Alkenes, Alcohols and Carboxylic Acids » Reactions of Alkenes

The C=C double bond decides everything

Alkenes are hydrocarbons with the functional group C=C. The functional group is the part of the molecule that decides how it reacts. Every alkene has the same C=C bond, so every alkene reacts in the same ways. Learn the reactions of ethene and you can predict the reactions of propene, butene and pentene too.

Key terms:

• Addition reaction: a reaction where atoms are added across the C=C double bond, so the double bond becomes a single bond.
• Functional group: the part of a molecule that decides its reactions. For alkenes it is C=C.

Burning alkenes

Alkenes burn in oxygen in the same way as other hydrocarbons. With plenty of air they make carbon dioxide and water. But in air they tend to burn with smoky flames. This is because of incomplete combustion: there is not enough oxygen to burn all the carbon, so tiny bits of unburnt carbon (soot) are left in the flame.

Addition reactions

The C=C double bond is the reactive part of an alkene. In an addition reaction, one of the two bonds in the double bond breaks. Each carbon atom then picks up a new atom. The double bond becomes a single bond, and only one product is made. Nothing is lost, so the product is a bigger molecule than the alkene you started with.

Alkenes do this with three types of reactant: hydrogen, water and the halogens.

Adding hydrogen

An alkene reacts with hydrogen to make an alkane. The reaction needs a nickel catalyst and the mixture is heated. The double bond becomes a single bond, so the molecule becomes saturated.

Ethene + hydrogen → ethane

C₂H₄ + H₂ → C₂H₆

Propene gives propane (C₃H₆ + H₂ → C₃H₈). Butene gives butane (C₄H₈ + H₂ → C₄H₁₀).

Adding water

An alkene reacts with water, as steam, to make an alcohol. The conditions are a high temperature, high pressure and an acid catalyst (phosphoric acid). One H atom goes onto one carbon and an OH group goes onto the other.

Ethene + steam → ethanol

C₂H₄ + H₂O → C₂H₅OH

The alcohol has the functional group –OH. You will meet alcohols properly in the next lessons.

Adding halogens

Halogens such as chlorine, bromine and iodine add across the double bond very easily. This happens at room temperature with no catalyst. Each carbon of the old double bond gets one halogen atom.

Ethene + bromine → dibromoethane

C₂H₄ + Br₂ → C₂H₄Br₂

Chlorine and iodine work in the same way, giving dichloroethane and diiodoethane. Orange-brown bromine water turns colourless when it reacts. This is the test for alkenes you met earlier.

Drawing the products

You must be able to draw fully displayed formulae for the first four alkenes (ethene, propene, butene and pentene) and their products. A fully displayed formula shows every atom and every bond. Follow these steps:

1. Draw the alkene with every atom and bond, including the C=C.
2. Change the C=C into a single C–C bond.
3. Put one new atom (or group) on each of the two carbon atoms that were joined by the double bond.
4. Check every carbon has exactly four bonds.

For ethene, each carbon has two H atoms. After adding hydrogen, each carbon has three H atoms and one C–C bond, so you have ethane. After adding chlorine, each carbon has two H atoms, one Cl and the C–C bond, so you have dichloroethane. After adding water, one carbon gets an H and the other gets an OH, so you have ethanol.

The same method works for longer alkenes. The new atoms go onto the two carbons of the double bond and the rest of the molecule stays as it is. Adding bromine to propene gives CH₃CHBrCH₂Br, which is dibromopropane. In your drawing, the CH₃ group must show all three C–H bonds.

Exam-style question

Propene (C₃H₆) reacts with hydrogen. (a) Name the catalyst needed. (b) Give the name and formula of the product. (c) Explain why this is called an addition reaction.