Chemical properties of phenols. Chemical properties of phenols acid properties of phenols Phenol carbonic acid

Phenols- derivatives of aromatic hydrocarbons, which may contain one or more hydroxyl groups connected to a benzene ring.

What are phenols called?

According to IUPAC rules, the name " phenol" The numbering of atoms comes from the atom that is directly bonded to the hydroxy group (if it is the senior one) and is numbered so that the substituents receive the lowest number.

Representative - phenol - C 6 H 5 OH:

The structure of phenol.

The oxygen atom has a lone electron pair at its outer level, which is “pulled” into the ring system (+M effect HE-groups). As a result, 2 effects can occur:

1) increasing the electron density of the benzene ring to the ortho- and para- positions. Basically, this effect manifests itself in electrophilic substitution reactions.

2) the density on the oxygen atom decreases, as a result of which the bond HE weakens and may tear. The effect is associated with the increased acidity of phenol compared to saturated alcohols.

Mono-substituted derivatives phenol(cresol) can be in 3 structural isomers:

Physical properties of phenols.

Phenols are crystalline substances at room temperature. Poorly soluble in cold water, but well soluble in hot water and in aqueous solutions of alkalis. They have a characteristic odor. Due to the formation of hydrogen bonds, they have a high boiling and melting point.

Preparation of phenols.

1. From halobenzenes. When chlorobenzene and sodium hydroxide are heated under pressure, sodium phenolate is obtained, which, after reacting with acid, turns into phenol:

2. Industrial method: the catalytic oxidation of cumene in air produces phenol and acetone:

3. From aromatic sulfonic acids by fusion with alkalis. The reaction most often carried out to produce polyhydric phenols is:

Chemical properties of phenols.

R The -orbital of the oxygen atom forms a single system with the aromatic ring. Therefore, the electron density on the oxygen atom decreases, and on the benzene ring it increases. Communication polarity HE increases, and the hydrogen of the hydroxyl group becomes more reactive and can easily be replaced by a metal atom even under the action of alkalis.

The acidity of phenols is higher than that of alcohols, so the following reactions can be carried out:

But phenol is a weak acid. If carbon dioxide or sulfur dioxide is passed through its salts, phenol is released, which proves that carbonic and sulfurous acids are stronger acids:

The acidic properties of phenols are weakened by the introduction of type I substituents into the ring and enhanced by the introduction of type II.

2) Formation of esters. The process occurs under the influence of acid chlorides:

3) Electrophilic substitution reaction. Because HE-group is a substituent of the first kind, then the reactivity of the benzene ring in the ortho- and para-positions increases. When phenol is exposed to bromine water, a precipitate is observed - this is a qualitative reaction to phenol:

4) Nitration of phenols. The reaction is carried out with a nitrating mixture, resulting in the formation of picric acid:

5) Polycondensation of phenols. The reaction occurs under the influence of catalysts:

6) Oxidation of phenols. Phenols are easily oxidized by atmospheric oxygen:

7) A qualitative reaction to phenol is the effect of a solution of ferric chloride and the formation of a violet complex.

Application of phenols.

Phenols are used in the production of phenol-formaldehyde resins, synthetic fibers, dyes and medicines, and disinfectants. Picric acid is used as explosives.

The hydroxyl group in molecules of organic compounds can be associated with aromatic core directly, or can be separated from it by one or more carbon atoms. It can be expected that, depending on this property, substances will differ significantly from each other due to the mutual influence of groups of atoms. Indeed, organic compounds containing the aromatic radical phenyl C 6 H 5 - directly bonded to the hydroxyl group exhibit special properties, different from the properties of alcohols. Such connections are called phenols.

Organic substances whose molecules contain a phenyl radical linked to one or more hydroxo groups. Just like alcohols, phenols are classified according to their atomicity, i.e., according to the number of hydroxyl groups.

Monohydric phenols contain one hydroxyl group in the molecule:

Polyhydric phenols contain more than one hydroxyl group in molecules:

There are other polyhydric phenols containing three or more hydroxyl groups on the benzene ring.

Let's take a closer look at the structure and properties of the simplest representative of this class - phenol C 6 H 5 OH. The name of this substance formed the basis for the name of the entire class - phenols.

Phenol is a solid, colorless crystalline substance, t° = 43 °C, t° = 181 °C, with a sharp characteristic odor. Poisonous. Phenol is slightly soluble in water at room temperature. An aqueous solution of phenol is called carbolic acid. Upon contact with skin it causes burns, so phenol must be handled with care!

