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Alkenes

Got some unripe fruit that you just can't wait to eat? Perhaps a rock-solid kiwi, or a stubbornly hard avocado. There's a simple fix: put the fruit in a paper bag with a banana, and they'll ripen much more quickly. This is because bananas release a hormone called ethene, which speeds up the ripening process up considerably. Ethene is just one example of an alkene.

  • This article is about alkenes in organic chemistry.
  • We'll define alkene and look at their general formula.
  • After that, we'll turn our attention to alkene nomenclature and alkene isomerism.
  • We'll then explore the properties of alkenes, alkene production, and testing for alkenes.
  • Before we finish, we'll compare alkanes and alkenes.
  • Finally, we'll look at some examples of alkenes.

Alkene definition

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 (C=C) double bond.

Simply put, alkenes are molecules made up of just carbon and hydrogen atoms, with at least one C=C double bond. Alkenes are used to make polymers such as polystyrene and PVC, and can also be found in products such as antifreeze and paints.

Alkene general formula

Alkenes form a homologous series, characterised by their C=C double bond. Alkenes with just one C=C double bond are represented by the general formula CnH2n. Once you know the number of carbon atoms in an alkene, you can easily find out the number of hydrogen atoms - simply double the number of carbons.

Propene, an alkene, has 3 carbon atoms. How many hydrogen atoms does it have?

Let's look at our general formula for an alkene: CnH2n. Propene has 3 carbon atoms, and so n equals 3. Therefore, propene has (2 x 3) = 6 hydrogen atoms.

Check out Organic Compounds for the definition of a homologous series, as well as their properties.

Alkene nomenclature

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, just like numbers are used to show the position of other side chains or functional groups.

Let's consider an example.

Name this alkene:

Alkenes nomenclature example 3-methylbut-1-ene StudySmarterAn unknown alkene. StudySmarter Originals

We can see that this molecule 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-.

To show the position of the side chain and the C=C double bond, we use numbers. 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 3 + 1 = 4 or 3 + 2 = 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. This gives us 3-methylbut-1-ene.

Alkenes nomenclature example 3-methylbut-1-ene numbered StudySmarterOur unknown alkene, numbered correctly (green) and incorrectly (red). StudySmarter Originals

For more information on nomenclature, see Organic Nomenclature.

Alkene 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. They might have side chains in different places, for example, or perhaps side chains of different lengths. For example, 3-methylbut-1-ene and pent-1-ene are chain isomers - count the number of carbon atoms to make sure.

Alkenes chain isomers StudySmarterAlkene chain isomers. StudySmarter Originals

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. For example, but-1-ene and but-2-ene are position isomers.

Alkenes position isomers examples StudySmarterAlkene position iosmers. StudySmarter Originals

Geometric isomerism

Geometric isomerism is a form of stereoisomerism.

Stereoisomers have the same structural formula but different spatial arrangements of atoms.

Geometric isomerism 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 a 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, take but-2-ene. Each carbon atom involved in the C=C double bond is joined to a -CH3 group, and a hydrogen atom. In both cases, the -CH3 group takes higher priority. Hence, but-2-ene displays the following geometric isomerism:

Alkenes stereoisomers StudySmarterAlkene stereoisomerism. StudySmarter Originals

Note that in the molecule on the right, the -CH3 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.

If you combine structural isomerism and stereoisomerism together, the number of potential isomers of alkenes climbs rapidly as you increase the length of the carbon chain. For example, C5H10 has just six alkene isomers. C12H24, on the other hand, has 2,281 alkene isomers whilst C31H62 has 193,706,542,776!

For more information on stereoisomers and assigning priority, see Isomerism.

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 alkenes more reactive than alkanes, as explained below. Let's explore their solubility, melting and boiling points, shape, and reactivity.

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. However, they are soluble in other organic solvents.

Melting and boiling points

Alkenes have relatively low melting and boiling points. This is because the weak van der Waals forces between molecules do not require much energy to overcome. As the number of carbon atoms increases, boiling point increases, and as branching of the molecule increases, boiling point decreases.

