📖 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.