Alkenes are valuable "transitional" substances. With their help, you can get alkanes, alkynes, halogen derivatives, alcohols, polymers and others. The main problem of unsaturated hydrocarbons is their almost complete absence in nature, for the most part substances of this particular series are extracted in the laboratory by chemical synthesis. In order to understand the peculiarities of the reactions for the production of alkenes, it is necessary to understand their structure.
What are alkenes?
Alkenes are organic substances that are made up of carbon and hydrogen atoms. A feature of this series are double covalent bonds: sigma and pi. They determine the chemical and physical properties of substances. Their melting point is lower than that of the corresponding alkanes. Also, alkenes differ from this “basic” series of hydrocarbons by the presence of an addition reaction, which occurs by breaking the pi bond. They are characterized by four types of isomerism:
- by the position of the double bond;
- on changes in the carbon skeleton;
- interclass (with cycloalkanes);
- geometric (cis and trans).
Another name for this series of substances is olefins. This is due to their similarity with polyhydric carboxylic acids, which in their composition have a double bond. The nomenclature of alkenes is characterized in that the determination of the first atom in the carbon chain is carried out by placing a multiple bond, the position of which is also indicated in the name of the substance.
Cracking is the main way to get alkenes
Cracking is a type of oil refining at high temperature conditions. The main goal of this process is the extraction of substances with a lower molecular weight. Cracking to produce alkenes occurs during the breakdown of alkanes, which are part of petroleum products. This occurs at temperatures from 400 to 700 ° C. In the course of this reaction for the production of alkenes, in addition to the substance which was the aim of its carrying out, alkane is formed. The total number of carbon atoms before and after the reaction is the same.
Other industrial methods for producing alkenes
You can not continue talking about alkenes without mentioning the dehydrogenation reaction. To carry it out, an alkane is taken in which a double bond can form after the removal of two hydrogen atoms. That is, methane will not enter into this reaction. Therefore, a number of alkenes are read from ethylene. Particular conditions for the reaction are elevated temperature and catalyst. The latter may be nickel or chromium oxide (III). The reaction will produce an alkene with an appropriate number of carbon atoms and a colorless gas (hydrogen).
Another industrial method for the extraction of substances of this series is the hydrogenation of alkynes. This reaction of producing alkenes takes place at elevated temperatures and with the participation of a catalyst (nickel or platinum). The hydrogenation mechanism is based on breaking one of the two pi bonds of the provided alkyne, after which hydrogen atoms join at the sites of destruction.
Laboratory method using alcohol
One of the simplest and most inexpensive methods is intramolecular dehydration, that is, the removal of water. When writing the reaction equation, it is worth remembering that it will be carried out according to the Zaitsev rule: hydrogen is split off from the least hydrogenated carbon atom. The temperature should be above 150 ° C. As a catalyst, it is necessary to use substances with hygroscopic properties (capable of drawing moisture), for example, sulfuric acid. At the place of separation of the hydroxyl group and hydrogen, a double bond will form. The reaction will result in a corresponding alkene and one water molecule.
Laboratory halogenated production
There are two more laboratory methods. The first is the action of an alkali solution on alkane derivatives, which in their composition have one halogen atom. This method is called dehydrohalogenation, that is, the elimination of hydrogen compounds with non-metallic elements of the seventh group (fluorine, bromine, chlorine, iodine). The implementation of the reaction mechanism, as in the previous case, proceeds according to the Zaitsev rule. The catalytic conditions are alcohol solution and elevated temperature. After the reaction, alkene, a salt of a metal element of alkali and halogen, water are formed.
The second method is very similar to the previous one. It is carried out using an alkane, which has two halogens in it. Such a substance is acted upon by an active metal (zinc or magnesium) in the presence of an alcohol solution and elevated temperature. The reaction will take place only if hydrogen is replaced by halogen at two adjacent carbon atoms, if the condition is not met, then a double bond will not form.
Why is it necessary to take zinc and magnesium? During the reaction, the metal oxidizes, which can give off two electrons, and the removal of two halogens. If you take alkaline elements, they will react with water, which is in the alcohol solution. As for the metals that are in the Beketov series after magnesium and zinc, they will be too weak.