Reactions of Alkenes | Chemistry

📖 In-Depth Theory

Addition Reactions

The C=C double bond in alkenes makes them highly reactive — they undergo ADDITION REACTIONS.

In an ADDITION REACTION:

A small molecule adds across the C=C double bond.

The double bond becomes a single bond.

Only ONE product is formed — hence 100% atom economy.

General pattern:

alkene + small molecule → single product (no by-product)

ALKENES ALSO BURN:

Alkenes combust in air like other hydrocarbons.

Complete combustion: alkene + O₂ → CO₂ + H₂O

Tend to burn with SMOKY FLAMES in air — more carbon relative to hydrogen means incomplete combustion is common.

Key Addition Reactions

1. HYDROGENATION — addition of hydrogen:

Alkene + H₂ → alkane (saturated)

CH₂=CH₂ + H₂ → CH₃-CH₃ (ethene + hydrogen → ethane)

Conditions: nickel catalyst, ~200°C.

Industrial use: hydrogenation of vegetable oils (C=C bonds) to make margarine.

2. HYDRATION — addition of water (steam):

Alkene + H₂O → alcohol

CH₂=CH₂ + H₂O → CH₃CH₂OH (ethene + water → ethanol)

Conditions: phosphoric acid catalyst (H₃PO₄), high temperature (~300°C), high pressure.

Industrial method for making ethanol.

3. HALOGENATION — addition of bromine (or other halogens):

Alkene + Br₂ → dibromoalkane

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (ethene + bromine → 1,2-dibromoethane)

This is why alkenes decolourise bromine water — bromine is used up in an addition reaction.

4. ADDITION POLYMERISATION:

Many alkene monomers join together to form a long polymer chain.

nCH₂=CH₂ → (—CH₂—CH₂—)ₙ (ethene → poly(ethene))

100% atom economy — no by-products.

Comparing Alkene Reactions

WHY ADDITION REACTIONS ARE IMPORTANT:

The C=C double bond is a high-energy bond — energy released when it becomes two single bonds drives reactions.

Addition reactions have 100% atom economy — very efficient, no waste.

SUMMARY TABLE:

Reactant added | Conditions | Product

H₂ (hydrogen) | Ni catalyst, 200°C | Alkane (saturated)

H₂O (steam) | H₃PO₄ catalyst, 300°C, high pressure | Alcohol

Br₂ (bromine) | Room temperature | Dibromoalkane

Polymerisation | High pressure, catalyst | Polymer

ALL ALKENES undergo these reactions because they all have the C=C functional group.

INDUSTRIAL SIGNIFICANCE:

Hydrogenation: food industry (margarine production).

Hydration: production of ethanol for industrial solvents and fuels.

Polymerisation: plastics industry — poly(ethene), poly(propene), PVC.

⚠️ Common Mistake

Addition reactions produce only ONE product — the small molecule adds ACROSS the double bond, breaking it. Do not confuse with substitution reactions which swap atoms. Hydrogenation uses a NICKEL catalyst at 200°C; hydration uses a phosphoric acid catalyst at higher temperature and pressure.

📐 Key Equations

Hydrogenation: CH₂=CH₂ + H₂ → CH₃CH₃ (Ni catalyst)

Hydration: CH₂=CH₂ + H₂O → CH₃CH₂OH

Halogenation: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

📌 Key Note

Alkenes undergo addition reactions — small molecule adds across C=C, one product, 100% atom economy. Key reactions: + H₂ (→ alkane, Ni catalyst), + H₂O (→ alcohol, H₃PO₄ catalyst), + Br₂ (→ dibromoalkane, explains bromine water test), + monomers (→ polymer).

🎯 Matching Activity — Alkene Addition Reactions

Match each addition reaction to the product and conditions. — drag the symbols on the right to match the component names on the left.

+ H₂ (hydrogen)

Drop here

+ H₂O (steam)

Drop here

+ Br₂ (bromine)

Drop here

Polymerisation

Drop here

Dibromoalkane — room temperature — decolourises bromine water

Polymer — high pressure, catalyst — poly(ethene) from ethene

Alcohol — phosphoric acid catalyst, ~300°C, high pressure — industrial ethanol

Alkane — nickel catalyst, ~200°C — used to harden vegetable oils

🔬 Triple Science Only

Reactions of alkenes (4.7.2.2) is chemistry-only — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. What product forms when ethene reacts with steam? What are the conditions?

2. Why do addition reactions of alkenes have 100% atom economy?

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🧪 Reactions of Alkenes

Addition Reactions

Key Addition Reactions

Comparing Alkene Reactions

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