Cycloalkanes

Opening history

In 1885, Adolf Von Bayer put forward a theory to confirm certain aspects of the chemistry of cyclic elements. Based on the fact that the bonds with the carbon atom take the form of a tetrahedron with angles between the bonds of 109.5°, bonds in cycloalkanes with a number of carbon atoms less than 5 and more than 6 should be under the influence of high voltage and such molecules cannot exist under normal conditions.

In the molecule of cyclobutane(C₃H₆) and cyclopropane (C₄H₈), the angles between the bonds atoms are very different from 109.5, the angles between the bonds in cyclobutane are 60 and 90 in cyclopropane. Thus, the bonds in the cyclobutane and cyclopropane molecules must be energized, so the molecules cyclobutane and cyclopropane react with substances that break the ring, since such reactions relieve tension with connections.

Conformations - geometry

Cyclopropane C₃H₆

The carbon atoms in the cyclopropane molecule are practically in the same plane, the angles between the C-C bonds are 60 degrees, so the bonds are under a lot of stress. Molecule Cyclopropane is easily hydrogenated in the presence of a catalyst, forming propane already at 50°C.

Cyclobutane C₄H₈

The angular stress acting on the bonds in the cyclobutane molecule is less than in cyclopropane, since the angles between the connections are 90 degrees. The electron clouds of atoms overlap more strongly than in cyclopropane, forming a torque on the C-C bonds, which also makes the molecule active. Cyclobutane undergoes hydrogenation in the presence of a catalyst, but at higher temperatures than cyclopentane, about 50°C.

Cyclopentane C₅H₁₀

In a cyclopentane molecule, all bonds overlap with each other, so the molecule is forced to change its shape, thereby reducing torsion stresses and slightly increasing angular. In the cyclopentane molecule , the angular the voltage is very low, since the angles between the bonds are close to 109 degrees. Cyclopentane reacts weakly with other molecules. The hydrogenation of cyclopentane is carried out at higher temperatures, from 300°C.

Cyclohexane C₆H₁₂

In cyclohexane, the angles between the bonds are 120 degrees, which leads to a large angular stress, and the overlap of the bonds forms a large torsion stress. In this regard , the cyclohexane molecule takes the shape of the chair, thus reducing the angle between the carbon bonds to 111.4 degrees, minimizing the angular voltage, other forms of the molecule are less stable, for example, the form of a bath, in this form only occur about 2% of the molecules.

The bond with six hydrogen atoms has an arrangement parallel to the axis of symmetry, these atoms hydrogen is called axial, the other six atoms are located almost parallel to the plane carbons are called equatorial. The most stable compounds are those in which the substituents are in the equatorial position.

Reactions

Cycloalkanes in reactions are similar to their saturated hydrocarbon homologues, but cyclopropane and cyclobutane They are exceptions to the general rule and in addition reactions behave like alkenes, breaking the C-C bond to form a new bond.

Cyclopropane + H₂ → C₃H₈ (propane)
Cyclopropane +Br₂ → C₃H₆Br₂ (1,3-dibromopropane)
Cyclopropane + XH→ C₃H₇X (X-propane halogen)

Since the stresses in cyclobutane are less than in cyclopropane, the cyclobutane molecule reacts worse, for example, under normal conditions, cyclobutane does not react in aqueous solutions of sulfuric acid and no addition reaction is performed. Halogenation occurs at higher temperatures and in the presence of a catalyst. Conversion to butane occurs in the presence of nickel at a temperature of 200°C. At high temperatures, the C-C bond breaks in cyclobutane and butene is formed.

Cyclopentane and cyclohexane do not produce addition reactions, in substitution reactions they behave the same as corresponding alkanes. Isomerization reactions occur with Cl₃Al catalysts, in which the ring breaks and joins, for example, the isomers dimethylcyclopentane and methylcyclohexane, or cyclohexane and methylcyclopentane.

Getting

The most widely used cycloalkanes in the industry are methylcyclopentane, 1,2-dimethylcyclopentane, cyclohexane and methylcyclohexane. The main source of cycloalkanes is oil. Reactions of formation of cycloalkanes from aliphatic compounds are called cyclization and are mainly used for cyclopropane and cyclobutane. Thus, a hydrocarbon with halogens on the terminal carbons in reaction with zinc powder forms cycloalkane:

Br-CH₂-CH₂-CH₂-Br +Zn→ C₃H₆ (cyclopropane) + Br₂Zn

Cyclohexane

Cyclohexane is a colorless liquid with a special smell. Cyclohexane is insoluble in water, it is soluble in alcohols, acetones and benzenes. The boiling point is 80.7°C. Cyclohexane takes 43 the place in terms of chemical production, nylon is synthesized from cyclohexane. The main source of cyclohexane is oil, during the distillation of naphtha, two main masses can be distinguished - one with cyclohexane and the other - benzene and methylcyclohexane.

The mass with cyclohexane undergoes catalytic reforming, resulting in a mixture of benzene, pentane, methylpentane and dimethylpentane. During the distillation process, more volatile pentane and less volatile dimethylpentane are removed.

The resulting mixtures of benzene with methylpentane and benzene with methylcyclohexane are mixed and subjected to catalytic hydrogenation, in which methylpentane is isomerized and benzene is hydrogenated into cyclohexane.

Cyclohexanol and cyclohexanone are obtained from cyclohexane by catalytic oxidation, which are used nylon, rubber, polyester and polyurethane are produced as raw materials. To a lesser extent, cyclohexanol and cyclohexanone are used as organic solvents.