Physical properties

Benzene and its closest homologues are colorless liquids with a specific odor. Aromatic hydrocarbons are lighter than water and do not dissolve in it, but they easily dissolve in organic solvents - alcohol, ether, acetone.

Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high carbon content of their molecules.

The physical properties of some arenes are presented in the table.

Table. Physical properties of some arenas

Name	Formula	t°.pl., °C	t°.bp., °C
Benzene	C 6 H 6	5,5	80,1
Toluene (methylbenzene)	C 6 H 5 CH 3	95,0	110,6
Ethylbenzene	C 6 H 5 C 2 H 5	95,0	136,2
Xylene (dimethylbenzene)	C 6 H 4 (CH 3) 2
ortho-		25,18	144,41
meta-		47,87	139,10
pair-		13,26	138,35
Propylbenzene	C 6 H 5 (CH 2) 2 CH 3	99,0	159,20
Cumene (isopropylbenzene)	C 6 H 5 CH(CH 3) 2	96,0	152,39
Styrene (vinylbenzene)	C 6 H 5 CH \u003d CH 2	30,6	145,2

Benzene - low-boiling ( tkip= 80.1°C), colorless liquid, insoluble in water

Attention! Benzene - poison, acts on the kidneys, changes the blood formula (with prolonged exposure), can disrupt the structure of chromosomes.

Most aromatic hydrocarbons are life threatening and toxic.

Obtaining arenes (benzene and its homologues)

In the laboratory

1. Fusion of salts of benzoic acid with solid alkalis

C 6 H 5 -COONa + NaOH t → C 6 H 6 + Na 2 CO 3

sodium benzoate

2. Wurtz-Fitting reaction: (here G is halogen)

From 6H 5 -G+2Na + R-G →C 6 H 5 - R + 2 NaG

WITH 6 H 5 -Cl + 2Na + CH 3 -Cl → C 6 H 5 -CH 3 + 2NaCl

In industry

• isolated from oil and coal by fractional distillation, reforming;
• from coal tar and coke oven gas

1. Dehydrocyclization of alkanes with more than 6 carbon atoms:

C 6 H 14 t , kat→C 6 H 6 + 4H 2

2. Trimerization of acetylene(only for benzene) – R. Zelinsky:

3С 2 H2 600°C, Act. coal→C 6 H 6

3. Dehydrogenation cyclohexane and its homologues:

Soviet Academician Nikolai Dmitrievich Zelinsky established that benzene is formed from cyclohexane (dehydrogenation of cycloalkanes

C 6 H 12 t, cat→C 6 H 6 + 3H 2

C 6 H 11 -CH 3 t , kat→C 6 H 5 -CH 3 + 3H 2

methylcyclohexanetoluene

4. Alkylation of benzene(obtaining homologues of benzene) – r Friedel-Crafts.

C 6 H 6 + C 2 H 5 -Cl t, AlCl3→C 6 H 5 -C 2 H 5 + HCl

chloroethane ethylbenzene

Chemical properties of arenes

I. OXIDATION REACTIONS

1. Combustion (smoky flame):

2C 6 H 6 + 15O 2 t→12CO 2 + 6H 2 O + Q

2. Benzene at normal conditions does not decolorize bromine water and water solution potassium permanganate

3. Benzene homologues are oxidized by potassium permanganate (discolor potassium permanganate):

A) in an acidic environment to benzoic acid

Under the action of potassium permanganate and other strong oxidants on the homologues of benzene, the side chains are oxidized. No matter how complex the chain of the substituent is, it is destroyed, with the exception of the a -carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid:

Homologues containing two side chains give dibasic acids:

5C 6 H 5 -C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 → 5C 6 H 5 COOH + 5CO 2 + 6K 2 SO 4 + 12MnSO 4 + 28H 2 O

5C 6 H 5 -CH 3 + 6KMnO 4 + 9H 2 SO 4 → 5C 6 H 5 COOH + 3K 2 SO 4 + 6MnSO 4 + 14H 2 O

Simplified :

C 6 H 5 -CH 3 + 3O KMnO4→C 6 H 5 COOH + H 2 O

B) in neutral and slightly alkaline to salts of benzoic acid

C 6 H 5 -CH 3 + 2KMnO 4 → C 6 H 5 COO K + K OH + 2MnO 2 + H 2 O

II. ADDITION REACTIONS (harder than alkenes)

1. Halogenation

C 6 H 6 + 3Cl 2 h ν → C 6 H 6 Cl 6 (hexachlorocyclohexane - hexachloran)

2. Hydrogenation

C 6 H 6 + 3H 2 t , PtorNi→C 6 H 12 (cyclohexane)

