Preparation of alkenes

Alkenes are unsaturated acyclic hydrocarbons that include only one double bond of carbon atoms. They form a single homologous complex (series), which is displayed by the equation of the formula for the substance CnH2n. For example, the formula of the starting material in this series - ethylene - C2H4, the fifth formula in the pentene series is C5H10, the tenth decene is C10H20.

The carbon atoms in these substances in the presence of a double bond are in the state of sp² hybridization, while the valence angle of their molecular structure is 120 °. The simplest of the alkenes is the ethylene compound with the formula C2H4. The homology of a number of alkenes also reflects their names, which are formed by adding an appropriate prefix.

It should be noted that the production of alkenes is entirely determined by its chemical properties and features. For example, those of them that have more than two carbons in their composition create isomers. In this case, there is a regularity that the more the number of carbon atoms contained in a particular alkene, the more isomers it is able to form.

The physical properties of these compounds also directly depend on the position in the homologous series. For example, the melting point, boiling point and density for ethylene are, respectively, -169.1, -103.7 and 0.5700. The same parameters of the eighth in the alkene-octene series are, respectively, -101.7, -121.3 and 0.7140.

The first receipt of alkenes took place in 1669, when the German chemist Bekher singled it out during the exposure of sulfuric acid to ethyl alcohol. This time, the scientist could not accurately determine the gas released, that's why he did not give him the name, but simply called "air". Later, "the air Becher" exactly the same method received the Dutch Damien and Pot-van Troostwick in 1795. This time, the scientists described in detail their ways of obtaining alkenes and gave them a unique name - "oil gas". This name was born because, during the reaction with chlorine, an oily liquid was formed, only later identified as dichloroethane by the chemist from France, Antoine Fourcroix.

Further, at the very beginning of the century before last, J. Gay-Lussac established that ethanol includes, in addition to the "oil-gas" gas proper, and water molecules.

J. Dumas and P. Bullay in 1928 opened the receipt of alkenes from ethyl chloride. It is interesting that for a very long time chemists have not been able to correctly write down the formula for this compound. Only in 1848 Kosby brought it out for ethylene and recorded it as C4H4.

In nature, alkenes are practically not present. Ethylene, the simplest representative of the homologous series, is in the form of a hormone in some plants.

Lower alkenes are toxic and can cause a narcotic effect in high concentrations. In case of ingestion of higher alkenes, convulsions and swelling of the respiratory tract may occur.

The molecular weight determines the boiling and melting points of these compounds and their aggregate state. Alkenes from the first to the fourth in the series - gases, from the fifth to the seventeenth have the form of a liquid, and the next - solid bodies. All alkenes without exception are insoluble in water, but are perfectly soluble in organic solvents.

In modern conditions there are quite a lot of different ways of obtaining alkenes. Thus, the following industrial methods are referred to methods for the preparation of alkenes.

The main method is catalytic high-temperature cracking, in which natural gas and hydrocarbon compounds contained in crude oil participate. As applied to the lower, it is possible to apply the method of dehydration of alcohols.

In the laboratories this method is also used, but only here the reaction is carried out in the presence of strong acids, and the methods of synthesizing Hoffmann and Chugayev are also used. Other methods of synthesis include the reaction of Burda, which consists in the elimination of bromine.

Alkenes are an indispensable raw material for the chemical industry, where they are used to produce a huge number of chemical compounds. Therefore, the production of alkenes is an important direction in the development of chemical industry in all industrially developed countries.