We explain what alkenes are and the physical and chemical properties they present. In addition, we explain what their characteristics are and what examples they provide.
What are alkenes?
It is known as alkenes a type of unsaturated hydrocarbon (which have lost hydrogens bonded to adjacent carbon atoms) in whose molecules it is possible to find carbon-carbon double bonds.
Alkenes or olefinic compounds They are organic substances which, together with alkanes (or paraffins), alkynes (or acetylenic compounds) and cycloalkanes, constitute aliphatic hydrocarbons, that is, those that are not aromatic (derived mainly from benzene/C6H6).
In turn, aliphatic hydrocarbons can be classified as linear, monocyclic (a single ring composed of carbon atoms) and polycyclic (with several linked rings), so there may be alkenes belonging to all classifications.
See also: Alkali metals
Nomenclature of alkenes
Alkenes They were formerly called “olefins” because its simplest representatives, such as ethene, were reacted with halogens to produce oils.
According to systematic nomenclature, these compounds are named using the same rule as for alkanes. We look for the longest carbon chain that contains the double bond. If it has more than one double bond, the chain that has the smallest combination of the positions of these double bonds is searched. If the molecule has substituents, they are named by giving them the numerical prefix that corresponds to the position of the carbon to which they are bonded (1,2,3…), but if it has several identical substituents, the prefix is also given (di, tri , tetra…) depending on whether they are two, three or four.
However, Many alkenes are known by their non-systematic name (as is often the case with hydrocarbons) and in this case the official ending (–ene) is replaced by –ylene, as is the case with ethene, also called ethylene, or with propene, called propylene.
On the other hand, if there is more than one double bond, the endings are used respectively –diene (two double bonds) or –triene (three double bonds), etc.
General formula of alkenes
The general formula of linear alkenes is CnH2n, where n represents the number of the respective carbon atoms. This means that for each new double bond there will be two fewer hydrogen atoms than indicated in this formula.
Difference with alkanes and alkynes
Alkenes differ from alkanes and alkynes in the double bond that appears between its carbon atoms. It is often said, in fact, that an alkene is an alkane that has lost hydrogen atoms, thus releasing two electrons that make up the second bond of each double bond.
Alkanes They present a simple bond between their carbons (CC); alkenes at least one double bond (C=C) and alkynes at least one triple bond (C≡C).
Physical properties of alkenes
Alkenes have the following physical properties:
- Depending on their number of carbon atoms, alkenes can be gases (fewer atoms) or liquids (more atoms).
- They are insoluble in water but soluble in ether, chloroform or benzene.
- They are less dense than water.
- They present some molecular polarity or none depending on the geometry of each molecule around the double bond.
- They have a higher acidity than the corresponding alkanes, a product of the polarity of the bond.
- Its boiling and melting points do not vary much with respect to the corresponding alkanes.
Chemical properties of alkenes
Alkenes are characterized by a high chemical reactivity with respect to alkanes and its majority reactions are addition. The most frequent reactions are:
Abundance of alkenes
The alkenes They are much less abundant than alkanesin fact they are found in natural petroleum but as a very minor component.
Oil cracking processes, on the other hand, are an important source of usable alkenes for the industry. However, the alkenes are common in organic chemistry and are part of the reactions common to living beings, especially plants and vegetables.
Synthesis of alkenes
The reactions that produce alkenes as a result and that, therefore, can be used to synthesize them are:
Dehydrohalogenation.
Dehalogenation.
Water removal from alcohols.
Pyrolysis.
They can also be obtained as a result of the Chungaev reaction and the Grieco reaction, by forming carbon-carbon bonds (Wittig Reaction, Julia Olefination, etc.), by pericyclic reactions or coupling reactions with metals (Suzuki Reaction, Heck Reaction).
Dienes and polyenes
If there is more than one double bond in the molecule, we will be in the presence of a diene (two bonds), a triene (three bonds) or a polyene (multiple bonds).
These compounds, however, lack special properties and They behave like normal alkenesexcept that they exhibit very particular reactivities, depending on the location in the molecule of the multiple double bonds.
Stability of alkenes
Alkenes are more or less stable, depending on where in the molecule the double bond is located. Internal alkenes, whose double bond is located towards the central part of the molecule, are much more stable than those with the double bond located at the ends. In this way, the isomers trans They are more stable than cis.
Examples of alkenes
Some common alkenes are:
- Ethene (CH2=CH2)
- Propene (CH2=CH-CH3)
- 1-butene (CH2=CH-CH2-CH3)
- Cholesterol (C27H46O6)
- Lanosterol (C30H50O)
- Camphor (C10H16O)
- Myrcene (C10H16)
It may help you: Polymers
References:
- Introduction to the IUPAC Nomenclature of Organic Compounds. Dr. Eugenio Alvarado. University of Costa Rica. 2000
- Introduction to Organic Chemistry. Juan Carlos Autino, Gustavo Romanelli and Diego Manuel Ruiz. National University of La Plata. 2013. ISBN: 978-950-34-0998-5
- Carbon Chemistry. Dr. Pedro Ambielle. 1937.
- “Alkenes” in Wikipedia.