In organic chemistry one can meet hydrocarbon substances with different amount of carbon in the chain and C=C-bond. They are homologues and are called alkenes. Because of their structure, they are chemically more reactive than alkanes. But what exactly are their reactions? Consider their distribution in nature, different methods of obtaining and application.
What are they?
Alkenes, which are also called olefins (oily), got their name from ethene chloride, a derivative of the first representative of this group. All alkenes have at least one C=C double bond. C H2n - the formula of all olefins, and the name is formed from an alkane with the same number of carbons in the molecule, only the suffix -an changes to -ene. The Arabic numeral at the end of the name through a hyphen indicates the carbon number from which the double bond begins. Consider the main alkenes, the table will help you remember them:
| Alkane | Name | Alkene | Name |
| C2H6 | ethane | C2H4 | ethene (ethylene) |
| C3H8 | propane | C3H6 | propene (propylene) |
| C4H10 | butane | C4H8 | butene-1 |
| C5H12 | pentane | C5H10 | pentene-1 (amylene) |
| C6H14 | hexane | C6H12 | hexene-1 (hexylene) |
| C7H16 | heptane | C7H14 |
heptene-1 (heptylene) |
| C8H18 | octane | C8H16 | octene |
| C9H20 | nonane | C9H18 | nonen |
If the molecules have a simple unbranched structure, then add the suffix -ylene, this is also reflected in the table.
Where can you find them?
Since the reactivity of alkenes is very high, their representatives in nature are extremely rare. The principle of life of the olefin molecule is "let's be friends." There are no other substances around - it does not matter, we will be friends with each other, forming polymers.
But they exist, and a small number of representatives are included in the associated petroleum gas, and higher ones are in oil produced in Canada.
The very first representative of alkenes ethene isa hormone that stimulates the ripening of fruits, so it is synthesized in small quantities by representatives of the flora. There is an alkene cis-9-tricosene, which in female houseflies plays the role of a sexual attractant. It is also called Muscalur. (Attractant - a substance of natural or synthetic origin, which causes attraction to the source of smell in another organism). From the point of view of chemistry, this alkene looks like this:
Since all alkenes are very valuable raw materials, the ways to obtain them artificially are very diverse. Consider the most common.
And if you need a lot?
In industry, the class of alkenes is mainly obtained by cracking, i.e. splitting of the molecule under the influence of high temperatures, higher alkanes. The reaction requires heating in the range from 400 to 700 °C. The alkane splits as it wants, forming alkenes, the methods for which we are considering, with a large number of molecular structure options:
C7H16 -> CH3-CH=CH 2 + C4H10.
Another common method is called dehydrogenation, in which a hydrogen molecule is separated from a representative of the alkane series in the presence of a catalyst.
In laboratory conditions, alkenes and methods of obtaining are different, they are based on elimination reactions (elimination of a group of atoms without replacing them). Most often, water atoms are eliminated from alcohols, halogens, hydrogen or hydrogen halide. The most common way to obtain alkenes is from alcohols in the presence ofacid as a catalyst. Other catalysts may be used
All elimination reactions are subject to Zaitsev's rule, which reads:
A hydrogen atom is split off from the carbon adjacent to the carbon bearing the -OH group, which has fewer hydrogens.
After applying the rule, answer which reaction product will prevail? Later you will know if you answered correctly.
Chemical properties
Alkenes actively react with substances, breaking their pi-bond (another name for the C=C bond). After all, it is not as strong as a single (sigma bond). An unsaturated hydrocarbon turns into a saturated one without forming other substances after the reaction (addition).
The following are the most common reactions of alkenes that are carried out in different types of human activities:
- addition of hydrogen (hydrogenation). The presence of a catalyst and heating is needed for its passage;
- attachment of halogen molecules (halogenation). It is one of the qualitative reactions to a pi bond. After all, when alkenes react with bromine water, it becomes transparent from brown;
- reaction with hydrogen halides (hydrohalogenation);
- water addition (hydration). The reaction conditions are heating and the presence of a catalyst (acid);
Reactions of unsymmetrical olefins with hydrogen halides and water obey Markovnikov's rule. This means that hydrogen will join that carbon from the carbon-carbon double bond, which already has morehydrogen atoms.
- burning;
- partial oxidation catalytic. The product is cyclic oxides;
- Wagner reaction (oxidation with permanganate in a neutral medium). This alkene reaction is another high quality C=C bond. When flowing, the pink solution of potassium permanganate discolors. If the same reaction is carried out in a combined acidic environment, the products will be different (carboxylic acids, ketones, carbon dioxide);
- isomerization. All types are characteristic: cis- and trans-, double bond displacement, cyclization, skeletal isomerization;
- polymerization is the main property of olefins for industry.
Medical applications
The reaction products of alkenes are of great practical importance. Many of them are used in medicine. Glycerin is obtained from propene. This polyhydric alcohol is an excellent solvent, and if used instead of water, the solutions will be more concentrated. For medical purposes, alkaloids, thymol, iodine, bromine, etc. are dissolved in it. Glycerin is also used in the preparation of ointments, pastes and creams. It prevents them from drying out. By itself, glycerin is an antiseptic.
When reacting with hydrogen chloride, derivatives are obtained that are used as local anesthesia when applied to the skin, as well as for short-term anesthesia with minor surgical interventions, using inhalation.
Alkadienes are alkenes with two double bonds in one molecule. Their main application- production of synthetic rubber, from which various heating pads and syringes, probes and catheters, gloves, nipples and much more are then made, which is simply indispensable when caring for the sick.
Industrial applications
| Type of industry | What is used | How can they be used |
| Agriculture | ethene | accelerates the ripening of vegetables and fruits, defoliation of plants, films for greenhouses |
| Paint-colourful | ethene, butene, propene, etc. | for obtaining solvents, ethers, solvent |
| Engineering | 2-methylpropene, ethene | synthetic rubber production, lubricating oils, antifreeze |
| Food industry | ethene | production of Teflon, ethyl alcohol, acetic acid |
| Chemical industry | ethene, polypropylene | receive alcohols, polymers (polyvinyl chloride, polyethylene, polyvinyl acetate, polyisobutylene, acetaldehyde |
| Mining | ethen et al | explosives |
Alkenes and their derivatives have found wider application in industry. (Where and how alkenes are used, see table above).
This is just a small part of the use of alkenes and their derivatives. Every year the need for olefins only increases, which means that the need for their production also increases.
(Answer: butene-2.)
