Chemical properties of monohydric phenols. Preparation, chemical properties and use of phenol. Examples of problem solving
Carbolic acid is one of the names of phenol, indicating its special behavior in chemical processes. This substance undergoes nucleophilic substitution reactions more easily than benzene. The inherent acidic properties of the compound are explained by the mobility of the hydrogen atom in the hydroxyl group associated with the ring. Studying the structure of the molecule and qualitative reactions to phenol make it possible to classify the substance as an aromatic compound - benzene derivatives.
Phenol (hydroxybenzene)
In 1834, the German chemist Runge isolated carbolic acid from coal tar, but was unable to decipher its composition. Later, other researchers proposed a formula and classified the new compound as an aromatic alcohol. The simplest representative of this group is phenol (hydroxybenzene). In its pure form, this substance is transparent crystals with a characteristic odor. When exposed to air, the color of phenol may change, becoming pink or red. Aromatic alcohol is characterized by poor solubility in cold water and good solubility in organic solvents. Phenol melts at a temperature of 43°C. It is a toxic compound and causes severe burns upon contact with skin. The aromatic part of the molecule is represented by the phenyl radical (C6H5—). The oxygen of the hydroxyl group (—OH) is directly bonded to one of the carbon atoms. The presence of each particle is demonstrated by a corresponding qualitative reaction to phenol. The formula showing the total content of atoms of chemical elements in a molecule is C6H6O. The structure is reflected by the inclusion of the Kekule cycle and the functional group - hydroxyl. A visual representation of the aromatic alcohol molecule is provided by ball-and-stick models.
Features of the structure of the molecule
The interaction of the benzene ring and the OH group determines the chemical reactions of phenol with metals, halogens, and other substances. The presence of an oxygen atom associated with the aromatic ring leads to a redistribution of electron density in the molecule. The O-H bond becomes more polar, which leads to an increase in the mobility of hydrogen in the hydroxyl group. The proton can be replaced by metal atoms, which indicates the acidity of the phenol. In turn, the OH group increases the reaction properties of the benzene ring. The delocalization of electrons and the ability for electrophilic substitution in the nucleus increases. In this case, the mobility of hydrogen atoms associated with carbon in the ortho and para positions increases (2, 4, 6). This effect is due to the presence of an electron density donor—the hydroxyl group. Thanks to its influence, phenol behaves more actively than benzene in reactions with certain substances, and new substituents are oriented to ortho- and para-positions.
Acid properties
In the hydroxyl group of aromatic alcohols, the oxygen atom acquires a positive charge, weakening its bond with hydrogen. The release of the proton is facilitated, so phenol behaves like a weak acid, but stronger than alcohols. Qualitative reactions to phenol include testing with litmus paper, which changes color from blue to pink in the presence of protons. The presence of halogen atoms or nitro groups associated with the benzene ring leads to an increase in hydrogen activity. The effect is observed in molecules of nitro derivatives of phenol. Substituents such as amino group and alkyl (CH3-, C2H5- and others) reduce acidity. Compounds that combine a benzene ring, a hydroxyl group and a methyl radical include cresol. Its properties are weaker than carbolic acid.
Reaction of phenol with sodium and alkali
Like acids, phenol interacts with metals. For example, it reacts with sodium: 2C6H5—OH + 2Na = 2C6H5—ONa + H2. Hydrogen gas is formed and released. Phenol reacts with soluble bases. Occurs with the formation of salt and water: C6H5–OH + NaOH = C6H5–ONa + H2O. The ability to donate hydrogen in the hydroxyl group of phenol is lower than that of most inorganic and carboxylic acids. Even carbon dioxide (carbonic acid) dissolved in water displaces it from salts. Reaction equation: C6H5—ONa + CO2 + H2O = C6H5—OH + NaHCO3.
Benzene ring reactions
The aromatic properties are due to the delocalization of electrons in the benzene ring. Hydrogen from the ring is replaced by halogen atoms and a nitro group. A similar process in the phenol molecule occurs more easily than in benzene. One example is bromination. The halogen acts on benzene in the presence of a catalyst, producing bromobenzene. Phenol reacts with bromine water under normal conditions. As a result of the interaction, a white precipitate of 2,4,6-tribromophenol is formed, the appearance of which makes it possible to distinguish the test substance from similar aromatic compounds. Bromination is a qualitative reaction to phenol. Equation: C6H5–OH + 3Br2 = C6H2Br3 + HBr. The second product of the reaction is hydrogen bromide. When phenol reacts with a dilute solution, nitro derivatives are obtained. The product of the reaction with concentrated nitric acid, 2,4,6-trinitrophenol or picric acid, is of great practical importance.
