Alkenes and Their Reactions

This lesson covers alkenes — unsaturated hydrocarbons with a C=C double bond — and their key reactions, as required by the Edexcel GCSE Chemistry specification (1CH0). You need to know the general formula, the first three members, how to test for unsaturation, and the important addition reactions (hydrogenation and hydration).

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What Are Alkenes?

Alkenes are a homologous series of hydrocarbons that contain at least one carbon–carbon double bond (C=C). Because they have a double bond, they have fewer hydrogen atoms than the equivalent alkane and are described as unsaturated.

Feature	Detail
General formula	CₙH₂ₙ
Bonding	Contains a C=C double bond
Saturation	Unsaturated (not all carbon atoms are bonded to the maximum number of hydrogen atoms)
Reactivity	More reactive than alkanes because of the double bond

Comparing Alkanes and Alkenes

Property	Alkanes	Alkenes
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Bonding	Single C–C bonds only	Contains a C=C double bond
Saturation	Saturated	Unsaturated
Reactivity	Relatively unreactive	More reactive (undergo addition reactions)

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The First Three Alkenes

Name	Molecular formula	Number of C atoms	Displayed formula description
Ethene	C₂H₄	2	H₂C=CH₂ — two carbon atoms joined by a double bond, each with two hydrogen atoms
Propene	C₃H₆	3	CH₃–CH=CH₂ — three carbon chain with the double bond between C-2 and C-3
Butene	C₄H₈	4	CH₃–CH₂–CH=CH₂ — four carbon chain with the double bond between C-3 and C-4 (but-1-ene)

Exam Tip: Notice that there is no "methene" — you need at least two carbon atoms to form a C=C double bond. The simplest alkene is ethene.

Drawing Displayed Formulae

When drawing the displayed formula of an alkene:

• Show the double bond as two parallel lines between the two carbon atoms: C=C.
• Each carbon in the double bond forms four bonds in total (the double bond counts as two bonds).
• The two carbons in the double bond each have fewer hydrogen atoms than they would in the equivalent alkane.

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Testing for Unsaturation — The Bromine Water Test

The test to distinguish between alkanes (saturated) and alkenes (unsaturated) uses bromine water.

Method

1. Add a few drops of bromine water (an orange/brown solution) to the hydrocarbon.
2. Shake the mixture.
3. Observe the colour change.

Results

Hydrocarbon	Observation	Conclusion
Alkene (unsaturated)	Bromine water changes from orange/brown to colourless (decolourises)	The C=C double bond reacts with bromine — addition reaction occurs
Alkane (saturated)	Bromine water stays orange/brown (no colour change)	No double bond to react with — no addition reaction

Exam Tip: The key phrase is "bromine water is decolourised" (not "turns clear"). Colourless and clear are different things in chemistry — a coloured solution can still be clear. Always say decolourised.

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Addition Reactions

Alkenes are more reactive than alkanes because the C=C double bond can open up, allowing atoms to add across the double bond. This type of reaction is called an addition reaction.

In an addition reaction:

• The double bond (C=C) opens to become a single bond (C–C).
• Atoms from the other reactant bond to each of the two carbon atoms.
• Only one product is formed (no atoms are lost).

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Hydrogenation

Hydrogenation is the addition of hydrogen (H₂) across the double bond of an alkene to form an alkane.

Conditions

• Reactants: Alkene + hydrogen gas (H₂)
• Temperature: About 150 °C
• Catalyst: Nickel catalyst

Equation (Ethene)

Word equation: ethene + hydrogen → ethane

Symbol equation: C₂H₄ + H₂ → C₂H₆

Uses of Hydrogenation

Hydrogenation is used industrially to convert vegetable oils (which contain C=C double bonds) into solid fats for making margarine and other spreads. The process hardens the oil by converting unsaturated fats into saturated fats.

Before hydrogenation	After hydrogenation
Liquid vegetable oil (unsaturated)	Solid/semi-solid fat (saturated)
Contains C=C double bonds	C=C bonds converted to C–C single bonds

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Hydration

Hydration is the addition of water (as steam) across the double bond of an alkene to produce an alcohol.

Conditions

• Reactants: Alkene + steam (H₂O)
• Temperature: About 300 °C
• Pressure: High pressure (~60–70 atm)
• Catalyst: Phosphoric acid (H₃PO₄) catalyst

Equation (Ethene → Ethanol)

Word equation: ethene + steam → ethanol

Symbol equation: C₂H₄ + H₂O → C₂H₅OH

This is the industrial method for producing ethanol (the alcohol found in alcoholic drinks and used as a solvent and fuel). Ethanol produced this way is sometimes called synthetic ethanol because it is made from a petrochemical feedstock (ethene) rather than from fermentation.

Exam Tip: Make sure you can recall the catalyst for each reaction: nickel for hydrogenation and phosphoric acid for hydration. These are commonly tested.

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Reaction of Alkenes with Bromine

The reaction of an alkene with bromine is also an addition reaction (this is the basis of the bromine water test):

Word equation: ethene + bromine → dibromoethane

Symbol equation: C₂H₄ + Br₂ → C₂H₄Br₂

The product, 1,2-dibromoethane, is colourless, which is why the orange bromine water is decolourised.

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Summary of Addition Reactions