3. Polymerization

III. SUBSTITUTION REACTIONS – ionic mechanism (lighter than alkanes)

1. Halogenation -

a ) benzene

C 6 H 6 + Cl 2 AlCl 3 → C 6 H 5 -Cl + HCl (chlorobenzene)

C 6 H 6 + 6Cl 2 t ,AlCl3→C 6 Cl 6 + 6HCl( hexachlorobenzene)

C 6 H 6 + Br 2 t,FeCl3→ C 6 H 5 -Br + HBr( bromobenzene)

b) benzene homologues upon irradiation or heating

In terms of chemical properties, alkyl radicals are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst, when heated or UV irradiated, radical reaction side chain substitutions. The influence of the benzene ring on alkyl substituents leads to the fact that the hydrogen atom is always replaced at the carbon atom directly bonded to the benzene ring (a-carbon atom).

1) C 6 H 5 -CH 3 + Cl 2 h ν → C 6 H 5 -CH 2 -Cl + HCl

c) benzene homologues in the presence of a catalyst

C 6 H 5 -CH 3 + Cl 2 AlCl 3 → (mixture of orta, pair of derivatives) +HCl

2. Nitration (with nitric acid)

C 6 H 6 + HO-NO 2 t, H2SO4→C 6 H 5 -NO 2 + H 2 O

nitrobenzene - smell almond!

C 6 H 5 -CH 3 + 3HO-NO 2 t, H2SO4→ WITH H 3 -C 6 H 2 (NO 2) 3 + 3H 2 O

2,4,6-trinitrotoluene (tol, trotyl)

The use of benzene and its homologues

Benzene C 6 H 6 is a good solvent. Benzene as an additive improves the quality of motor fuel. It serves as a raw material for the production of many aromatic organic compounds - nitrobenzene C 6 H 5 NO 2 (solvent, aniline is obtained from it), chlorobenzene C 6 H 5 Cl, phenol C 6 H 5 OH, styrene, etc.

Toluene C 6 H 5 -CH 3 - a solvent used in the manufacture of dyes, drugs and explosives (trotyl (tol), or 2,4,6-trinitrotoluene TNT).

Xylene C 6 H 4 (CH 3) 2 . Technical xylene is a mixture of three isomers ( ortho-, meta- And pair-xylenes) - is used as a solvent and starting product for the synthesis of many organic compounds.

Isopropylbenzene C 6 H 5 -CH (CH 3) 2 serves to obtain phenol and acetone.

Chlorine derivatives of benzene used for plant protection. Thus, the product of substitution of H atoms in benzene with chlorine atoms is hexachlorobenzene C 6 Cl 6 - a fungicide; it is used for dry seed dressing of wheat and rye against hard smut. The product of the addition of chlorine to benzene is hexachlorocyclohexane (hexachloran) C 6 H 6 Cl 6 - an insecticide; it is used to control harmful insects. These substances refer to pesticides - chemical means of combating microorganisms, plants and animals.

Styrene C 6 H 5 - CH \u003d CH 2 polymerizes very easily, forming polystyrene, and copolymerizing with butadiene - styrene-butadiene rubbers.

VIDEO EXPERIENCES

DEFINITION

Benzene(cyclohexatriene - 1,3,5) - organic matter, the simplest representative of a number of aromatic hydrocarbons.

Formula - C 6 H 6 ( structural formula- rice. 1). Molecular weight - 78, 11.

Rice. 1. Structural and spatial formulas of benzene.

All six carbon atoms in the benzene molecule are in the sp 2 hybrid state. Each carbon atom forms 3σ bonds with two other carbon atoms and one hydrogen atom lying in the same plane. Six carbon atoms form a regular hexagon (σ-skeleton of the benzene molecule). Each carbon atom has one unhybridized p-orbital, which contains one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered cycle. The hydrocarbon radical derived from benzene is called C 6 H 5 - - phenyl (Ph-).

Chemical properties of benzene

Benzene is characterized by substitution reactions proceeding according to the electrophilic mechanism:

- halogenation (benzene interacts with chlorine and bromine in the presence of catalysts - anhydrous AlCl 3, FeCl 3, AlBr 3)

C 6 H 6 + Cl 2 \u003d C 6 H 5 -Cl + HCl;

- nitration (benzene easily reacts with a nitrating mixture - a mixture of concentrated nitric and sulfuric acids)

- alkylation with alkenes

C 6 H 6 + CH 2 \u003d CH-CH 3 → C 6 H 5 -CH (CH 3) 2;

Addition reactions to benzene lead to the destruction of the aromatic system and proceed only under harsh conditions:

- hydrogenation (the reaction proceeds when heated, the catalyst is Pt)

- addition of chlorine (occurs under the action of UV radiation with the formation of a solid product - hexachlorocyclohexane (hexachlorane) - C 6 H 6 Cl 6)

Like any organic compound benzene enters into a combustion reaction with the formation as reaction products carbon dioxide and water (burns with a smoky flame):

2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O.