Qualitative reactions to phenol. List
When substances interact, certain products are obtained that make it possible to establish the qualitative composition of the starting substances. A number of color reactions indicate the presence of particles and functional groups, which is convenient for chemical analysis. Qualitative reactions to phenol prove the presence of an aromatic ring and an OH group in the molecule of the substance:
- In a phenol solution, blue litmus paper turns red.
- Color reactions to phenols are also carried out in a weak alkaline medium with diazonium salts. Yellow or orange azo dyes are formed.
- Reacts with brown bromine water, producing a white precipitate of tribromophenol.
- As a result of the reaction with a solution of ferric chloride, ferric phenoxide is obtained - a substance with a blue, violet or green color.
Preparation of phenols
The production of phenol in industry occurs in two or three stages. At the first stage, cumene (the trivial name for isopropylbenzene) is obtained from propylene and benzene in the presence. Friedel-Crafts reaction equation: C6H5—OH + C3H6 = C9H12 (cumene). Benzene and propylene in a 3:1 ratio are passed over an acid catalyst. Increasingly, instead of the traditional catalyst - aluminum chloride - environmentally friendly zeolites are used. At the final stage, oxidation is carried out with oxygen in the presence of sulfuric acid: C6H5—C3H7 + O2 = C6H5—OH + C3H6O. Phenols can be obtained from coal by distillation and are intermediate compounds in the production of other organic substances.
Use of phenols
Aromatic alcohols are widely used in the production of plastics, dyes, pesticides and other substances. The production of carbolic acid from benzene is the first step in the creation of a number of polymers, including polycarbonates. Phenol reacts with formaldehyde to produce phenol-formaldehyde resins.
Cyclohexanol serves as a raw material for the production of polyamides. Phenols are used as antiseptics and disinfectants in deodorants and lotions. Used to produce phenacetin, salicylic acid and other drugs. Phenols are used in the production of resins, which are used in electrical products (switches, sockets). They are also used in the preparation of azo dyes such as phenylamine (aniline). Picric acid, which is a nitro derivative of phenol, is used for dyeing fabrics and making explosives.
Malyshenkova E., Shikula E. Municipal educational institution lyceum No. 102 of Chelyabinsk. /2010 Chemistry, 10B.
TCS. Dependence of properties... on the structure.
Phenols (II)
The hydroxyl group in phenol is directly bonded to the carbon atom of the phenyl hydrocarbon radical, which influences it. Unlike the radicals of saturated hydrocarbons, which are electron donors, the benzene ring, more precisely, its C6H5- radical, called phenyl, has the ability to attract electrons to itself, in this case from the oxygen atoms of the hydroxyl group –OH. This leads to the appearance of a positive charge on the hydrogen atom of the hydroxyl group, which makes it more mobile compared to the hydrogen atom in the –OH group of alcohols, and the phenol substance itself exhibits acidic properties.
In its turn, the hydroxyl group affects the radical. Under the influence of the –OH functional group in the benzene ring of phenol, the electron density is distributed unevenly: a partial negative charge is concentrated on the carbon atoms located in the 2,4,6-positions. This facilitates the substitution reactions of hydrogen atoms of the benzene ring precisely in the indicated positions. As a result of substitution reactions, 2,4,6-phenol derivatives are obtained.
Physical properties
Phenol is a solid, colorless, crystalline substance, low-melting, very hygroscopic, with a characteristic odor. Phenol oxidizes in air. Slightly soluble in water. Phenol is fusible, melting point 43°C. PHENOL IS TOXIC!!!
Chemical properties
The presence in the phenol molecule of both a hydroxyl group and a benzene ring in the hydrocarbon radical phenyl determines its chemical properties. The presence of the –OH group in a molecule gives some of its properties similar to the properties of alcohols.
Phenol reacts with alkali metals
Unlike monohydric alcohols, phenol reacts with alkalis.
Phenol reacts with bromine water.
Phenol reacts with nitric acid.
Phenol reacts with hydrogen.
Phenol reacts with sulfuric acid.
Reaction of phenol with sodium
Reaction of phenol with sodium hydroxide
Reaction of phenol with bromine water
Reaction of phenol with nitric acid
Reactions of phenol with sulfuric acid
Reaction of phenol with hydrogen
Qualitative reactions to phenol /with bromine water/
Reaction of phenol with iron chloride (III 
conclusions
Phenols are derivatives of aromatic hydrocarbons (primarily benzene), in the molecules of which one or more hydroxyl groups are directly bonded to the carbon atoms of the benzene ring.
