What are alkenes?
It is known as alkenes to a type of aromatic hydrocarbons in whose molecules it is possible to find carbon-carbon bonds, that is, double bonds in its structure.
Alkenes or olefins are obviously organic substances which, together with alkanes or paraffinics, alkynes or acetylenics and cycloalkanes, constitute aromatic hydrocarbons, that is to say those whose central element is the benzene ring (C 6 H 6 ). They are distinguished in the latter from the aliphatic, whose foundational stone is methane (CH 4 ).
In turn, aromatic hydrocarbons can be classified as monocyclic (a single ring of benzene) and polycyclic (with several interlaced rings), so there may be alkenes belonging to both classifications.
Alkenes were formerly called “olefins” because their simplest representatives, such as ethene, were used to produce oils along with halogen elements. Systemically these compounds are named using the same rule as for alkanes, that is, using the Latin particle that alludes to the total amount of carbons , but with the suffix -one.
However, many alkenes are known by their non-systematic name (as is usually the case with hydrocarbons) and in that case the official ending (-eno) is replaced by -ylene, as is the case with ethene, also called ethylene, or with the propene, called propylene.
On the other hand, if there is more than one double bond, the terminations -dieno or -trieno, etc. are used respectively.
The general formula of the alkenes is C n H 2n , where N represents the quantity of the respective atoms. This means that for every double bond between the first, there will be twice as many hydrogen atoms less than those indicated in this formula.
Difference with alkanes and alkynes
Alkenes differ from alkanes and alkynes in the double bond that appears between their 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 new double bond.
Alkanes have a simple bond between their carbons (CC); the alkenes at least one double bond (C = C) and the alkynes a triple bond (C?C).
Alkenes have the following physical properties:
- Depending on the amount of carbon atoms , they may 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 molecular polarity due to the double bond.
- They have a higher acidity compared to alkanes, the result of the polarity of the link.
- Its boiling and melting point does not vary with respect to alkanes.
Alkenes are characterized by a high chemical reactivity and their majority reactions are addition, contrary to subtraction, predominant in alkanes, much less reactive.
The addition reactions of hydrogen halides, sulfuric acid, oxidation, hydroxylation and polymerization are frequent, the latter vital in the petroleum industry and its derivatives, since long and resistant chains of malleable material (plastics) are achieved .
Alkenes are far less abundant than alkanes, in fact they are found in natural oil but as a very minor component.
The oil “cracking” processes, on the other hand, are an important source of alkenes that can be used by the industry. However, 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 can therefore be used to synthesize them, are: dehydroalogenation, dehalogenation, the elimination of water from alcohols, pyrolysis, the Chungaev reaction and the Grieco reaction.
They can also be obtained by forming carbon-carbon bonds (Wittig Reaction, Julia Olefin, etc.), by pericyclic reactions or metal coupling reactions (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 trieno (three bonds) or a polyene (multiple bonds).
These compounds, however, lack special properties and behave like normal alkenes, except that they have very particular reactivities, depending on the location in the molecule of the two double bonds.
Alkenes are more or less stable, depending on where the double bond is in the molecule. Thus, the inmates are much more stable than those at the extremes, and the trans isomers (mirror molecules) are more stable than the cis .
Examples of alkenes
Some common alkenes are:
- Ethene (CH 2 = CH 2 )
- Propene (CH 2 = CH-CH 3 )
- Butene (CH 2 = CH-CH 2 -CH 3 )
- Cholesterol (C 27 H 4 O 6 )
- Lanosterol (C 30 H 50 O)
- Camphor (C 10 H 16 O)
- Mycenae (C 10 H 16 )