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Alkenes, also known as olefins, are unsaturated hydrocarbons.
Let's look at that definition in more detail. Hydrocarbons are organic molecules containing just carbon and hydrogen atoms. The term unsaturated means that they contain at least one carbon-carbon double bond.
The homologous series for alkenes is represented by the general molecular formula 
.
An alkene, ethene. It has two carbon atoms and therefore has four hydrogen atoms.commons.wikimedia.org
Alkenes are used to make polymers such as polystyrene and PVC, and can be found in products such as antifreeze and paints.
Alkenes are named using the suffix -ene and standard nomenclature rules. A number between the root name and the suffix indicates the position of the double bond within the chain.
Two alkenes: but-2-ene and 3-methylbut-1-ene Anna Brewer, StudySmarter Originals
Let’s take the second molecule as an example. It contains a carbon backbone four atoms long, a methyl side chain and a C=C double bond. This means that it takes the prefix methyl-, the suffix -ene and the root name -but-.
If we number the carbons from both directions, either the methyl group is attached to carbon 3 and the double bond is joined to carbon 1, or the methyl group is joined to carbon 2 and the double bond is attached to carbon 3. If we add those numbers up, we get 1 +3 = 4 or 2 + 3 = 5. Remember the ‘lowest numbers’ rule - we want any constituents on the molecule to be attached to the lowest numbered carbons possible.
So in this case, we number the carbon atoms from right to left.
Left: 3-methylbut-1-ene. This is the correct numbering and nomenclature. Right: 2-methylbut-3-ene. This is the incorrect numbering and nomenclature. Anna Brewer, StudySmarter Originals
For more information on nomenclature, see Organic Compounds.
Alkenes show three types of isomerism.
Chain isomerism is a type of structural isomerism.
Structural isomers are molecules with the same molecular formula but different structural formulae.
In the case of chain isomers, these molecules have different arrangements of the hydrocarbon chain.
Positional isomerism is also a type of structural isomerism. In this case, the functional group differs in its position within the carbon chain.
Taking the two above forms of isomerism into account, the alkene represented by 
could be either but-1-ene, but-2-ene or 2-methylprop-1-ene:
Top left: but-1-ene. Top right: but-2-ene. Bottom: methylpropene. Anna Brewer, StudySmarter Originals
Geometric isomerism is a form of stereoisomerism.
Stereoisomers have the same structural formula but different spatial arrangements of atoms.
In geometric isomerism, this occurs if two different groups are attached to each of the atoms involved in a double bond, as the double bond limits rotation of the molecule.
To name geometric isomers, each carbon in the C=C double bond is taken in turn and the two atoms directly attached to it are looked at. The group containing the atom with higher molecular mass is assigned first priority. If both groups with first priority from each carbon are on the same side of the double bond, the molecule is known as the Z-isomer. If the highest priority groups are on opposite sides of the double bond, the molecule is known as the E-isomer. E- and Z- isomers are also known as trans- and cis- isomers respectively.
For example, but-2-ene displays the following geometric isomerism:
Left: in E-but-2-ene, the highest priority groups (in this case, methyl groups) are on opposite sides of the double bond. Right: in Z-but-2-ene, the highest priority groups are on the same side of the double bond. Anna Brewer, StudySmarter Originals
The groups with highest priority are shown in red. In this example, the methyl group 
takes priority over the hydrogen atom on each of the carbons in the C=C bond. In the molecule on the left, the 
groups are both on the same side of the double bond. This is therefore the Z- isomer.
E- and Z- come from the German words entgegen, meaning opposite, and zusammen, meaning together.
For more information on stereoisomers and assigning priority, see Isomerism.
Alkenes have some similar properties to alkanes. They are comparable in mass and, like alkanes, contain only non-polar bonds. Therefore, the only forces present between molecules are van der Waals forces. However, their C=C double bond makes them more reactive than alkanes, as explained below.
Study tip: Remember the differences between alkanes and alkenes. We’ll look at these in more detail at the end of this article.
Alkenes are insoluble in water. Because they contain only non-polar bonds, they cannot bond to polar water molecules, but are soluble in other organic solvents.
Alkenes have relatively low melting and boiling points as the weak van der Waals forces between molecules do not require much energy to overcome. As chain length increases, boiling point increases, and as branching of the molecule increases, boiling point decreases.
For example, but-1-ene has a higher boiling point than propene as it has a longer carbon chain.
But-1-ene and propene, shown with their respective boiling points.Anna Brewer, StudySmarter Originals
For more information on the effect of van der Waals forces on physical properties, see Alkanes.
Alkenes are trigonal planar molecules. They have an angle of roughly 120° between each bond.
The bond angle in an ethene molecule.Anna Brewer, StudySmarter Originals.
Alkenes are relatively reactive. This is because the C=C double bond is an area of high electron density and is attractive to electrophiles.
Electrophiles are electron pair acceptors that contain a positive ion or δ+ atom with an empty orbital.
This means that alkenes frequently undergo electrophilic addition reactions. Examples of this include:
Testing for alkenes relies on an electrophilic addition reaction, as mentioned above. Shake an unknown substance with orange-brown bromine water. If the solution decolourises, there is an alkene present. This is because the bromine water adds on to the double bond, forming a halogenoalkane.
Testing for alkenes. Here, ethene reacts with bromine water to produce 1,2-dibromoethane. Anna Brewer, StudySmarter Originals
The following table compares alkenes to alkanes:
A table comparing alkanes and alkenes. Anna Brewer, StudySmarter Originals
An alkene, also known as an olefin, is an unsaturated hydrocarbon containing one of more carbon-carbon double bonds (C=C).
Alkenes are unsaturated hydrocarbons.
Alkenes are used to make polymers like polystyrene and PVC, and are found in products such as antifreeze and paints.
Alkanes are saturated hydrocarbons and contain only C-H and C-C single bonds, whereas alkenes are unsaturated and also contain one or more C=C double bonds.
Question
What is the general formula of an alkene?
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Which group takes first priority?
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Name the following molecule:
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Z-pent-2-ene
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Name the following molecule:
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2-chloropropene
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Name this molecule:
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3-methylpent-1-ene
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Draw and label the two geometric isomers of 2-chlorobut-2-ene.
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Which of the following molecules are an electrophile?
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