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 mined in the laboratory by chemical synthesis. In order to understand the features of the reactions for obtaining alkenes, you need to understand their structure.
What are alkenes?
Alkenes are organic substances that consist 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:
- according to the position of the double bond;
- for changes in the carbon skeleton;
- interclass (with cycloalkanes);
- geometric (cis- and trans-).
Another name for thisa number of substances - olefins. This is due to their similarity with polyhydric carboxylic acids, which have a double bond in their composition. The nomenclature of alkenes differs in that the first atom in the carbon chain is determined by the placement of a multiple bond, the position of which is also indicated in the name of the substance.
Cracking is the main way to extract alkenes
Cracking is a type of oil refining at high temperatures. 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 to obtain alkenes, in addition to the substance that was the purpose of its implementation, an alkane is formed. The total number of carbon atoms before and after the reaction is the same.
Other industrial methods for obtaining alkenes
You can't continue talking about alkenes without mentioning the dehydrogenation reaction. For its implementation, an alkane is taken, in which a double bond can form after the elimination of two hydrogen atoms. That is, methane will not enter into this reaction. Therefore, a number of alkenes start from ethylene. Special conditions for the reaction are elevated temperature and a catalyst. Nickel or chromium (III) oxide can act as the latter. The result of the reaction will be an alkene with the appropriate number of carbon atoms and a colorless gas (hydrogen).
Another industrial method for obtaining substances of this series is the hydrogenation of alkynes. This reaction for obtaining 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 are added at the sites of destruction.
Laboratory method using alcohol
One of the simplest and most inexpensive ways is intramolecular dehydration, that is, the elimination of water. When writing the reaction equation, it is worth remembering that it will be carried out according to the Zaitsev rule: hydrogen will split off from the least hydrogenated carbon atom. The temperature must be above 150°C. Substances with hygroscopic properties (capable of attracting moisture), such as sulfuric acid, must be used as a catalyst. A double bond will form at the site of the separation of the hydroxyl group and hydrogen. The result of the reaction will be the corresponding alkene and one water molecule.
Laboratory-based halo derivatives
There are two more laboratory methods. The first is the action of an alkali solution on alkane derivatives, which have one halogen atom in their composition. 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 ruleZaitsev. The catalytic conditions are an alcoholic solution and an elevated temperature. After the reaction, an alkene, a s alt of an alkali metal element and a halogen, water is formed.
The second method is very similar to the previous one. It is carried out using an alkane, which has two halogens in its composition. Such a substance is affected by an active metal (zinc or magnesium) in the presence of an alcohol solution and elevated temperature. The reaction will take place only if the hydrogen is replaced by a halogen at two neighboring carbon atoms, if the condition is not met, then the double bond is not formed.
Why take zinc and magnesium? During the reaction, the metal is oxidized, which can donate two electrons, and two halogens are eliminated. If you take alkaline elements, they will react with water, which is part of the alcohol solution. As for the metals that are after magnesium and zinc in the Beketov series, they will be too weak.
