Unsaturated hydrocarbons: alkenes, chemical properties and application

Alkenes are unsaturated hydrocarbons, in molecules of which there is one double bond between the two carbon atoms. They are often called hydrocarbons of the ethylene series, since its simple representative is ethylene HC2 = CH2. Sometimes such hydrocarbons are called olefins, because gaseous alkenes, reacting with bromine or chlorine, form oily compounds that do not dissolve in water. The word "alkenes" in the IUPAC nomenclature is derived from the word "alkanes", in which the suffix -an is replaced by the suffix -ene, which indicates the presence of a double bond in the molecule.

Unsaturated hydrocarbons - alkenes: structure

All alkenes are considered as derivatives of ethylene, or ethene - C2H4.

In the molecule of ethylene there are 2 carbon atoms and four hydrogen atoms, the carbon atoms being unsaturated. Each unsaturated carbon atom hybridizes one es and two pi-orbitals, and one pi-orbit remains "pure", which ensures maximum separation of the hybridized electron orbitals. Their axes form 120 °. At the same time, optimal conditions are created for the overlapping of electronic orbitals. Three such orbitals of the carbon atom form three sigma bonds (two C-H, one C-C). Consequently, the ethylene molecule has five sigma bonds, which are localized in one plane, forming an angle of 120 ° between themselves. The P-electrons, which remained in the "pure form", form yet another bond between the two carbon atoms. The described electronic structure of the ethylene molecule is typical for all alkenes. Unsaturated hydrocarbons should be considered as derivatives of ethylene.

For alkenes, there are two types of isomerism - geometric and structural.

The structural isomerism of alkenes begins with butene C4H8. Distinguish and its varieties - the isomerism of the chain (or carbon skeleton) and the isomerization of the double bond in such a chain.

Geometric isomerism is due to the unequal placement of hydrogen substituents in the ethylene molecule with respect to the double bond plane. Unsaturated hydrocarbons have cis and trans isomers.

Alkenes in the free state are very rare in nature. They are synthesized using industrial and laboratory methods.

Chemical properties of hydrocarbons: alkenes

They are caused by a double bond between two carbon atoms in the alkene molecule.

Hydrogenation reaction

Alkenes readily react with hydrogen. It takes place in the presence of catalysts or when heated:

CH2 = CH2 = H2-H3C-CH3

Addition of halogenated hydrogen

The reaction proceeds according to Markovnikov's rule, that is, during the interaction of hydrogen halides with asymmetric alkene molecules, hydrogen joins at the location of the double bond mainly to the carbon atom near which there are more hydrogen atoms and the halogen atom to the carbon atom near which there are fewer hydrogen atoms or at all They are not here:

CH3-CH + CH2 = HCl-CH3-CHCl-CH3.

The use of hydrocarbons

Ethane is a gas that dissolves well in alcohol and is bad in water, explosive.

They are rich in cracking gases (up to 20% of the total mass) and coke oven gas (about 5%). In the laboratory, ethylene is produced by the methods of dehydration of ethanol and ethane dehydrogenation. Ethylene is used to produce ethyl alcohol, vinyl chloride, ethylbenzene, polyethylene, antifreezes (substances that reduce the freezing point of water in motors) and other organic substances. In medicine and veterinary medicine, ethylene is used as a narcotic drug, in plant growing - to accelerate ripening of fruits (tomatoes, lemons, etc.).

Propylene is produced together with ethylene during pyrolysis and cracking of various types of petroleum feedstocks. Propylene is a component of motor fuel. It is used as raw material for the production of polypropylene, isopropylbenzene, isopropyl alcohol. With isopropanol, acetone is obtained, with isopropylbenzene - acetone and phenol. Propylene is used as a narcotic for the synthesis of acrylonitrile, cumene, butanol, etc.