Alkenes

Alkene definition

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 C_nH_2n. 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.

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:

An 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.

Our unknown alkene, numbered correctly (green) and incorrectly (red). StudySmarter Originals

Alkene isomerism

Alkenes show three types of isomerism.

• Chain isomerism.
• Position isomerism.
• Geometric isomerism

Chain isomerism

Chain isomerism is a type of structural isomerism.

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.

Alkene 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.

Alkene position iosmers. StudySmarter Originals

Geometric isomerism

Geometric isomerism is a form of stereoisomerism.

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:

Alkene 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.

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.

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.

The 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.

The boiling points of but-1-ene and methylpropene. StudySmarter Originals

Shape

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

Alkene 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.

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.

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.

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.

Testing 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.

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.