Physical properties of benzene

Benzene is a colorless liquid, but has a specific pungent odor. Forms an azeotropic mixture with water, mixes well with ethers, gasoline and various organic solvents. Boiling point - 80.1C, melting point - 5.5C. Toxic, carcinogen (i.e. contributes to the development of cancer).

Obtaining and using benzene

The main methods for obtaining benzene:

— dehydrocyclization of hexane (catalysts - Pt, Cr 3 O 2)

CH 3 -(CH 2) 4 -CH 3 → C 6 H 6 + 4H 2;

- dehydrogenation of cyclohexane (the reaction proceeds when heated, the catalyst is Pt)

C 6 H 12 → C 6 H 6 + 4H 2;

– trimerization of acetylene (the reaction proceeds when heated to 600C, the catalyst is activated carbon)

3HC≡CH → C 6 H 6 .

Benzene serves as a raw material for the production of homologues (ethylbenzene, cumene), cyclohexane, nitrobenzene, chlorobenzene, and other substances. Previously, benzene was used as an additive to gasoline to increase its octane number, however, now, due to its high toxicity, the content of benzene in fuel is strictly regulated. Sometimes benzene is used as a solvent.

Examples of problem solving

EXAMPLE 1

Exercise

Write down the equations with which you can carry out the following transformations: CH 4 → C 2 H 2 → C 6 H 6 → C 6 H 5 Cl.

Solution

To obtain acetylene from methane, the following reaction is used: 2CH 4 → C 2 H 2 + 3H 2 (t = 1400C). Obtaining benzene from acetylene is possible by the reaction of trimerization of acetylene, which occurs when heated (t = 600C) and in the presence of activated carbon: 3C 2 H 2 → C 6 H 6 . The chlorination reaction of benzene to obtain chlorobenzene as a product is carried out in the presence of iron (III) chloride: C 6 H 6 + Cl 2 → C 6 H 5 Cl + HCl.

EXAMPLE 2

Exercise	To 39 g of benzene in the presence of iron (III) chloride was added 1 mol of bromine water. What amount of the substance and how many grams of what products did this result in?
Solution	Let us write the equation for the reaction of benzene bromination in the presence of iron (III) chloride: C 6 H 6 + Br 2 → C 6 H 5 Br + HBr. The reaction products are bromobenzene and hydrogen bromide. Molar mass benzene calculated using the table chemical elements DI. Mendeleev - 78 g/mol. Find the amount of benzene substance: n(C 6 H 6) = m(C 6 H 6) / M(C 6 H 6); n(C 6 H 6) = 39/78 = 0.5 mol. According to the condition of the problem, benzene reacted with 1 mol of bromine. Consequently, benzene is in short supply and further calculations will be made for benzene. According to the reaction equation n (C 6 H 6): n (C 6 H 5 Br) : n (HBr) \u003d 1: 1: 1, therefore n (C 6 H 6) \u003d n (C 6 H 5 Br) \u003d: n(HBr) = 0.5 mol. Then, the masses of bromobenzene and hydrogen bromide will be equal: m(C 6 H 5 Br) = n(C 6 H 5 Br)×M(C 6 H 5 Br); m(HBr) = n(HBr)×M(HBr). Molar masses of bromobenzene and hydrogen bromide, calculated using the table of chemical elements of D.I. Mendeleev - 157 and 81 g/mol, respectively. m(C 6 H 5 Br) = 0.5×157 = 78.5 g; m(HBr) = 0.5 x 81 = 40.5 g.
Answer	The reaction products are bromobenzene and hydrogen bromide. The masses of bromobenzene and hydrogen bromide are 78.5 and 40.5 g, respectively.

C6H6 + Cl2 → C6H6Cl + HCl

In this case, iron chloride or bromide (III) is usually used as a catalyst. Other metal chlorides, such as AlCl3, SbCl3, SbCl5, as well as iodine, can also be used as catalysts.

The role of the catalyst is to activate (polarize) the halogen, which performs electrophilic substitution in the benzene ring. In the presence of FeCl3

chlorination goes, for example, according to the scheme:

FeCl3 + :Cl::Cl: ↔ FeCl-4 + Cl:+

۠۠۠ ۠ ۠۠۠۠۠ ۠ ۠ ۠۠

C6H6 + Cl+ → C6H5Cl + H+;

H+ + Cl2 → HCl + Cl+ etc.

