Alkenes

Alkene definition

Unsaturated hydrocarbons containing a C=C double bond

General formula

CₙH₂ₙ

Bond type

One double bond (C=C) + single bonds

Unsaturated meaning

Can undergo addition reactions because of C=C

Homologous series

Alkenes form a homologous series with similar properties

Ethene formula

C₂H₄

Propene formula

C₃H₆

Butene formula

C₄H₈

Boiling point trend

Increases with chain length

Flammability trend

Decreases with chain length

Reactivity

More reactive than alkanes due to C=C bond

Combustion

Burns with smoky flame due to incomplete combustion

Addition reaction definition

Atoms add across the double bond

Hydrogenation

Alkene + hydrogen → alkane (Ni catalyst, 150°C)

Halogenation

Alkene + bromine → dibromoalkane

Bromine water test

Decolourises from orange to colourless with alkenes

Hydration

Alkene + steam → alcohol (H₃PO₄ catalyst)

Polymerisation

Many alkene molecules join to form addition polymers

Example polymerisation

Ethene → poly(ethene)

Key idea

Alkenes are reactive due to the C=C bond and undergo addition reactions

ethene displayed formula

propene displayed formula

but-1-ene displayed formula

pent-1-ene displayed formula