Alkenes

What are alkenes?

• unsaturated hydrocarbons that have a double bond, which can break to allow atoms to be added to the molecule

• Added together to make polymers

• More reactive than alkanes

Why are alkenes a homologous series?

• contains same functional group (C=C)

• General formula = CnH2n

• Share similar chemical properties

• Graduated physical properties

Names of alkenes and their molecular formulas

Ethene - C2H4

Propene - C3H6

Butene - C4H8

Pentene - C5H10

Hexene - C6H12

Reactions of alkenes with bromine water

• Bromine forms an orange-brown colour in water

• Addition reaction occurs, a double bond will break and the bromine atoms will be added to the alkene

• The product formed is called a dibromo(alkanes) and is colourless

• Small amount of bromine water will react with the dibromo(alkane) to form a colourless solution

• Alkanes are already saturated, no room for bromine to be added

• Therefore no reaction and solution remains orange-brown

What is an addition reaction

A chemical reaction where two or more molecules join together to form a larger molecule with no leftover products

What is an addition polymer?

• formed by many small molecules called monomers joining up together

What are rules when drawing addition polymers?

• need to draw monomers facing up and down rather than out at angles

• Repeating unit - draw the empty bonds pointing out to the left or right through the brackets

• (N)s after the brackets to present no of repeats of monomers

What do addition polymerisation require?

• high pressure and a catalyst

How to name an addition polymer?

poly(alkene)

What are problems with an addition polymer?

• Non-biodegradable because difficult to break the long chains of strong C=C double bonds

• Therefore takes up lots of space in landfill

• When burned, CO2 released and toxic CO gas in incomplete combustion

• Some addition polymers are thermosetting (doesn’t melt when heated)

• Therefore cannot be recycled