Alkenes

How is the double bond formed?

The double bond results from the sideways overlap of a spare un-bonded singly filled p-orbital

Name and Angle of the C=C

• Planar

• 120 degrees

Structural Isomerism

Molecules with the same molecular formula but different structural formula

Stereoisomerism

Molecules with the same molecular formula but different arrangements of atoms in space

Why does stereoisomerism occur?

It occurs as a result of restricted rotation about the C=C double bond

Why do some molecules not show stereoisomerism?

On one of the carbon atoms of the C=C bond it has two of the same groups attached,

Z isomerism or Cis Isomerism

the priority groups are on the same side

E isomerism or Trans Isomerism

the priority groups are on different sides

Reaction of Alkenes: Hydrogenation

• Addition of hydrogen

• Reagents: H_2/Ni

• Importance: used in the manufacture of margarine

Reaction of Alkenes: Halogenation

• Addition of a halogen

• Reagents: Br₂ or Cl₂

• Importance used in the test tube lab to test for the presence of an alkene

Reaction of Alkenes: Addition of an acid

• Reagents: HBr or HCl

• Importance: used in organic synthesis when making a haloalkane

Reaction of Alkenes: Hydration

• Addition of water

• Reagents: Steam, add a catalyst

• Importance: used to make industrial ethanol for fuel and solvent purposes

Electrophile

Electron pair acceptor

Electrophilic addition

Why does hydrogen react more readily with alkenes than with alkanes?

The bond enthalpy of C-C sigma bonds is higher than that of pi bonds

What happens when an unsymmetrical alkene is added to HBr or HCl

A major product and minor product is form

The major product is formed via the more stable carbocation

Carbocations

What increases the stability of carbocations?

The presence of alkyl groups

Addition polymerisation

Uses of Polymers

• Poly(ethene) - washing up bowls, plastic bags

• Poly(propene) - Rope

Poly(chlororethene) PVC

• Hard and rigid as there is PDD, Plasticisers are added to make the plastic more flexible

Waste polymers and alternatives: Combustion for energy production

Polymers can be burned to release heat energy for generating electricity

Adv/Disadv of using combustion for energy production

• Advantage - Reduces the need for combusting fossil fuels

• Disadvantage - Produces HCl gas which is toxic. Can be prevented from entering atmosphere by reacting with bases e.g. CaCO₃ and CaO

  - Releases CO2

  - Releases CO. Ensure there is plenty of oxygen to avoid incomplete combustion.

Waste polymers and Alternatives: Organic Feedstock

Waste polymers can be converted into other chemicals such as alkenes, that can be used in manufacturing processes

Advantage of Organic Feedstock

Reduces the need for cracking crude oil

Biodegradable and Photodegradable Polymers

Plastics can be made from renewable food sources - corn starch - this reduces the dependency on crude oil

• Biodegradable - naturally broken down by bacteria

• Photodegradable - naturally broken down by sunlight