Alkenes are unsaturated hydrocarbons in the molecules of which there is one double bond between two carbon atoms. They are often called ethylene hydrocarbons, since its simple representative is ethylene 2 = 2. Sometimes such hydrocarbons are called olefins, since gaseous alkenes, reacting with bromine or chlorine, form oily compounds that do not dissolve in water. The word "alkenes" according to the IUPAC nomenclature is a derivative of the word "alkanes" in which the suffix -an is replaced by the suffix -en, which indicates the presence of a double bond in the molecule of the substance.
Unsaturated hydrocarbons - alkenes: structure
All alkenes are considered derivatives of ethylene, or ethene - C2H4.
The ethylene molecule contains 2 carbon atoms and four hydrogen atoms, and the carbon atoms are unsaturated. Each unsaturated carbon atom hybridizes one es and two pi-orbitals, and one pi-orbital remains in its “pure form”, which ensures maximum remoteness of hybridized electronic orbitals. Their axes form 120 °. In this case, optimal conditions are created for overlapping electronic orbitals. Three such orbitals of a carbon atom form three sigma bonds (two C — H, one C — C). Therefore, the ethylene molecule has five sigma bonds, which are localized in one plane, forming an angle of 120 ° between them. P-electrons that remain in their “pure form” form another bond between two carbon atoms. The described electronic structure of the ethylene molecule is typical of all alkenes. Unsaturated hydrocarbons should be considered as derivatives of ethylene.
Alkenes are characterized by 2 types of isomerism - geometric and structural.
The structural isomerism of alkenes begins with C4H8 butene. Its varieties are distinguished - the isomerism of the chain (or carbon skeleton) and the isomerism of the double bond in such a chain.
The geometric isomerism is due to the unequal distribution of hydrogen substituents in the ethylene molecule with respect to the double bond plane. Unsaturated hydrocarbons have cis and trans isomers.
Free alkenes are very rare in nature. They are synthesized using industrial and laboratory methods.
Chemical properties of hydrocarbons: alkenes
They are due to a double bond between two carbon atoms in an alkene molecule.
Hydrogenation reaction
Alkenes easily react with the addition of hydrogen. It proceeds in the presence of catalysts or when heated:
CH2 = CH2 = H2-H3C-CH3
The addition of hydrogen halides
The reaction proceeds according to the Markovnikov rule, that is, when hydrogen halides interact 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 less hydrogen atoms or completely They are not here:
CH3-CH + CH2 = HC1-CH3-CHC1-CH3.
The use of hydrocarbons
Ethene is a gas that is highly soluble in alcohol and poorly 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 ethanol dehydration and ethane dehydrogenation. Ethylene is used to produce ethanol, 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 crop production - to accelerate the ripening of fruits (tomatoes, lemons, etc.).
Propylene is produced together with ethylene during the pyrolysis and cracking of various types of crude oil. Propylene is a component of motor fuel. It is used as a raw material for the production of polypropylene, isopropylbenzene, isopropyl alcohol. Acetone is obtained from isopropanol, acetone and phenol are obtained from isopropylbenzene. Propylene is used as a narcotic for the synthesis of acrylonitrile, cumene, butanol, etc.