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Alkenes

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Chemistry

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 .

Alkenes, ethene, StudySmarterAn 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.

How are alkenes named?

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.

Alkenes, examples of alkenes but-2-ene 3-methylbut-1-ene, StudySmarterTwo 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.

Alkenes, 3-methylbut-1-ene nomenclature, StudySmarterLeft: 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.

How do alkenes show isomerism?

Alkenes show three types of isomerism.

  • Chain isomerism.
  • Position isomerism.
  • Geometric isomerism

Chain 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

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:

Alkenes, isomers of butene, StudySmarterTop left: but-1-ene. Top right: but-2-ene. Bottom: methylpropene. Anna Brewer, StudySmarter Originals

Geometric isomerism

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:

Alkenes stereoisomers StudySmarterLeft: 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.

What are the properties of alkenes?

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.

Solubility

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.

Melting and boiling points

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.

Alkenes, but-1-ene propene boiling points, StudySmarterBut-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.

Shape

Alkenes are trigonal planar molecules. They have an angle of roughly 120° between each bond.

Alkenes, ethene bond angle, StudySmarterThe bond angle in an ethene molecule.Anna Brewer, StudySmarter Originals.

Reactivity

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:

  • A hydration reaction with steam and phosphoric acid catalyst to form an alcohol.
  • A reaction with a hydrogen halide to form a halogenoalkane.
  • Hydrogenation in the presence of a nickel catalyst to form an alkane.

How do you test for alkenes?

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.

Alkenes, testing for alkenes bromine water, StudySmarterTesting for alkenes. Here, ethene reacts with bromine water to produce 1,2-dibromoethane. Anna Brewer, StudySmarter Originals

How do alkenes compare to alkanes?

The following table compares alkenes to alkanes:

A table comparing alkanes and alkenes. Anna Brewer, StudySmarter Originals

Alkenes - Key takeaways

  • Alkenes, also known as olefins, are unsaturated hydrocarbons containing one or more carbon-carbon double bond (C=C).
  • Alkenes are named using the suffix -ene and standard nomenclature rules.
  • Alkenes can show positional, chain and geometric isomerism due to their C=C double bond.
  • Geometric isomers are named using E/Z or cis/trans notation.
  • Alkenes have similar solubility and boiling points to comparable alkanes, but are more reactive due to their C=C double bond.
  • Alkenes can react in electrophilic addition reactions.

Alkenes

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.

Final Alkenes Quiz

Question

What is an alkene?

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Answer

An alkene is an unsaturated hydrocarbon containing one or more carbon-carbon double bonds (C=C).


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Question

Define unsaturated.


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Answer

An unsaturated compound contains at least one C=C double bond.

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What is the general formula of an alkene?


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Answer


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What type/s of isomerism can alkenes show?


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Answer

  • Positional.
  • Chain.
  • Geometric.


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Question

What feature of alkenes means that they show geometric isomerism?

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Answer

C=C double bond. This prevents the molecule rotating. If two different groups are present on each side of the C=C double bond, the molecule shows geometric isomerism.

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Question

Which group takes first priority?

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Answer


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Question

Which group takes first priority?

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Answer

Cl

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Question

Name the following molecule:


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Answer

Z-pent-2-ene

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Name the following molecule:


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Answer

2-chloropropene

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Name this molecule:


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Answer

3-methylpent-1-ene

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Draw and label the two geometric isomers of 2-chlorobut-2-ene.


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How do you determine which group bonded to a C=C double bond takes first priority? 

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Answer

The group with the higher molecular mass takes first priority.

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What two letters are used to show geometric isomerism?

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A

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Alkenes are ____ in water.

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Soluble

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Alkenes have relatively ____ melting and boiling points.

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Low

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What shape are alkenes?

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Answer

Trigonal planar

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Question

Name the type/s of intermolecular force/s present between alkene molecules.


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Answer

Van der Waals forces only

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Question

Predict which alkene will have a higher boiling point:


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Answer

Propene

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Question

Predict which alkene will have a higher boiling point


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Answer

Butene

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Question

Why are alkenes more reactive than alkanes?


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Answer

  • Alkenes contain at least one C=C double bond. 
  • This is an area of high electron density and is attractive to electrophiles.
  • In contrast, alkanes contain only C-H and C-C single bonds, which aren't attractive to electrophiles.

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Name a common reaction type involving alkenes.

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Answer

Electrophilic addition.

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What is the bond angle around the C=C double bond?

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Answer

120°


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Question

Compare and contrast propene and propane.


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Answer

  • Both molecules contain only three carbon atoms, but propane has eight hydrogen atoms whilst propene has six.
  • Propane is saturated and has only C-C and C-H single bonds whilst propene is unsaturated and contains a C=C double bond. 
  • Both molecules have relatively low melting and boiling points, are insoluble in water and are nonpolar.
  • Propane is relatively unreactive but propene readily undergoes electrophilic addition reactions due to its C=C double bond. 


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Question

What is an electrophile?

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Answer

An electron pair acceptor with a vacant orbital and a positive or partially positive charge.

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Which of the following molecules are an electrophile?


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Question

What is the product of the electrophilic addition reaction involving HCl and ethene?

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Answer

Chloroethene.

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What is a carbocation?


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A positively charged carbon atom.

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What is the product of the reaction between propene, sulfuric acid, and water?

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Propanol

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Question

Give the conditions for the reaction between ethene and steam.


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Answer

  • 60 atm.
  • 300 ℃.
  • Phosphoric acid catalyst.

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Question

What is the colour change seen when cold acidic potassium permanganate solution reacts with propene?


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Answer

Purple to colourless.

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What is the colour change seen when cold alkaline potassium permanganate solution reacts with propene?

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Answer

Purple to dark green.

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Why is the reaction with potassium permanganate solution not a useful test for alkenes?


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Answer

Potassium permanganate can oxidise a variety of compounds and so will also change colour in reaction with molecules other than alkenes.


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Question

Describe the test for alkenes.

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Answer

Orange-brown bromine water decolourises.

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Question

What is the product of the reaction between an alkene and hydrogen in the presence of a nickel catalyst?


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Answer

An alkane.

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