Chemical properties of phenols

Acid properties. The hydrogen atom of the hydroxyl group is acidic in nature. The acidic properties of phenol are more pronounced than that of water and alcohols. Unlike alcohols and water, phenol reacts not only with alkali metals, but also with alkalis to form phenolates:

However, the acidic properties of phenols are less pronounced than those of inorganic and carboxylic acids. For example, the acidic properties of phenol are approximately 3000 times less than those of carbonic acid. Therefore, by passing carbon dioxide through an aqueous solution of sodium phenolate, free phenol can be isolated.

Adding hydrochloric or sulfuric acid to an aqueous solution of sodium phenolate also leads to the formation of phenol:

Phenol reacts with iron (III) chloride to form an intensely purple complex compound.

This reaction allows it to be detected even in very limited quantities. Other phenols containing one or more hydroxyl groups on the benzene ring also produce bright blue-violet colors when reacted with iron(III) chloride.

The presence of a hydroxyl substituent greatly facilitates the occurrence of electrophilic substitution reactions in the benzene ring.

1. Bromination of phenol.

Unlike benzene, the bromination of phenol does not require the addition of a catalyst (iron (III) bromide). In addition, the interaction with phenol proceeds selectively: bromine atoms are directed to the ortho and para positions, replacing the hydrogen atoms located there. The selectivity of substitution is explained by the features of the electronic structure of the phenol molecule discussed above.

Thus, when phenol reacts with bromine water, a white precipitate of 2,4,6-tribromophenol is formed:

This reaction, like the reaction with iron (III) chloride, serves for the qualitative detection of phenol.

2. Nitration of phenol also occurs more easily than benzene nitration. The reaction with dilute nitric acid occurs at room temperature. As a result, a mixture of ortho- and para-isomers of nitrophenol is formed:

When concentrated nitric acid is used, 2,4,6-trinitrophenol is formed - picric acid, an explosive:

3. Hydrogenation of the aromatic core of phenol in the presence of a catalyst occurs easily:

4. Polycondensation of phenol with aldehydes, in particular, with formaldehyde occurs with the formation of reaction products - phenol-formaldehyde resins and solid polymers.

The interaction of phenol with formaldehyde can be described by the following scheme:

The dimer molecule retains “mobile” hydrogen atoms, which means that further continuation of the reaction is possible with a sufficient number of reagents:

The polycondensation reaction, i.e., the reaction of producing a polymer that occurs with the release of a low-molecular-weight by-product (water), can continue further (until one of the reagents is completely consumed) with the formation of huge macromolecules. The process can be described by the summary equation:

The formation of linear molecules occurs at ordinary temperatures. Carrying out this reaction when heated leads to the fact that the resulting product has a branched structure, it is solid and insoluble in water. As a result of heating a linear phenol-formaldehyde resin with an excess of aldehyde, hard plastic masses with unique properties are obtained. Polymers based on phenol-formaldehyde resins are used for the manufacture of varnishes and paints, plastic products that are resistant to heating, cooling, water, alkalis and acids. They have high dielectric properties. The most critical and important parts of electrical appliances, power unit housings and machine parts, and the polymer base of printed circuit boards for radio devices are made from polymers based on phenol-formaldehyde resins. Adhesives based on phenol-formaldehyde resins are capable of reliably connecting parts of a wide variety of natures, maintaining the highest joint strength over a very wide temperature range. This glue is used to attach the metal base of lighting lamps to a glass bulb. Thus, phenol and products based on it are widely used.

DEFINITION

Phenols- derivatives of aromatic hydrocarbons, in the molecules of which hydroxyl groups are directly bonded to the carbon atoms of the benzene ring. The functional group, like that of alcohols, is OH.

Phenol is a solid, colorless, crystalline substance, low-melting, very hygroscopic, with a characteristic odor. In air, phenol oxidizes, so its crystals initially acquire a pinkish tint (Fig. 1), and during long-term storage they darken and become more red. It is slightly soluble in water at room temperature, but dissolves quickly and well at 60 - 70 o C. Phenol is low-melting, its melting point is 43 o C. It is toxic.

Rice. 1. Phenol. Appearance.

Preparation of phenol

On an industrial scale, phenol is obtained from coal tar. Among the laboratory methods I most often use 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 occur mainly with splitting:

1) O-H 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 carboxylic acid anhydrides

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, which disappears when acid is added)

6C 6 H 5 OH + FeCl 3 → (C 6 H 5 OH) 3 + 3Cl - .

