Alkenes

Why are alkenes more reactive than alkanes?

• Due to the carbon-carbon double bond

  • This is a region of high electron density so it attracts electrophiles easily

How is the π bond formed?

It is formed from the sideways overlap of a spare un-bonded singly filled p-orbital

Why does hydrogen react more readily with alkenes than alkanes?

The bond enthalpy of the C-C σ bond is higher than that of the π bonds

Why does the C=C bond give rise to stereoisomers?

There is no free rotation around it

Shape and Bond angles of Alkenes

• Shape - Planar

• Bond Angle - 120°

Structural Isomerism

Molecules with the same molecular formula but different structural formula

Stereoisomerism

Molecules with the same structural formula but different arrangement of atoms in space

Why do some molecules not show stereoisomerism?

• One of the carbon atoms of the C=C bond has two of the same groups attached

Z isomerism

The same groups are on the same side

E isomerism

The same groups are on either side

Cis isomerism (Two of the same groups present)

Same groups are on the same side

Trans isomerism (Two of the same groups presents)

Same groups are on either side

Cahn-Ingold-Prelog

• Look at left carbon and see which group takes priority

• Look at right carbon and see which group takes priority

Reaction of Alkenes: Hydrogenation

• Addition of hydrogen

• Reagents - H₂, Nickel catalyst

• Used in the manufacture of margarine

• Turns Alkene to Alkane

Reaction of Alkenes: Halogenation

• Addition of a halogen

• Reagents - Br₂ or Cl₂

• 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

• Used in organic synthesis when making a haloalkane

Reaction of Alkenes: Hydration

• Addition of water

• Reagents - Steam, Add a catalyst H₂PO₄, H₃PO₄

• Used to make industrial ethanol for fuel and solvent purposes

Electrophile

Electron pair acceptor

Electrophilic Addition Mechanism

Unsymmetrical alkenes

The major product is formed the from the more stable carbocation

How is the stability of carbocations increased?

By the presence of alkyl groups - CH₃, CH₂, CH

Addition Polymerisation

Uses of Poly(ethene)

• Washing up bowls, Plastic bags

• Monomer - H₂C=CH₂

Uses if Poly(propene)

• Rope

• Monomer - CH₃CH=CH₂

Why is PVC hard and rigid?

There are permanent-dipole-dipole forces between the chains

What is added to PVC to make plastics more flexible?

Plasticisers

Waste polymers and alternatives: Combustion for energy production

Polymers can be burned to release heat energy for generating electricity

Combustion for energy production: Advantage

Reduces the need for combusting fossil fuels

Combustion for energy production: Disadvantage and how to minimise them

• Can produce HCl gas which is toxic and corrosive

  • Can be prevented by reacting it with bases , CaCO3 or CaO, to neutralise it

• Releases CO2

• Releases CO

  • Ensure plenty of oxygen to prevent incomplete combustion

Waste polymers and alternatives: Organic Feedstock

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

Organic Feedstock: Advantage

Reduces the need for cracking crude oil

Biodegradable polymers

Naturally broken down by bacteria

Photodegradable polymers

Naturally broken down by sunlight