In the presence of substituents in the benzene core, the place of entry of the nitro group and the reaction conditions are determined, as in other reactions of electrophilic substitution by the nature of the existing substituent.

Orientants of the first kind(OH, OR, OCOR, NH 2, NHR, NR 2, NHCOR, –N=N–, CH 2 Cl, CH 3, F, Cl, Br, I) direct the nitro group mainly to the ortho-para position and facilitate (except halides) its introduction.

Orientants of the second kind(SO 3 H, NO 2 , COOH, COOR, CN, CCl 3) direct the nitro group mainly to the meta position and make it difficult to introduce it into the nucleus.

Compounds containing orientants of the first kind are nitrated under milder conditions: nitration of toluene to mononitro compounds occurs at a temperature not exceeding 40°C; mononitration of phenol is carried out even with dilute nitric acid upon cooling. The nature of the substituents present in the benzene core also determines the rate of the reaction. A comparative change in the reaction rate under the influence of the existing substituent can be schematically represented next to where groups to the right of chlorine accelerate the reaction, and to the left slow it down.

NO 2 > SO 3 H > COOH > Cl< CH 3 < OCH 3 < OC 2 H 5 < OH

slow down speed up

In the case of di- and polysubstituted benzenes, the effect of substituents is additive. In the presence of substituents of different types, the place of entry of the electrophile determines the orientant of the first kind, since it activates the nucleus. If both substituents are of the same gender, then the place of entry is determined by the stronger one:

Nitration of toluene with a nitrating mixture at 20-30°C results in a mixture of o- and p-nitrotoluenes with an almost quantitative total yield.

Further nitration of nitrotoluenes to nitro compounds is carried out at a higher temperature.

Under the action of fuming nitric and sulfuric acids at 110 ° C on dinitrotoluenes, 2,4,6-trinitrotoluene (TNT) is formed, which is used as an explosive. With an increase in the number of alkyl groups in the benzene ring, nitration is facilitated. Xylenes are nitrated more easily than toluene, and mesitylene, under even milder conditions, with acetyl or benzoyl nitrate.

Obtaining via a diazo compound

The methods of this group are much less numerous, but they are distinguished by high yields, low contents of by-product impurities, simplicity and variety in execution.

The simplest and most reliable method of this group is to carry out the Sandmeyer reaction. We can give 2 examples of different implementation of this stage only:

2.3 Other Methods

PhBr + TfOMe, anthranilic acid in the Borodin-Hunsdieker reaction, the reaction of o-dibromobenzene and MeMgBr, etc. - have advantages over other methods and have a lower preparative value, although they are of interest.

The chosen path of synthesis - analysis, description of methods for performing the stages of the experiment

The main criterion for choosing one or another method, described in detail above, is reliability and availability. This corresponds to the path toluene - nitrotoluene - o-toluidine - o-bromotoluene.

Nitration of toluene

0.15 mol of an aromatic nitro compound is placed in a 250 ml three-necked flask equipped with a stirrer, addition funnel, internal thermometer (the device must not be sealed). The nitrating mixture, pre-cooled to at least 10 °C, is then added slowly, with good stirring and cooling with an ice bath, the temperature of the reaction mixture should be in the range of 5-10 °C.

Further, at room temperature, stir for another 2-3 hours. The reaction mixture is then carefully poured into 300 ml of ice-cold water and stirred well. The organic layer is separated, the aqueous layer is extracted with ether. The combined organic extracts are washed with water, 2N. sodium bicarbonate solution until neutral, then again with water. heater. The extracts are dried over CaCl 2 and distilled. The p-isomer is frozen out with a mixture of ice and salt, washed with a small amount of cold petroleum ether. (Careful separation is sufficient, this method leaves about 4% of the p-isomer: freezing for 8 hours with a mixture of ice and salt (2:1). A good separation method is the reduction of the p-isomer with an alkaline reducing agent. P-toluidine can be separated due to its basic properties Separation is best achieved by fractional distillation followed by crystallization 11). From the filtrate by vacuum distillation on a 30 cm column In Game with electrical heating, the o-isomer is isolated. The yield of the o-isomer is 40%. The boiling points of o- and p-nitrotoluene are 96°C/9 mm, respectively. and 105°C /10 mm, melting point of p-toluidine 52-54°C.

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, however, 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:

3C 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 under normal conditions does not decolorize bromine water and an aqueous solution of 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

The chemical properties of alkyl radicals are similar to those of alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst, heating or UV irradiation leads to a radical substitution reaction in the side chain. 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.

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