Alkenes are called aliphatic hydrocarbons, the molecules of which have one double bond between carbon atoms. They are also commonly called olefins (from the Latin olefiant - oil), because of the ethylene chloride obtained back in the 18th century, which is a liquid oily substance. The general formula of alkenes has the form C n Η 2n . These substances, in comparison with alkanes, contain fewer hydrogen atoms with an equal carbon chain length. Therefore, they are also called unsaturated hydrocarbons.
The simplest alkene
The simplest representative of a number of alkenes is ethene (aka ethylene). The carbon atoms in its molecule are in the sp 2 hybrid state. This means that one s-orbital and two p-orbitals are involved in the formation of a double bond in alkenes.
Each carbon atom in a double bond has three orbitals in the state of sp 2 hybridization and one p-orbital. When the electron clouds overlap, one σ-bond and one π-bond are formed, which form a double bond. In this case, the π-bond is much weaker and easily breaks under the action of reagents, determining the characteristic properties of alkenes. At the same time, it makes it impossible to rotate adjacent carbon atoms around the bond axis.
The ethylene molecule has a symmetrical structure. All atoms are in the same plane, and the angles between them are 120 °. The bond length C = C is 0.134 nm.
Isomerism
Alkenes are characterized by the same types of isomerism as alkanes. But one more view is added to them - spatial. Due to the presence of the π-bond, and as a result of the rigid attachment of carbon atoms to each other, rotation along the axis of the double bond is impossible without breaking it. But in the molecules of alkenes cis-trans isomerism is observed. It is characterized by a different arrangement of substituents with respect to the π-bond plane. This leads to differences in the physical and chemical properties of such isomers. Examples include trans-butene-2 and cis-butene-2.
The structural formulas of alkenes in the isomeric position of the double bond are distinguished by the location of C = C in the main chain. For example, in the hexene-2 molecule, a multiple bond is located between the second and third C atoms, and in the hexene-3 molecule it is between the third and fourth.
When the carbon skeleton isomerism, the multiple bond has the same arrangement, but the mutual arrangement of carbon atoms in the molecules is different. So, hexene-1 has a linear structure, and its structural isomer (2,3-dimethylbutene-1) is branched.
Interclass isomerism is also characteristic. Cycloalkanes have the identical general formula with alkenes. For example, the interclass butene-1 isomer will be cyclobutane.
Physical properties
In their physical properties, alkenes are close to alkanes, and a certain regularity is also observed in the change in the melting and boiling points of ethene homologs. With increasing molecular weight, that is, with an extension of the hydrocarbon chain, t bales and t PL , as well as density, increase.
The first three members of the ethylene series are gases, the next 13 are liquids, and further from the alkene with the formula C 18 Η 36 are solids. All olefins are poorly soluble in water, but perfectly soluble in organic solvents (such as gasoline and benzene). Lighter than water.
Chemical properties
Such substances are characterized by various types of reactions. Most of them are accompanied by a cleavage of the double bond as a result of the interaction of alkenes with any agents.
Addition reactions:
- Hydrogenation (addition of a hydrogen molecule):
Η 3 -Η = Η 2 + Η 2 -> Η 3 -Η 2 -Η 3.
- Halogenation (addition of a halogen molecule - Cl 2 , Br 2 , F 2 ):
Η 3 -Η = Η 2 + Cl 2 -> Η 3 -CΗCl-CΗ 2 Cl.
- Hydration (addition of a water molecule):
Η 3 -Η = Η 2 + Η 2 -> Η 3 -Η () -CH 3.
- Hydrohalogenation (addition of a hydrogen halide molecule):
Η 3 -Η = Η 2 + ΗCl -> Η 3 -CΗCl-Η 3.
Polymerization
The polymerization reaction is usually called the process of combining low molecular weight monomer molecules into a large polymer molecule. This process is caused by various factors, for example, a change in pressure or temperature, radiation, the action of free radicals. The polymerization of alkenes is also accompanied by a cleavage of the double bond, and due to the appearance of free orbitals, neighboring molecules are interconnected. So polyethylene is obtained from ethylene, polypropylene is obtained from propene (propylene) .
n CH 2 = Η 2 -> (- Η 2 - Η 2 -) n
n CH 2 = CH-Η 3 -> (- Η 2 - Η (Η 3 ) -) n
However, not only ethylene homologs are important in the preparation of polymers, but also derivatives of alkenes with the formulas C 2 F 4 (tetrafluoroethylene), C 6 H 5 -CH = CH 2 (styrene), CH 2 = CHCN (acrylonitrile), from which Teflon, polystyrene and synthetic fibers.
Oxidation
Combustion. Like all hydrocarbons, alkenes burn with the formation of CO 2 and Η 2 O:
C 2 Η 4 + 3O 2 -> 2CO 2 + 2Η 2 O.
Mild oxidation with potassium permanganate is a qualitative reaction, since the pink color of the solution is discolored:
Η 2 = Η 2 + [O] + Η 2 -> Η-Η 2 -Η 2 -Η.
Catalytic oxidation used in industry. For example, in the presence of PdCl 2 and CuCl 2, ethylene oxidizes to acetaldehyde:
2Η 2 = Η 2 + 2 -> 2Η 3 - Η.
Getting
The first four representatives of alkenes with the formulas C 2 Η 2 , C 3 Η 6 , C 4 Η 8 and C 5 Η 10 are isolated from gases generated during the cracking and pyrolysis of petroleum products, as well as coal coking:
C 7 Η 16 -> C 3 Η 6 + C 4 H 10.
In industry, the method of dehydrogenation of alkanes at elevated temperature in the presence of catalysts is also important:
Η 3 -Η 2 -Η 3 -> Η 3 -Η = Η 2 + Η 2.
In laboratories, ethylene is obtained by heating ethanol in the presence of Al 2 O 3 :
CΗ 3 –CΗ 2 –OΗ -> CΗ 2 = CΗ 2 + Η 2 O.