Reactions of Alkenes [H]

This lesson covers the reactions of alkenes as required by the AQA GCSE Chemistry specification (5.8.2). This is Higher Tier only [H] content. Alkenes are much more reactive than alkanes because of their C=C double bond, which can open up to allow other atoms to add across it. These addition reactions are the key reactions of alkenes.

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Why Are Alkenes Reactive?

Alkenes contain a C=C double bond, which makes them much more reactive than alkanes. The double bond is a region of high electron density that attracts other molecules and atoms.

The C=C double bond can break open, allowing atoms to add across the bond. This is why alkenes undergo addition reactions — the double bond breaks, and two new single bonds form.

Feature	Alkanes	Alkenes
Bonding	Single C–C bonds only	Contains C=C double bond
Saturation	Saturated	Unsaturated
Reactivity	Relatively unreactive	More reactive
Main reaction type	Combustion	Addition reactions

Exam Tip: [H] When explaining why alkenes are more reactive than alkanes, always refer to the C=C double bond. State that the double bond provides a site for addition reactions, making alkenes more reactive.

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Addition Reactions

An addition reaction is a reaction in which atoms or groups of atoms are added across the C=C double bond of an alkene. The double bond breaks and forms two new single bonds. No atoms are lost — the product is a single, larger molecule.

General Scheme

graph LR
    A["Alkene<br/>(C=C double bond)"] -->|"+ Reactant"| B["Addition Product<br/>(Single bonds only)"]
    B --> C["No atoms lost"]
    B --> D["One product only"]
    B --> E["Product is saturated"]

    style A fill:#27ae60,color:#fff
    style B fill:#2980b9,color:#fff

The key feature of addition reactions is that the unsaturated alkene becomes a saturated product.

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Reaction 1: Addition of Hydrogen (Hydrogenation)

When an alkene reacts with hydrogen gas in the presence of a nickel catalyst, the hydrogen adds across the double bond to form an alkane. This is called hydrogenation.

Example: Hydrogenation of Ethene

ethene + hydrogen → ethane

C2H4 + H2 → C2H6

Conditions

Condition	Detail
Temperature	Approximately 150°C
Catalyst	Nickel
Pressure	Moderate

Uses of Hydrogenation

Hydrogenation is used industrially to convert vegetable oils (which contain C=C double bonds, making them unsaturated liquids) into margarine and other solid fats. The process hardens the oils by converting them from unsaturated to saturated molecules, raising their melting point so they become solid at room temperature.

Exam Tip: [H] Remember that hydrogenation converts an unsaturated compound to a saturated one. The C=C double bond breaks, and the hydrogen atoms add across it. This is why vegetable oils (liquids) become solid fats (margarine).

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Reaction 2: Addition of Water (Hydration)

When an alkene reacts with steam in the presence of a phosphoric acid catalyst, the water adds across the double bond to form an alcohol. This is called hydration.

Example: Hydration of Ethene

ethene + steam → ethanol

C2H4 + H2O → C2H5OH

Conditions

Condition	Detail
Temperature	Approximately 300°C
Catalyst	Phosphoric acid (H3PO4)
Pressure	High (approximately 60–70 atm)

This is one of two industrial methods of producing ethanol (the other being fermentation of sugars).

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Reaction 3: Addition of Halogens (Halogenation)

When an alkene reacts with a halogen (such as bromine or chlorine), the halogen atoms add across the double bond to form a dihalogenoalkane.

Example: Bromination of Ethene

ethene + bromine → 1,2-dibromoethane

C2H4 + Br2 → C2H4Br2

This is the basis of the bromine water test for unsaturated compounds:

• Bromine water (orange) is decolourised by alkenes
• Bromine water remains orange with alkanes (no reaction)

Example: Chlorination of Ethene

ethene + chlorine → 1,2-dichloroethane

C2H4 + Cl2 → C2H4Cl2

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Reaction 4: Addition of Hydrogen Halides

When an alkene reacts with a hydrogen halide (such as HBr or HCl), the hydrogen halide adds across the double bond to form a halogenoalkane.

Example: Ethene with Hydrogen Bromide

ethene + hydrogen bromide → bromoethane

C2H4 + HBr → C2H5Br

The hydrogen atom bonds to one carbon, and the bromine atom bonds to the other.

Exam Tip: [H] In all addition reactions of alkenes, the C=C double bond breaks and is replaced by two new single bonds. The product is always a saturated molecule with no double bond. Make sure you can draw the displayed formula of the product.

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Summary of Addition Reactions

Reaction	Reactant Added	Product	Conditions
Hydrogenation	H2	Alkane	Nickel catalyst, ~150°C
Hydration	H2O (steam)	Alcohol	Phosphoric acid catalyst, ~300°C, high pressure
Halogenation (Br2)	Br2	Dihalogenoalkane	Room temperature
Halogenation (Cl2)	Cl2	Dihalogenoalkane	Room temperature
Addition of HBr	HBr	Halogenoalkane	Room temperature

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Combustion of Alkenes

Like alkanes, alkenes also undergo combustion. However, because alkenes have a higher ratio of carbon to hydrogen compared to alkanes with the same number of carbons, they tend to burn with a smokier flame (more soot is produced due to incomplete combustion).

Complete Combustion of Ethene

ethene + oxygen → carbon dioxide + water

C2H4 + 3O2 → 2CO2 + 2H2O

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Distinguishing Alkanes from Alkenes

Test	Alkane	Alkene
Bromine water	Stays orange (no reaction)	Decolourised (turns colourless)
Flame	Burns with a clean blue flame	Burns with a smokier, more luminous flame
Reaction type	Mainly combustion	Addition reactions + combustion

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Summary