Halogen can be introduced into the side chain in the absence of catalysts in the light or by heating. The substitution mechanism in this case is radical. For toluene, these transformations can be expressed by the scheme:

Halogens are substituents of the first kind, and therefore, when benzene is halogenated, the second halogen atom enters predominantly in the n-position to the first. However, halogens, unlike other substituents of the first kind, make substitution difficult (compared to benzene).

When n-fluorochlorobenzene is chlorinated, the third halogen atom enters the o-position to chlorine, and not to fluorine. Therefore, the inductive effect of the halogen has a decisive influence on the order of substitution (the o-position to the fluorine atom has a large positive charge, since –IF > -ICl): AAAAAAAAAAAAAAAAAAAAAAAAAAA

2. Replacement of the amino group with a halogen through the intermediate formation of diazo compounds. This method allows you to get any halogen derivatives, including fluorine derivatives:

───→ C6H5Cl + N2

C6H5NH2───→ C6H5N2Cl ────→ C6H5I + KCl +N2

───→ C6H5Br + Cu2Cl2 + N2

BF4 → C6H5F + N2 + BF3

2.2 Adamantane

The structural features of adamantane determine its unusual physical and Chemical properties. Adamantane has the highest melting point of 269°C for hydrocarbons and a density of 1.07 g/cm3. It is thermally stable in the absence of oxygen when heated up to 660°C. At a pressure of 20 kilobars and a temperature of 480°C and above, it gradually graphitizes. Adamantane is extremely resistant to aggressive chemical environments and does not interact with potassium permanganate, chromic and concentrated nitric acid, even at elevated temperatures.

Table 1 shows the yield of adamantane as a function of the catalyst used.

Table 1. Results of liquid-phase isomerization of TMNB to adamantane

Reaction conditions	Yield of adamantane, %
BF3, HF, 23 at H2, 50°C
SbF5, HF, 120°C, 5 h
A1C13, HC1.40 at H2, 120°C
A1C13, HC1, tert-C4H9Cl
A1Br3, tert-C4H9Br

The isomerization of TMNB to adamantane is carried out according to the scheme:

Due to spatial considerations, only the endo-isomer is capable of further rearrangement into adamantane, and its equilibrium concentration is about 0.5 wt. %.

In kinetic terms, endo-TMNB isomerization is one of the slowest rearrangements of saturated hydrocarbons under these conditions: geometric TMNB isomerization (Wagner-Meerwein rearrangement) proceeds approximately 10,000 times faster.

This synthesis method became the basis for the industrial technology of adamantane. The ease of such a rearrangement is explained by the high thermodynamic stability of adamantane; therefore, the treatment of all known C10H16 isomers with Lewis acids inevitably leads to this polycyclic frame hydrocarbon.

Synthesis of adamantanecarboxylic acids

To obtain acids of the adamantane series, the Koch-Haaf reaction is widely used. Adamantane, 1-bromo-, 1-hydroxyadamantane, and 1-hydroxyadamantane nitrate are used as starting materials.

Adamantane -1-carboxylic acid was obtained by reacting 1-bromo- or 1-hydroxyadamantane with formic acid in sulfuric acid or adamantane with formic or sulfuric acid in the presence of tert-butyl alcohol.

It has been shown that the maximum yield of adamantane-1-carboxylic acid is achieved at a ratio of AdOH:HCOOH:H2SO4 = 1:1:24. The yield decreases with a lack of formic acid.

Adamantane-1-carboxylic acid can be obtained from adamantane in 20% oleum. It is assumed that the reaction proceeds through the formation of adamantyl cation

To obtain carboxylic acids from adamantane, its reaction with CO2 in sulfuric acid or oleum is used (autoclave, 90-160ºС) . This produces a mixture of adamantane-1-carboxylic and adamantane-1,3-dicarboxylic acid in a ratio of 1:6.

Synthesis of (1-adamantyl)acetic acid from 1-bromo or 1-hydroxyadamantane and dichloroethylene is carried out in 80-100% H2SO4 in the presence of BF3 at 0-15ºС.

When adamantane and its derivatives react with trichlorethylene in the presence of 90% sulfuric acid, the corresponding α-chloroacetic acids are formed.

3-alkyladamantane-1-carboxylic acids are obtained from alkyladamantanes in sulfuric acid in the presence of tert-butyl alcohol and 95% formic acid.

Adamantane nitrates.

The reaction of adamantane with an excess of 96-98% nitric acid leads to 1-nitroxyadamantane as the main reaction product 1.3-dinitroxiadamantane.

Adamantane reacts with a mixture of nitric and acetic acids at a slower rate than with nitric acid, and the maximum nitrate yield of 80% is reached in 3 hours. The only by-product of the reaction is adamantol-1.