The chemical properties of phenol are determined by both the functional group -OH and the hydrocarbon aromatic radical - phenyl (C6H5-). The properties of phenol are affected by the mutual influence of the hydroxyl group and the benzene ring: unlike alcohols, it is able to interact as a weak acid with alkalis, unlike benzene, in reactions of substitution of hydrogen atoms of the benzene ring, phenol forms 2,4,6-derivatives (tribromophenol, trinitrophenol, etc.).
Compounds with one or more hydroxyl groups attached to a benzene ring; are called phenols. The most important of these is phenol itself:
Phenol was discovered in 1834, when it was isolated from coal tar. It was first called carbolic acid, and this name is still used today for liquid phenol containing 5% water. Phenol received its current name in 1841.
All the simplest phenols under normal conditions are solids with a low melting point. Phenol is a colorless crystalline substance with a melting point of 43°C. It has a characteristic smell. Like alcohols, phenols have higher boiling points than would be expected from their relative molecular weight. This is due to the formation of intermolecular hydrogen bonds in phenols. It was already noted above that 2-nitrophenol has a lower boiling point than 4-nitrophenol. This is explained by the existence of an intramolecular hydrogen bond in the first of these compounds, while in the second compound there are intermolecular hydrogen bonds, making it less volatile (see Section 2.2).
Phenols are poorly soluble in water, but are highly soluble in organic solvents, in particular in alcohols and ethers. Phenol has limited miscibility with water only at temperatures below 66°C. Above 66°C, phenol mixes with water in any proportions (see Fig. 6.22 and Section 6.2).
Laboratory methods of obtaining
To obtain phenol in laboratory conditions, anhydrous sodium salt of benzenesulfonic acid is fused with solid sodium hydroxide at 300-350°C, and then dilute hydrochloric acid is added to the mixture:
Benzenesulfonic acid is prepared by sulfonation of benzene (see Section 18.2). Neutralization of this acid with sodium hydroxide leads to the formation of its sodium salt.
Phenol is also obtained by heating an aqueous solution of phenyldiazonium chloride above 10°C:
Phenyldiazonium chloride is prepared by diazotization of phenylamine (see Section 19.4).
Chemical properties of phenols
Hydroxyl group reactions. Acidity. Phenol has an acidity constant of 9.95. Thus, it has the properties of a weak acid, although stronger than Methanol, ethanol and water (see Table 19.4). The phenolation resulting from the elimination of the ion is stabilized due to delocalization
negative charge:
It can be considered as a hybrid of the indicated resonant forms (see sections 2.1 and 18.2).
Like alcohols, phenol reacts with strongly electropositive metals, such as sodium, releasing hydrogen:
However, unlike alcohols, phenols react with sodium hydroxide:
Phenol is not as acidic as carboxylic acids. Carboxylic acids, such as acetic or benzoic acids, are capable of displacing carbon dioxide from sodium bicarbonate or sodium carbonate, but phenol is not. This reaction is used for analytical purposes to distinguish carboxylic acids from phenols.
Formation of esters. Although phenol does not react with carboxylic acids to form esters, it does react with carboxylic acid chlorides in alkaline solutions:
This type of reaction is called acylation.
Formation of ethers. Phenol reacts with haloalkanes in an alkaline medium, forming ethers:
This reaction is an example of the Williamson synthesis (see previous section).
Reaction with phosphorus pentachloride. Unlike alcohols, phenol does not react with hydrogen halides and phosphorus trihalides. However, it reacts slowly with phosphorus pentachloride to form chlorobenzene in low yield:
Reaction with iron (III) chloride. When a neutral solution of iron (III) chloride is added to phenol, a complex with a violet color is formed. This reaction is used as an analytical sample for phenol. This reaction is typical for compounds containing an enol group.
Reaction in the benzene ring. The benzene ring in the phenol molecule undergoes electrophilic substitution more easily than benzene itself. This is because the non-bonding electrons on the oxygen atom are drawn into the benzene ring and thereby activate it. The hydroxyl group of phenol has a 2,4-directing effect towards electrophilic substituents (see Section 18.2).
Halogenation. The halogenation of phenols is carried out under much milder conditions than the halogenation of benzene. For example, when bromine water is added to an aqueous solution of phenol, a white precipitate of 2,4,6-tribromophenol is formed:
In Sect. 18.2 it was indicated that the bromination of benzene requires the presence of a catalyst.