2) bonds C sp 2 -H predominantly in O- 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 = Ni, t 0 = 130 - 150, p = 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 contains 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

EXAMPLE 2

Phenols, like other enols, are characterized by keto-enol tautomerism. However, most monohydric phenols exist exclusively in the enol form. This stability of the enol form compared to ketones is due to the aromaticity of the benzene ring, as well as the high degree of conjugation of a pair of electrons of the hydroxyl oxygen atom with the $\pi$-electron system of the benzene ring ($\rho-\pi$-conjugation):

Picture 1.

As a result of this conjugation, the electron density of oxygen shifts to the benzene ring and enhances its electron-donating properties, despite the $-I$ effect of the $OH$ group. In this case, the $O-H$ bond becomes more polar, which promotes its ionization with the formation of a proton and a phenolate anion. So, phenols are typical $OH$ acids:

Figure 2.

In the phenolate anion, the negative charge of the oxygen atom is significantly delocalized, which increases its stability, but reduces its basicity compared to the $RO^-$ alkoxide anion. Negative charge dispersion can be conveyed using I-IV resonance structures, which are not possible for aliphatic alcohols:

Figure 3.

Phenols as acids

As acids, phenols are stronger than water and alcohols, but weaker than carbonic, cyanide and carboxylic acids:

Figure 4.

Therefore, phenols interact not only with sodium, but also, unlike alkanols, with strong bases - alkalis. Phenols do not interact with weak bases such as alkali metal bicarbonates:

Figure 5.

This is used to separate mixtures of alcohols, phenols and carboxylic acids according to the following scheme:

Figure 6.

The above separation method cannot be applied if all three starting compounds are soluble in water.

Alkali metal phenolates, like salts of strong bases and weak acids, are easily hydrolyzed by water, and the solution exhibits a strongly alkaline reaction:

Figure 7.

The dependence of the acidity of phenols depends on the substituent in the nucleus

The acidity of phenols depends on the nature of the substituent in the nucleus. Electron-withdrawing substituents increase acidity, and electron-donating substituents decrease acidity. The effect of the nature of the substituent on phenols is opposite to its effect on the basicity of aromatic amines. Thus, the introduction of a strong acceptor nitro group changes the value of the acidity index $pK_a$ of phenols as follows:

Figure 8.

The increase in acidity is explained by the ability of the $NO_2$ group to participate in the delocalization of the negative charge of the phenoxide anion. If there are three nitro groups in the phenol molecule, the acidic properties of, for example, picric acid reach the level of mineral ones.

Table 1 shows the acidity index $pK_a$ of some substituted phenols in water at 25$^\circ$С with the substituent located in ortho-, meta- And pair- position.

$pK_a$ value ortho-, meta- And pair-substituted phenols in water at 25$^\circ$С:

Figure 9.

Table 2 shows the acidity index $pK_a$ of some substituted phenols in water at 25$^\circ$С.

$pK_a$ value of substituted phenols in water at 25$^\circ$С:

Figure 10.

Heavy metal phenolates

Phenols are capable of forming phenolates not only with alkali metals, but also with salts of heavy metals. Thus, with iron (III) salts they give purple-colored complex compounds, therefore $Fe$(III) salts act as reagents for enol (phenyl) hydroxyl:

Figure 11.

Phenols - organic substances whose molecules contain a phenyl radical linked to one or more hydroxo groups. Just like alcohols, phenols are classified by atomicity, i.e. by the number of hydroxyl groups.

Monohydric phenols contain one hydroxyl group in the molecule:

Polyhydric phenols contain more than one hydroxyl group in molecules:

There are also polyhydric phenols containing three or more hydroxyl groups in the benzene ring.

Let's take a closer look at the structure and properties of the simplest representative of this class - phenol C 6 H 5 OH. The name of this substance formed the basis for the name of the entire cass - phenols.

Physical properties of phenol

Phenol is a solid, colorless crystalline substance, melting point = 43°C, boiling point = 181°C, with a sharp characteristic odor. Toxic. Phenol is slightly soluble in water at room temperature. An aqueous solution of phenol is called carbolic acid. On contact with skin it causes burns, Therefore, phenol must be handled very carefully!

