Alkenes

What are alkenes?

Alkenes are a homologous series of unsaturated hydrocarbons, which have:

• The same functional group - at least one carbon-carbon double bond / C=C
• The same general formula - C_nH_2n
• Similar chemical properties
• Differ in their chemical formula by one -CH₂- group
• Have gradually changing physical properties

What is the difference between alkanes, alkenes and alkynes?

Alkanes, alkenes and alkynes are all hydrocarbons.

Alkanes are saturated containing only carbon-carbon single bonds. All of the bonds in an alkane are sigma / σ bonds and each carbon has a tetrahedral shape (bond angle = 109.5 ^o).

Alkenes and alkynes are unsaturated.

Alkenes contain at least one carbon-carbon double bond / C=C which is comprised of one sigma / σ bond and one pi / π bond. The shape around an alkene carbon is trigonal planar (bond angle = 120 ^o).

Alkynes contain at least one carbon-carbon triple bond / C≡C which is comprised of one sigma / σ and two pi  / π bonds. The shape around an alkyne carbon is linear (bond angle = 180 ^o).

More information: Shapes & Bond Angles in Organic Molecules

Where do alkenes come from?

Two main methods to produce alkenes are:

• Cracking of long-chain hydrocarbons
• Chemical reactions such as the elimination reaction of a halogenoalkane to form an alkene.

More information: “Elimination”

What are the names of the first alkenes?

The systematic nomenclature of the alkenes follows the IUPAC rules, just like alkanes, with a prefix to indicate the number of carbon atoms in the chain but the ‘-ene’ suffix to indicate the presence of at least one carbon-carbon double bond. 

More information: “Nomenclature of organic compounds”

Number of carbon atoms	Prefix of the alkene	Name of the alkene	Molecular formula of the alkene
1*
2	eth-	ethene	C2H4
3	prop-	propene	C3H6
4	but-	butene	C4H8
5	pent-	pentene	C5H10

* ‘Methene’ does not exist because alkenes require a minimum of two carbon atoms to form the double bond / C=C.

What types of structural isomerism do alkenes show?

Alkenes can show the following types of structural isomerism because they have the same molecular formula but a different structural formula.

Functional group isomerism

Alkenes and cyclic alkanes both have the general formula C_nH_2n, which means that cyclic alkanes are functional group isomers of alkenes. For example, propene and cyclopropane.
 

Propene, C3H6	Cyclopropane, C3H6

Alkenes that contain four or more carbons in their main chain can exhibit position or branched chain isomerism.

Position isomerism

For example, but-1-ene has the double bond / C=C between carbons 1 and 2, while but-2-ene has the double bond / C=C between carbons 2 and 3.

But-1-ene, C4H8	But-2-ene, C4H8

Branched chain isomerism

For example, but-1-ene, but-2-ene and methylpropene all have the molecular formula C₄H₈ but methylpropene is the branched chain isomer of but-1-ene and but-2-ene.

But-1-ene	But-2-ene	Methylpropene

More information: "Structural Isomerism"

Stereoisomerism / Geometrical isomerism in alkenes

Stereoisomerism is split into two categories:

Geometrical isomerism

• This is dependent on the atoms / groups of atoms surrounding the carbon-carbon double bond and relies on the fact that the carbon-carbon double bond / C=C cannot rotate.
• This is called E / Z isomerism
  • Although you may see the outdated term cis-trans isomerism.

Optical isomerism

• This is not relevant to the carbon atoms in the carbon-carbon double bond / C=C of an alkene

At first, it may appear that there are 2 possible isomers of dichloroethene, C₂H₂Cl₂:

• 1,1-dichloroethene
• 1,2-dichloroethene

1,1-dichloroethene does not show geometrical isomerism because both chlorine atoms are on the same side of the carbon-carbon double bond / C=C:

However, 1,2-dichloroethene does show geometrical isomerism because the chlorine atoms are on either side of the carbon-carbon double bond / C=C. There is no free rotation about the carbon-carbon double bond / C=C which means that the chlorine atoms could be:

• Both “above” the double bond.

• • This gives the outdated cis-1,2-dichloroethene name or the up-to-date Z-1,2-dichloroethene name.
• One chlorine atom “above” and one chlorine atom “below” the double bond.

• • This gives the outdated trans-1,2-dichloroethene name or the up-to-date E-1,2-dichloroethene name.

E comes from the German ‘entgegen’ meaning opposite and Z comes from the German ‘zusammen’ meaning together.

For more information about the E / Z nomenclature system, see: Stereoisomerism: Geometrical Isomerism

Chemical and physical properties of alkenes

Alkenes have similar chemical properties and, therefore, undergo similar chemical reactions.

Like alkanes, alkenes are not soluble in water because they are not polar and cannot overcome / disrupt the hydrogen bonding between water molecules.

The alkenes show patterns in physical properties such as boiling point:

As the alkene gets longer, the boiling point increases. This is because longer alkenes have increased intermolecular forces requiring more energy to overcome as the physical state changes from liquid to gas.

What reactions do alkenes do?

Combustion of alkenes

Like alkanes and other chemicals, alkenes can undergo complete to form water + carbon dioxide or incomplete combustion to form water and either carbon monoxide or carbon.

More information: “Combustion”

Due to the reactivity of the carbon-carbon double bond, alkenes undergo many reactions:

Two of the main reactions are electrophilic addition and polymerisation.

Electrophilic addition

This is the addition of an electrophile to a double bond and commonly includes:

• Hydrogenation with hydrogen
• Hydration with water vapour
• Halogenation with hydrogen halides
• Halogenation with halogens
  • The decolourisation of bromine water is used as the saturation test to identify the presence of an alkene

The electrophilic addition mechanism is a key mechanism. The pi / π bond of the alkene carbon-carbon double bond breaks open and donates a pair of electrons to an electrophile.

• If the alkene is symmetrical, then there will only be one product.
• If the alkene is not symmetrical, then there will be a mixture of major and minor products:

• The major product is always formed from the most stable carbocation intermediate.
  • The stability of the carbocation intermediate depends on groups surrounding the positive carbon atom
  • Alkyl groups donate electron density to make stabilise the ion (by the photoinductive effect)
  • A primary carbocation only has one alkyl group donating electron density, making it the least stable
  • A  secondary carbocation has two alkyl groups donating electron density
  • A tertiary carbocation has three alkyl groups donating electron density, making it the most stable

More information: "Electrophilic Addition"

Addition polymerisation

The industrially important reaction of addition polymerisation is where the pi / π bond of many alkene monomer carbon-carbon double bonds break open and join together producing a polymer chain.

The addition polymer chain is made up of many alkene monomers. These can be referred to as repeat units, which are the smallest group of atoms that connect together to form a polymer.

Note: The repeat unit of an addition polymer always contains a minimum of two carbon atoms.

More information: "Addition Polymers"

What keyword definitions do I need to know for alkenes?

Some keyword definitions you need to know are:

• Hydrocarbon - a compound that contains hydrogen and carbon only
• Unsaturated - a compound that contains at least one carbon-carbon double bond
• Sigma - a type of bond present in all of the bonds of a hydrocarbon
• Pi - the type of bond that, alongside sigma bonds, makes a double or triple bond
• Electrophile - an atom (or group of atoms) that is attracted to an area of high electron density
• Addition - a chemical reaction that adds two chemicals together to form one chemical
• Addition polymerisation - the reaction where many alkene monomers form long chains of polymers as the only product
• Repeat unit - the smallest group of atoms that connect together to form a polymer chain

This is a quick summary of some key concepts on alkenes - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!