The structural element of aromatic compounds is benzene. Benzene can be represented as six carbon atoms, located in the same plane and forming a regular hexagon. Previously, it was assumed that every carbon atom in the molecule benzene is connected to two neighbors by a double and single bond. But, the theory of resonance has made its own adjustments: if there are two possible the electronic configuration of the molecule, then the real configuration is a combination of these two. Therefore , the bonds between the carbons in the molecule benzene is "one and a half".
Unlike aliphatic hydrocarbons, aromatic compounds are characterized by electrophilic substitution reactions, since benzene the ring has an impressive electronic cloud. Electrophilic reactions - halogenation, sulfation, alkylation and acylation. Such reactions are characteristic of both benzene and other aromatic rings.
Electrophiles attack the electron cloud of the benzene ring and force the attacked carbon atom and its neighbor to take the sp3-form for education σ-connections. Thus, the electrophile occupies two electrons at the benzene ring. The carbon ring redistributes the remaining four electrons are between the five carbons. Then the proton H+ cleaves off, thus restoring the aromaticity of the molecule.
Halogenation of benzene with bromine or chlorine occurs with Lewis acid, acids are more often used FeCl3, FeBr3, AlCl3:
In the presence of FeCl3, at room temperature:
C6H6 + Cl2 → C6H6Cl (90%) + HCl (10%)
In the presence of FeBr3, at high temperature:
C6H6 + Br2 → C6H6Br (75%) + HBr (25%)
Nitriding of benzene is slow. Sulfuric acid is used to accelerate. Sulfuric acid, when mixed with nitric acid, increases the concentration of electrophilic ions NO2+, thereby accelerating the reaction of electrophilic substitution of benzene:
Nitric acid acts as a base compared to sulfuric acid, so sulfuric acid gives one proton to nitric acid:
HNO3 + H2SO4 ↔H2NO3 + H2SO4
Protonated nitric acid dissociates to form a nitronium cation:
H2NO3 + H2SO4 ↔ N+O2 + H3O + HSO4
At a temperature of 50-55°C
NO2 + C6H6 → C6H6NO2
Sulfonation of benzene occurs with sulfuric acid, or with sulfuric acid and sulfur anhydride, to accelerate the reaction:
2H2SO4 ↔ SO3 + H3O+ + HSO4-
C6H6 + SO3 ↔ (C6H6)+SO3-
(C6H6)+SO3- + HSO4- ↔ (C6H6)SO3- + H2SO4
(C6H6)+SO3- + H3O+ ↔ (C6H6)SO3H + H2O
The alkylation process begins with the formation of a carbation (step 1), which subsequently acts as an electrophile and attacks the electron ring of benzene, forming an arene ion (step 2), then the arene ion cleaves off a proton, forming alkylbenzene (step 3).
General scheme of alkylation (in the presence of AlCl3):
C6H6 + RX → C6H6R + HX
Stage 1. The compound of isopropyl chloride with aluminum chloride - the formation of carbcation:
CH3CH(Cl)CH3 (isopropyl chloride) + AlCl3 ↔ CH3-C+H-CH3 (Carbocation) + AlCl4-
Step 2. The compound of benzene with isopropyl forms an arene ion:
C6H6 + CH3-C+H-CH3 ↔ (C6H6)+CH(CH3)CH3 (aren-ion)
Stage 3. Proton cleavage and isopropylbenzene formation:
(C6H6)+CH(CH3)CH3 + AlCl4- ↔ (C6H6)+CH(CH3)CH3 (isopropylbenzene) + HCl + AlCl3
Any group attached to benzene affects the rate of electrophilic substitution. So, groups having unshared pairs of electrons, they increase the density of the electron ring, thereby accelerating the electrophilic substitution reactions, respectively, the groups pulling the electron cloud over themselves slow down the electrophilic substitution reactions. Groups that increase the rate of electrophilic substitution, they are called activating groups.
In most cases, substitution reactions in benzenes with an activating group occur in the ortho- and para- positions (ortho - opposite, a pair - through one group), whereas benzenes with a retarding group attach electrophiles to the meta- (meta - next) position. The greater the electronegativity of the attached group, the further away from it the electrophilic compound will join.
Aromatic compounds have a higher density, boiling point and melting point due to their placement in the plane. Aromatic hydrocarbons are insoluble in water, have a characteristic smell, sometimes pleasant. Benzene dissolves 1% of water, but it is soluble in any proportions with polar compounds such as gasoline, esters, ketones, alcohols and carboxylic acids, which makes it possible to use it (benzene) as a solvent in industrial organic reactions. Benzene is very volatile and flammable, and is also a carcinogen. 1,2-benzopyrene has the same properties.
Aromatic hydrocarbons are of great industrial importance, since 60% (by weight) of plastics, elastomers, synthetic fibers, dyes, insecticides, medicines, etc. are made from them. Three main aromatic hydrocarbons are ethanol, toluene and xylene, their mixture is called BTX. These arenas are mainly in coal and oil.
Dry distillation of coalThe process of dry distillation consists in heating coal in a vertical furnace at a temperature of about 1000 °C, while carbon bonds they are torn apart, forming many different products. First of all, gases are formed (CH4, NH3, SH2 and others), and then vapors condense into coal tar. From the resulting tar, by fractional distillation, is derived aromatic compounds. Naphthalene, anthracene and phenanthrene are obtained from denser fractions.
In the process of coking coal, a lot of carbon is formed, which is used in the production of steel, so the dry distillation method is not the main one for the production of BTX, and depends mainly on the demand for steel, therefore, BTX production methods have been developed from oil.