Chemical properties of phenol

In most reactions, phenols are more active at the O–H bond, since this bond is more polar due to the shift of electron density from the oxygen atom towards the benzene ring (participation of the lone electron pair of the oxygen atom in the p-conjugation system). The acidity of phenols is much higher than that of alcohols. For phenols, reactions of C-O bond cleavage are not typical, since the oxygen atom is firmly bonded to the carbon atom of the benzene ring due to the participation of its lone electron pair in the conjugation system. The mutual influence of atoms in the phenol molecule is manifested not only in the behavior of the hydroxy group, but also in the greater reactivity of the benzene ring. The hydroxyl group increases the electron density in the benzene ring, especially at the ortho and para positions (OH groups)

Acid properties of phenol

The hydrogen atom of the hydroxyl group is acidic in nature. Because Since the acidic properties of phenol are more pronounced than those of water and alcohols, phenol reacts not only with alkali metals, but also with alkalis to form phenolates:

The acidity of phenols depends on the nature of the substituents (electron density donor or acceptor), position relative to the OH group and the number of substituents. The greatest influence on the OH-acidity of phenols is exerted by groups located in the ortho- and para-positions. Donors increase the strength of the O-H bond (thereby reducing hydrogen mobility and acidic properties), acceptors reduce the strength of the O-H bond, while acidity increases:

However, the acidic properties of phenol are less pronounced than those of inorganic and carboxylic acids. For example, the acidic properties of phenol are approximately 3000 times less than those of carbonic acid. Therefore, by passing carbon dioxide through an aqueous solution of sodium phenolate, free phenol can be isolated.

Adding hydrochloric or sulfuric acid to an aqueous solution of sodium phenolate also leads to the formation of phenol:

Qualitative reaction to phenol

Phenol reacts with ferric chloride to form an intensely purple complex compound. This reaction allows it to be detected even in very limited quantities. Other phenols containing one or more hydroxyl groups on the benzene ring also give a bright blue-violet color in reaction with ferric chloride(3).

Reactions of the benzene ring of phenol

The presence of a hydroxyl substituent greatly facilitates the occurrence of electrophilic substitution reactions in the benzene ring.

1. Bromination of phenol. Unlike benzene, the bromination of phenol does not require the addition of a catalyst (iron(3) bromide). In addition, the interaction with phenol occurs selectively: bromine atoms are directed to ortho- And pair- positions, replacing the hydrogen atoms located there. The selectivity of substitution is explained by the features of the electronic structure of the phenol molecule discussed above.

Thus, when phenol reacts with bromine water, a white precipitate of 2,4,6-tribromophenol is formed:

This reaction, like the reaction with iron(3) chloride, serves to qualitative detection of phenol.

2.Nitration of phenol also occurs more easily than benzene nitration. The reaction with dilute nitric acid occurs at room temperature. As a result, a mixture is formed ortho- And paro isomers of nitrophenol:

When concentrated nitric acid is used, 2,4,6, trinitritephenol-picric acid, an explosive, is formed:

3. Hydrogenation of the aromatic ring of phenol in the presence of a catalyst passes easily:

4.Polycondensation of phenol with aldehydes, in particular, with formaldehyde it occurs with the formation of reaction products - phenol-formaldehyde resins and solid polymers.

The interaction of phenol with formaldehyde can be described by the following scheme:

The dimer molecule retains “mobile” hydrogen atoms, which means that further continuation of the reaction is possible with a sufficient number of reagents:

Reaction polycondensation, those. the polymer production reaction, which occurs with the release of a low-molecular-weight by-product (water), can continue further (until one of the reagents is completely consumed) with the formation of huge macromolecules. The process can be described by the summary equation:

The formation of linear molecules occurs at ordinary temperatures. Carrying out the same reaction when heated leads to the fact that the resulting product has a branched structure, it is solid and insoluble in water. As a result of heating a phenol-formaldehyde resin of a linear structure with an excess of aldehyde, solid plastic masses with unique properties are obtained. Polymers based on phenol-formaldehyde resins are used for the manufacture of varnishes and paints, plastic products that are resistant to heating, cooling, water, alkalis, and acids. They have high dielectric properties. The most critical and important parts of electrical appliances, power unit housings and machine parts, and the polymer base of printed circuit boards for radio devices are made from polymers based on phenol-formaldehyde resins. Adhesives based on phenol-formaldehyde resins are capable of reliably connecting parts of a wide variety of natures, maintaining the highest joint strength over a very wide temperature range. This adhesive is used to attach the metal base of lighting lamps to a glass bulb. Thus, phenol and products based on it are widely used.

Application of phenols

Phenol is a solid substance with a characteristic odor that causes burns if it comes into contact with the skin. Poisonous. It dissolves in water, its solution is called carbolic acid (antiseptic). She was the first antiseptic introduced into surgery. Widely used for the production of plastics, medicines (salicylic acid and its derivatives), dyes, explosives.