AS Chemistry A OCR - Alkenes - Revision Cards in A Level and IB Chemistry

Properties of Alkenes.

Alkenes are unsaturated (Contain at least one multiple bond) hydrocarbons (contain only hydrogen and carbon)
Alkenes contain a C=C double bond.
- Double bond is made up of a π bond and a σ bond.
  - π bond is the sideways overlap of adjacent P orbitals above and below the bonding atoms.
  - π bonds have restricted rotation.
  - σ bond is the overlap of orbitals directly between the two nuclei.
Alkenes are trigonal planer around one of the carbons of the double bond.
- 3 bonding regions, 4 bonding pairs, no lone pairs.
- Bond angle of 120 degrees.
Alkenes are relatively reactive.
- π bond has low bond enthalpy.
- double bond has high electron density that attracts electrophiles. (electron pair acceptors)

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Stereoisomerism
- Compounds with the same structural formula, but a different arrangement in space.
E/Z isomerism.
- Only happens when there are 2 different species on each of the carbon atoms in a double bond.
- The restricted rotation means that if two bonds are facing opposite, they can't spin to allign.
- E is when the two species with the highest total atomic number are facing opposite directions.
- Z is when the two species with the highest total atomic number are facing the same direction.
Cis/Trans Isomerism.
- When there is one of the same species on each of the carbons of the double bond.
- Cis is when they are on the same side.
- Trans is when they are on the opposite side.

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Addition of Dihydrogen (H2).
- Reagents: H2
- Conditions: Presence of Nickel catalyst
- Product: Alkane
Addition of Halogen (Cl2, Br2, etc)
- Reagents: Dihalogen
- Conditions: None
- Product: Dihaloalkane
- Use: Bromine Water (aqueous Br2) is used to check if a sample is unsaturated.
  - Colour Change: Orange/Brown to Colourless.
Addition of Hydrogen Halide (HBr, HCl, etc)
- Reagents: Hydrogen Halide
- Conditions: None
- Product: Haloalkane
Addition of H2O
- Reagents: H2O
- Conditions: H3PO4 Catalyst, high temperature
- Product: Alcohol

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Halogen
- Br2 is not polar.
- Br2 becomes polar because of electron repulsion by the double bond.
- One atom of Br is bonded to the Alkene and the other becomes a negative ion.
- The negative ion bonds to the carbocation.

Hydrogen Halide
- HBr is polar
- The slightly positive H atom is attracted to the double bond and reacts with it.
- The slightly negative Br atom becomes an negative Ion.
- The negative Br ion bonds with the carbocation.

(http://alevelchem.com/img/electrophilic_addition_hydrogen_bromide.gif)

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Types of stability
- Primary
  - The carbon that has the positive charge is connected to one other carbon.
  - The least stable.
- Secondary
  - The carbon that has the positive charge is connected to two other carbons.
- Tertiary
  - The carbon that has the positive charge is connected to three other carbons.
  - The most stable.
Markownikoff’s rule is that the major organic product of a reaction will be the one with the most stable carbocation step.

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Monomer
- A single alkene that will be made into a polymer.
Polymer
- Lots of monomers repeating by the double bond.
Repeating unit
- What one molecule of an alkene looks like as part of the polymer.

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The benifits of cheap oil based polymers is counteracted by the problems they cause the enviroment in landfills.
Combustion
- Burnt to create energy.
Used as a feedstock to create other plastic items.
- The plastic is cleaned, melted down, and recasted into a new item.
Removal of toxic waste products from incinerated halogenated plastics.
- A base or carbonate is added to the incinerator chimney.

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