Alkenes

This page covers content standard 10.4 of the Form 5 Carbon Compounds chapter: alkenes. Alkenes are the reactive cousins of the alkanes, and the contrast between the two families, saturated versus unsaturated, substitution versus addition, is one of the most heavily examined ideas in the chapter. The single feature that drives everything is the carbon–carbon double bond.

What are alkenes?

Alkenes are unsaturated hydrocarbons: they contain only carbon and hydrogen, but they have at least one carbon–carbon double bond (C=C). The C=C double bond is their functional group, the reactive site that gives the whole family its chemistry.

General formula: CₙH₂ₙ. For n = 2, C₂H₄ (ethene); for n = 3, C₃H₆ (propene); for n = 4, C₄H₈ (butene). Note that the smallest alkene is ethene; there is no “methene”, because you need at least two carbon atoms to form a C=C bond.

Name	Molecular formula	n
Ethene	C₂H₄	2
Propene	C₃H₆	3
Butene	C₄H₈	4

Physical properties

Alkenes follow the same homologous-series trends as alkanes: the first members are gases, boiling and melting points rise down the series as molecules get larger, and they are insoluble in water. In fact an alkene and the alkane with the same number of carbons have very similar physical properties, the difference between the families is chemical, not physical.

Why alkenes are more reactive than alkanes

The C=C double bond consists of one strong bond and one weaker bond. The weaker part of the double bond breaks easily, allowing other atoms to add on. This is why alkenes are more reactive than the saturated alkanes and undergo addition reactions, in which the double bond opens up and atoms add across it to give a single, saturated product.

Addition reactions of alkenes

Using ethene (C₂H₄) as the example:

• Addition of hydrogen (hydrogenation). With a nickel catalyst at about 180 °C, ethene adds hydrogen to form ethane (an alkane): C₂H₄ + H₂ → C₂H₆. This is how liquid vegetable oils are hardened into margarine.
• Addition of halogen. Ethene reacts with bromine (or chlorine) at room temperature: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane). The brown bromine is decolourised, this is the test for unsaturation.
• Addition of hydrogen halide. Ethene adds hydrogen bromide to give bromoethane: C₂H₄ + HBr → C₂H₅Br.
• Addition of water (hydration). With steam and a phosphoric acid catalyst, ethene adds water to form ethanol: C₂H₄ + H₂O → C₂H₅OH. This is an industrial route to ethanol.
• Polymerisation. Many ethene molecules add together to form the long-chain polymer poly(ethene), an addition polymerisation.

The test for unsaturation

To find out whether a compound is unsaturated (an alkene) or saturated (an alkane), shake it with brown bromine water:

• An alkene decolourises bromine water quickly at room temperature (addition reaction).
• An alkane does not decolourise bromine water under the same conditions; the brown colour stays.

Worked example

Question. Ethene is bubbled into brown bromine water. State what you would observe, name the type of reaction, and write the equation.

Step 1, Observation. The brown colour of the bromine water is decolourised (turns colourless).

Step 2, Type of reaction. Ethene is unsaturated, so the C=C double bond opens and bromine adds across it. This is an addition reaction (specifically, addition of bromine).

Step 3, Equation. The two bromine atoms add to the two carbon atoms of the double bond:

C₂H₄ + Br₂ → C₂H₄Br₂

Answer. The bromine water is decolourised; the reaction is an addition reaction; the equation is C₂H₄ + Br₂ → C₂H₄Br₂, forming 1,2-dibromoethane.

Practice question

Question. Explain, in terms of bonding, why ethene undergoes addition reactions while ethane undergoes only substitution. Give one example reaction for ethene.

Answer. Ethene contains a carbon–carbon double bond (C=C), so it is unsaturated. The double bond can open up, allowing atoms to add across it, so ethene undergoes addition reactions, for example, C₂H₄ + H₂ → C₂H₆ with a nickel catalyst, or C₂H₄ + Br₂ → C₂H₄Br₂ which decolourises bromine water. Ethane has only single carbon–carbon bonds and is saturated, so there is no double bond to open; it can only react by substitution (replacing a hydrogen atom), and needs ultraviolet light to do so.

Exam tip

The bromine water test is one of the most reliable marks in the paper: alkene decolourises bromine water; alkane does not. Always name alkene reactions as addition, never substitution, that is the single biggest slip in this topic. When you write an addition equation, remember the double bond becomes a single bond and the product is saturated, so add atoms to both carbons of the C=C. For combustion, note that alkenes burn with a more luminous, sootier flame than alkanes because they have a higher percentage of carbon by mass.

How this connects

Alkenes tie the chapter together. Their addition of water (hydration) is one way to make alcohols (standard 10.6); their addition polymerisation leads straight into the Polymers chapter; and the alkene-versus-alkane comparison is the classic “distinguish these two compounds” question. Because the bromine water test and addition reactions appear so often across SPM Chemistry, this standard rewards careful study.

Where this fits

This is content standard 10.4 of the Carbon Compounds chapter, following directly on from alkanes so the two can be compared. Practise the addition equations in the chapter worked examples and make sure you never write “substitution” for an alkene using the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers drill the bromine water test and every addition reaction until the alkane/alkene distinction is second nature.

Quick recap

• Alkenes are unsaturated hydrocarbons, general formula CₙH₂ₙ, functional group C=C.
• First members: ethene, propene, butene (C₂H₄, C₃H₆, C₄H₈); there is no methene.
• The C=C makes alkenes more reactive than alkanes → addition reactions.
• Additions: H₂ (Ni catalyst), Br₂/Cl₂, HBr, H₂O (steam), polymerisation.
• Test for unsaturation: an alkene decolourises bromine water; an alkane does not.