Nitration. Phenol can be nitrated using dilute nitric acid. This produces a mixture of 2-nitrophenol and 4-nitrophenol:
Let us again compare these mild conditions with the conditions for the corresponding reaction of benzene. Nitration of benzene must be carried out in a mixture of concentrated nitric acid and sulfuric acid (see section 18.2).
2-Nitrophenol and 4-nitrophenol are stronger acids than phenol. Both of them are characterized by values approximately equal to 7.2. The increased acidity of nitrophenols is explained by the fact that the nitro group withdraws electrons. As a result, the benzene ring withdraws electrons more strongly from the oxygen atom of the hydroxyl group.
Sulfonation. The reaction of phenol with concentrated sulfuric acid leads to the formation of a mixture of hydroxybenzenesulfonic acids:
Hydroxybenzenesulfonic acid (yield 85%)
Both products of this reaction react with concentrated nitric acid to form 2,4,6-trinitrophenol, a yellow crystalline substance known by the trivial name "picric acid":
Due to the common electron-withdrawing effect of the three nitro groups, picric acid is a relatively strong acid. It is characterized by an acidity constant close to 1, and when interacting with a solution of sodium carbonate, it displaces carbon dioxide from it.
Combination reactions. An alkaline solution of phenol reacts with a solution of phenyldiazonium chloride, resulting in the formation of an orange precipitate of 4-hydroxyphenylazo-benzene:
This product is an azo dye. This type of reaction is called a coupling reaction (in this case azo coupling).
Description of the presentation by individual slides:
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Purpose: To characterize the physical and chemical properties of phenol. To show the negative and positive role of phenol and its derivatives in nature and human life.
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Phenol (hydroxybenzene, obsolete carbolic acid) C6H5OH is the simplest representative of the class of phenols. Crystalline, colorless substance with a characteristic odor. In air it oxidizes easily, first acquiring a pink, then brown color. Needle crystals of phenol
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Classification of phenols One-, two-, and trihydric phenols are distinguished depending on the number of OH groups in the molecule:
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Preparation Cumene method for producing phenol (USSR, Sergeev P.G., Udris R.Yu., Kruzhalov B.D., 1949). Advantages of the method: waste-free technology (yield of useful products > 99%) and cost-effectiveness. By fusing salts of aromatic sulfonic acids with solid alkalis: C6H5-SO3Na + NaOH t → Na2SO3 + C6H5 – OH From coal tar: C6H5ONa + H2SO4 (diluted) → C6H5 – OH + NaHSO4 From halobenzenes: C6H5-Cl + NaOH t,p → C6H5 – OH + NaCl
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Physical properties Soluble in water (6 g per 100 g of water), in alkali solutions, in alcohol, in benzene, in acetone. Phenol is extremely toxic and dangerous to the human body
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Electronic structure The hydroxyl group -OH is a substituent of the first kind, that is, it helps to increase the electron density in the benzene ring (especially in the ortho and para positions). This is due to the fact that one of the lone pairs of electrons of the oxygen atom of the OH group enters into conjugation with the π-system of the benzene ring. The displacement of the lone pair of electrons of the oxygen atom towards the benzene ring leads to an increase in the polarity of the O-H bond.
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Chemical properties Interaction with metallic sodium: 2C6H5OH + 2Na 2C6H5ONa + H2 It has weak acidic properties; when exposed to alkalis, it forms salts - phenolates (for example, sodium phenolate - C6H5ONa): C6H5OH + NaOH C6H5ONa + H2O Interaction with bromine water (qualitative reaction to phenol) : C6H5OH + 3Br2 C6H2Br3OH + 3HBr (white solid 2,4,6-tribromophenol is formed) Reaction with concentrated nitric acid: C6H5OH + 3HNO3 C6H2(NO2)3OH + 3H2O (2,4,6-trinitrophenol is formed) Reaction with chloride iron(III) (qualitative reaction to phenol): 6C6H5OH + FeCl3 Cl3
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Biological role Essential oils (have strong bactericidal and antiviral properties, stimulate the immune system, increase blood pressure: - anethole in dill, fennel, anise - carvacrol and thymol in thyme - eugenol in cloves, basil Flavonoids (help remove radioactive elements from the body) Medicinal drugs (purgen, paracetamol) Antiseptics (3-5% solution - carbolic acid) Phenol is one of the industrial pollutants. Phenol is quite toxic to animals and humans. Phenol is destructive to many microorganisms, therefore industrial wastewater with a high phenol content is difficult to treat biologically. .