For example, but-1-ene (C4H8) has a higher boiling point than propene (C3H6) as it has a longer carbon chain with more atoms.

Alkenes but-1-ene propene boiling points StudySmarterThe boiling points of but-1-ene and propene. StudySmarter Originals

In addition, but-1-ene has a higher boiling point than methylpropene (also C4H8). Although they have the same number of carbon atoms, methylpropene is branched, and so has weaker intermolecular forces between molecules.

Alkenes but-1-ene methylpropene boiling points StudySmarterThe boiling points of but-1-ene and methylpropene. 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 bond angle trigonal planar StudySmarterAlkene bond angle. 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:

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

Alkenes also react in other ways:

  • Oxidation with KMnO4 to form varying products.
  • Addition polymerisation to form polymers.

Head over to Reactions of Alkenes to learn about electrophilic addition and oxidation reactions. We explore addition polymerisation in greater depth in Polymerisation Reactions.

Producing alkenes

We'll now move on to learn about how you produce alkenes. This is done in a variety of different ways:

  • Cracking of alkanes.
  • Elimination of a halogenoalkane using hot ethanolic NaOH or KOH.
  • Dehydration of an alcohol, using hot Al2O3 or a concentrated acid.

You'll find all the necessary information you need to know about these reactions in Cracking (Chemistry), Elimination Reactions, and Reaction of Alcohols respectively.

Testing for alkenes

Testing for alkenes relies on an electrophilic addition reaction, like the ones we mentioned above. The process is simple: Shake an unknown substance with orange-brown bromine water (Br2). If the solution decolourises, there is an alkene present. This is because the bromine water adds to the double bond, forming a dihalogenoalkane.

Alkenes testing for alkenes bromine water StudySmarterTesting for alkenes using bromine water. StudySmarter Originals

Alkanes and alkenes

Throughout this article, we've mentioned alkanes. They are similar to alkenes, but they don't have any C=C double bonds. Instead, they contain just C-C and C-H single bonds. It is easy to get the structure, properties, and reactivity of alkanes and alkenes mixed up, so we've created a handy table comparing the two types of molecules. But of course, if you just want information about alkanes, head over to the article dedicated specifically to these organic hydrocarbons: Alkanes.

NameAlkaneAlkene
General formulaCnH2n+2CnH2n
SaturationSaturatedUnsaturated
BondsC-H, C-CC-H, C-C, C=C
Intermolecular forcesVan der Waals forces onlyVan der Waals forces only
Boiling pointLowLow
SolubilityInsoluble in water but soluble in other organic solventsInsoluble in water but soluble in other organic solvents
ReactivityLowHigh

Examples of alkenes

Last but not least, let's introduce you to some examples of alkenes. We've actually already encountered lots in this article. Here's a reminder of some of those molecules, as well as a few further examples that should hopefully capture your interest.

  • Ethene (C2H4) is the simplest alkene and is the most produced organic compound in the entire world! It has a range of uses. For example, it is not only an important plant hormone, as we found out in the introduction, but is also the basis of many polymers such poly(ethene). In addition, it can be turned into the alcohol ethanol.
  • Many polymers are made from alkenes. These include Teflon, the non-stick coating found on saucepans, as well as plastics like poly(propene) and PVC.
  • Other alkenes, such as butene (C4H8), are used in fuels, paints, and detergents. We also use alkene derivatives to make products like anti-freeze.

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 position, 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. We test for alkenes using an electrophilic addition reaction involving bromine water (Br2).
  • Examples of alkenes include ethene (C2H4) and butene (C4H8). Alkenes can be found in a variety of products, from polymers to paints.

Frequently Asked Questions about 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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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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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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Predict which alkene will have a higher boiling point:


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Answer

Propene

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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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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 electrophiles?


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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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Give the conditions for the reaction between ethene and steam.


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

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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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Describe the test for alkenes.

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Answer

Orange-brown bromine water decolourises.

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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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