Alkene

Key Stage 4

Meaning

Alkenes are hydrocarbon compounds with one double bond and the general formula; C_nH_2n

About Alkenes

Alkenes are a homologous series of hydrocarbon compounds.

The functional group of the Alkenes is the double bonds between the Carbon atoms.

Alkenes are long chains of Carbon atoms covalently bonded together with double and single bonds and Hydrogen atoms taking the remaining bonds.

Alkenes are referred to as unsaturated hydrocarbons because the double bond means it is not saturated (full) with Hydrogen atoms.

Testing for alkenes can be done by adding them to Bromine Water. If the Bromine Water turns from orange to colourless, then an alkene is present. During this test the Bromine reacts with the alkene to form a Bromoalkane.

Ethene + Bromine → Dibromoethane

C₂H₄ + Br₂ -> C₂H₄Br₂

Examples

	Ethene	Propene	But-1-ene	But-2-ene
Chemical Formula (CnH2n)	C2H4	C3H6	C4H8	C4H8
Structural Formula	CH2CH2	CH2CHCH3	CH2CHCH2CH3	CH3CHCHCH3
Structural Diagram
Ball and Stick Model

Reactions of Alkenes

Combustion

During combustion of alkenes the Carbon and Hydrogen atoms are oxidised to produce Carbon Dioxide and Water.

Complete Combustion

Complete combustion occurs when there is enough Oxygen to completely Oxidise all of the atoms in the alkene. In the complete combustion of alkenes the only products are Carbon Dioxide and Water.

Ethene + Oxygen → Carbon Dioxide + Water

<chem>C2H4 + 3O2 -> 2CO2 + 2H2O</chem>

Propene + Oxygen → Carbon Dioxide + Water

<chem>2C3H6 + 9O2 -> 6CO2 + 6H2O</chem>

Butene + Oxygen → Carbon Dioxide + Water

<chem>C4H8 + 6O2 -> 4CO2 + 4H2O</chem>

Incomplete Combustion

Incomplete combustion occurs when there is not enough Oxygen to Oxidise all of the atoms in the alkene. During incomplete combustion of alkenes the products may include Carbon (soot) and Carbon Monoxide.

Ethene + Oxygen → Soot + Water

<chem>C2H4 + O2 -> 2C + 2H2O</chem>

Ethene + Oxygen → Carbon Monoxide + Water

<chem>C2H4 + 2O2 -> 2CO + 2H2O</chem>

Ethene + Oxygen → Soot + Carbon Monoxide + Water

<chem>2C2H4 + 3O2 -> 2C + 2CO + 4H2O</chem>

Reaction with Halogens

When halogens are added to alkenes the double bond in the alkenes breaks and the halogens bond in its place.

Ethene + Chlorine → Dichloroethane

<chem>C2H4 + Cl2 -> C2H4Cl2</chem>

Propene + Bromine → Dibromopropane

<chem>C3H6 + Br2 -> C3H6Br2</chem>

Butene + Iodine → Diiodobutane

<chem>C4H8 + I2 -> C4H8I2</chem>

Reaction with Hydrogen

Alkenes can be saturated with extra Hydrogen using a catalyst.

Ethene + Hydrogen → Ethane

<chem>C2H4 + H2 -> C2H6</chem>

Propene + Hydrogen → Propane

<chem>C3H6 + H2 -> C3H8</chem>

Butene + Hydrogen → Butane

<chem>C4H8 + H2 -> C4H10</chem>

Reaction with Steam

Alkenes can react with steam to produce alcohols using a catalyst.

Ethene + Steam ⇌ Ethanol

<chem>C2H4(g) + H2O(g) <=> C2H5OH(l)</chem>

Propene + Steam ⇌ Propanol

<chem>C3H6(g) + H2O(g) <=> C3H7OH(l)</chem>

Butene + Steam ⇌ Butanol

<chem>C4H8(g) + H2O(g) <=> C4H9OH(l)</chem>