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Toxic properties Refers to highly hazardous substances (Hazard Class II). When inhaled, it causes dysfunction of the nervous system. Dust, vapors and phenol solution irritate the mucous membranes of the eyes, respiratory tract, and skin, causing chemical burns. Once on the skin, phenol is very quickly absorbed even through intact areas and within a few minutes begins to affect brain tissue.
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Toxic properties First, short-term excitement occurs, and then paralysis of the respiratory center. Even when exposed to minimal doses of phenol, sneezing, coughing, headache, dizziness, pallor, nausea, and loss of strength are observed. Severe cases of poisoning are characterized by unconsciousness, cyanosis, difficulty breathing, insensitivity of the cornea, rapid, barely perceptible pulse, cold sweat, and often convulsions.
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Effect on the body When a phenolic solution gets on the skin, severe chemical burns immediately appear, turning into ulcers. If a quarter of the entire surface of the body is burned by exposure to phenol, then the probability of death is close to 100%. Entry of the substance into the body through the gastrointestinal tract makes movement difficult and can cause bleeding and ulcers. The lethal dose for humans when ingested is 1-10 g, for children 0.05-0.5 g. Despite the rapid elimination period from the body (only a day), phenol can cause irreparable damage, and treatment can take several years. The most serious consequences are the development of cancer, infertility, and heart problems.
The names of phenols are compiled taking into account the fact that for the parent structure, according to IUPAC rules, the trivial name “phenol” is retained. The numbering of the carbon atoms of the benzene ring starts from the atom directly bonded to the hydroxyl group (if it is the highest function), and continues in such a sequence that the available substituents receive the lowest numbers.
Mono-substituted phenol derivatives, for example methylphenol (cresol), can exist in the form of three structural isomers - ortho-, meta- and para-cresols.
Physical properties.
Phenols are mostly crystalline substances (-cresol - liquid) at room temperature. They have a characteristic odor, are rather poorly soluble in water, but dissolve well in aqueous solutions of alkalis (see below). Phenols form strong hydrogen bonds and have fairly high boiling points.
Methods of obtaining.
1. Preparation from halobenzenes. When chlorobenzene and sodium hydroxide are heated under pressure, sodium phenolate is obtained, upon further processing of which with acid, phenol is formed:
2. Preparation from aromatic sulfonic acids (see reaction 3 in the section “Chemical properties of benzene”, § 21). The reaction is carried out by fusing sulfonic acids with alkalis. The initially formed phenoxides are treated with strong acids to obtain free phenols. The method is usually used to obtain polyhydric phenols:
Chemical properties.
In phenols, the p-orbital of the oxygen atom forms a single -system with the aromatic ring. As a result of this interaction, the electron density of the oxygen atom decreases and that of the benzene ring increases. The polarity of the O-H bond increases, and the hydrogen of the OH group becomes more reactive and is easily replaced by a metal even under the action of alkalis (unlike saturated monohydric alcohols).
In addition, as a result of such mutual influence in the phenol molecule, the reactivity of the benzene ring in the ortho and cara positions in electrophilic substitution reactions (halogenation, nitration, polycondensation, etc.) increases:
1. The acidic properties of phenol manifest themselves in reactions with alkalis (the old name “carbolic acid” has been preserved):
Phenol, however, is a very weak acid. When carbon dioxide or sulfur dioxide gases are passed through a solution of phenolates, phenol is released - this reaction proves that phenol is a weaker acid than carbonic and sulfur dioxide:
The acidic properties of phenols are weakened by the introduction of substituents of the first kind into the ring and enhanced by the introduction of substituents of the second kind.
2. Formation of esters. Unlike alcohols, phenols do not form esters when exposed to carboxylic acids; For this purpose, acid chlorides are used:
3. Halogenation. When phenol is exposed to bromine water (compare with the conditions for the bromination of benzene - § 21), a precipitate of 2,4,6-tribromophenol is formed:
This is a qualitative reaction for the detection of phenol.
4. Nitration. Under the influence of 20% nitric acid, phenol is easily converted into a mixture of ortho- and para-nitrophenols. If phenol is nitrated with concentrated nitric acid, 2,4,6-trinitrophenol is formed - a strong acid (picric acid).
5. Oxidation. Phenols are easily oxidized even under the influence of atmospheric oxygen.
Thus, when standing in air, phenol gradually turns pinkish-red. During the vigorous oxidation of phenol with a chromium mixture, the main oxidation product is quinone. Diatomic phenols are oxidized even more easily. The oxidation of hydroquinone produces quinone:
