compound allow it to interact with acids, thus forming esters. For example, the interaction of phenol and acetic acid leads to the formation of phenyl ester (phenyacetate).
The nitration reaction is widely known, in which, under the influence of 20% nitric acid, phenol forms a mixture of para- and orthonitrophenols. If phenol is treated with concentrated nitric acid, 2,4,6-trinitrophenol is obtained, which is sometimes called picric acid.
Phenol in nature
As an independent substance, phenol is found in nature in coal tar and in certain grades of oil. But for industrial needs, this amount does not play any role. Therefore, obtaining phenol artificially has become a priority for many generations of scientists. Fortunately, this problem was solved and artificial phenol was obtained as a result.
properties, getting
The use of various halogens makes it possible to obtain phenolates, from which benzene is formed during further processing. For example, heating sodium hydroxide and chlorobenzene produces sodium phenolate, which decomposes into salt, water, and phenol when exposed to acid. The formula for this reaction is given here:
C 6 H 5 -CI + 2NaOH -> C 6 H 5 -ONa + NaCl + H 2 O
Aromatic sulfonic acids are also a source for the production of benzene. The chemical reaction is carried out with the simultaneous melting of alkali and sulfonic acid. As can be seen from the reaction, phenoxides are formed first. When treated with strong acids, they are reduced to polyhydric phenols.
Phenol in industry
In theory, obtaining phenol in the simplest and most promising way looks like this: using a catalyst, benzene is oxidized with oxygen. But so far, the catalyst for this reaction has not been found. Therefore, other methods are currently used in industry.
A continuous industrial method for producing phenol consists in the interaction of chlorobenzene and 7% sodium hydroxide solution. The resulting mixture is passed through a one and a half kilometer system of pipes heated to a temperature of 300 C. Under the influence of temperature and maintained high pressure, the starting materials react to obtain 2,4-dinitrophenol and other products.
Not so long ago, an industrial method for obtaining phenol-containing substances by the cumene method was developed. This process consists of two stages. First, isopropylbenzene (cumene) is obtained from benzene. To do this, benzene is alkylated with propylene. The reaction looks like this:
After that, the cumene is oxidized with oxygen. The output of the second reaction is phenol and another important product, acetone.
Production of phenol on an industrial scale is possible from toluene. To do this, toluene is oxidized on the oxygen contained in the air. The reaction proceeds in the presence of a catalyst.
Examples of phenols
The closest homologues of phenols are called cresols.
There are three types of cresols. Meta-cresol under normal conditions is a liquid, para-cresol and ortho-cresol are solids. All cresols are poorly soluble in water, and in their chemical properties they are almost similar to phenol. Cresols are naturally found in coal tar, they are used in industry in the production of dyes and some types of plastics.
Examples of dihydric phenols are para-, ortho- and meta-hydrobenzenes. All of them are solids, readily soluble in water.
The only representative of trihydric phenol is pyrogallol (1,2,3-trihydroxybenzene). Its formula is shown below.
Pyrogallol is a fairly strong reducing agent. It is easily oxidized, so it is used to obtain gases purified from oxygen. This substance is well known to photographers, it is used as a developer.
