Alkanes and Alkenes

Alkane

Alkane

A group of saturated hydrocarbons

the term saturated means that they only have single carbon-carbon bonds, there are no double bonds (remember carbon can only have a maximum of 4 bonds)

Structure of an alkane example (propane model)

general formula for alkanes

 C_nH_2n+2

How do alkanes react

Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller molecules and can react with halogens in the presence of light

List the alkanes (starting from the one with only one carbon to 5 carbons)

• Methane

• Ethane

• Propane

• Butane

• Pentane

Using the formula, what is propane molecular formula (Propane has 3 carbons)

 C_nH_2n+2 (n = number of carbons)

C₃H_(2×3)+2 = C₃H₈

What happens when Alkanes and halogens are in the presence of ultraviolet light

A substitution reaction, one atom is swapped with another atom. Alkanes undergo a substitution reaction with halogens in the presence of ultraviolet radiation

Alkene

Alkenes are a homologous series of hydrocarbon compounds with at least one double bond between two of the carbon atoms on the chain

The double bond can be written as carbon to carbon double bond or as C=C

Structure of an alkene model (Propene model used)

General formula for alkenes

C_nH_2n

Using the formula, what is propene molecular formula (Propene has 3 carbons)

C_nH_2n = C₃H_2×3

C₃H₆

Which is more reactive, alkanes or alkenes and why

The double bond alkenes have means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group 

Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds

This makes them much more reactive than alkanes

How to work out if a substance is an alkane or an alkene

bromine water test

Bromine water is an orange-coloured solution

When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in the solution

But when bromine water is added to an alkene, the bromine atoms add across the C=C bond, hence the solution no longer contains free bromine so it loses its colour