CHAPTER 13: ALKENES Flashcards
What are alkenes?
• Unsaturated hydrocarbons containing 1 or multiple C=C bonds, which contains a T-bond.
• T-bonds are sideways overlap of adjacent p-orbitals above & below the bonding C atoms & a -bond (overlap of orbitals directly between bonding atoms).
• There is restricted rotation around the T-bond.
• This gives a trigonal planar shape.
• 3 regions of electron density around each C atom.
• They repel each other as far apart
as possible.
• Gives 120° bond angle round each C in C=C of alkenes.
• All of the atoms are in the same plane.
What are stereoisomers?
• Compounds with same structural formula but different arrangement in space.
E/Z Isomerism:
• E/Z isomerism is an example of stereoisomerism.
• Needs restricted rotation around a C=C & 2 different groups attached to each C atom of the C=C group.
• Restricted rotation is due to π-bond’s electron density above & below plane of δ-bond.
Cis-Trans Isomerism:
• Cis–trans isomerism is a special case of E/Z isomerism.
• 2 of the substituent groups attached to each carbon atom of the C=C group are the same.
• Cis Isomer = Z-isomer & Trans Isomer = E-Isomer.
What are the CIP rules?
- Attached groups are prioritised by atomic number.
- If groups of higher priority are on the same side of the double bond, the compound is the Z isomer.
- If groups of higher priority are on opposite sides of the double bond, the compound is the E isomer.
What are the properties of alkenes?
Low Bond Enthalpy: • Alkenes contain C=C double bonds. • These consist of a π-bond & δ-bond. • π-bond has a lower bond enthalpy than δ-bonds. • Therefore, it breaks more readily. • Allows alkenes to react more easily.
π-Bond Position:
• Electron density concentrated above & below δ-bond.
• π-bond electrons are more exposed.
• This makes it more open to electrophile attacks.
How can alkanes be formed from alkenes?
Hydrogenation:
- Mix the alkene with hydrogen.
- Add a nickel catalyst at 423K.
- Addition reaction occurs to form an alkane.
- One mole of hydrogen is required per double bond.
How can you test for unsaturation in a Carbon Chain?
Bromination:
- Alkenes react with Cl or Br at room temperature.
- Add bromine water (orange) dropwise to sample.
- Bromine adds across the double bond.
- The orange colour disappears, indicating a C=C bond.
- Unsaturated carbon chains decolourise bromine water.
How do haloalkanes form from alkenes?
- Alkenes react with gaseous hydrogen halides at room temperature to produce haloalkanes.
- If alkene is a gas, 2 gases are mixed for reaction.
- If it’s liquid, the hydrogen halide is bubbled through it.
- Can also react with concentrated HCl (aq) or HBr (aq).
- These are solutions of hydrogen halides in water.
- Unsymmetrical alkenes react with hydrogen halides to give 2 possible products.
How can alcohols be formed from alkenes?
Hydration:
- React alkenes with steam, H₂O (g).
- Occurs with phosphoric acid (H₃PO₄) catalyst.
- Steam adds across the double bond.
- There are two possible products.
What happens in electrophilic addition of a hydrogen halide?
- High electron density of π-bond attracts electrophiles.
- Electrophiles are electron pair acceptors.
- Electrophilic addition occurs by heterolytic fission.
- HBr adds to but-2-ene to form a single product.
- Br is more electronegative than H.
- HBr is polar & contains dipole: Hδ+ ⎯Brδ-.
- Electron pair in bond is attracted to δ+ H atom.
- This causes the double bond to break.
- Bond forms between H atom & a carbocation.
- H⎯Br bond breaks by heterolytic fission.
- The electron pair goes to the Br atom.
- A Br- & carbocation are formed.
- A carbocation is a +ve charged C atom.
- Br- ion reacts with carbocation to form the product.
What happens in electrophilic addition of a halogen?
- Br₂ is a non-polar molecule.
- π-electrons interact with electrons in Br⎯Br bond.
- This causes polarisation of the Br⎯Br bond.
- One end becomes Brδ+ & the other is Brδ-.
- This is known as an induced dipole.
- Electron pair in π-bond is attracted to Brδ+ end.
- This causes the double bond to break.
- A bond forms between one C atom & Br atom.
- Br⎯Br breaks by heterolytic fission.
- Electron pair goes to Brδ- end of the molecule.
- This forms Br- & a carbocation.
- The Br- & carbocation react to give the product.
What are polymers?
- Large molecules formed from many monomers.
- Monomers are repeat units.
- Unsaturated alkene molecules make long, saturated chains via addition polymerisation.
- Different polymers formed from different monomers.
- Repeat units are specific arrangements of atoms in polymer molecules which repeat.
- The repeat unit is always in square brackets.
- ‘n’ after brackets show large number of repeats.
How are waste polymers processed?
Recycling:
• Recycled products are recycled & sorted.
• Once sorted, they’re chopped into flakes, washed, dried, melted, cut into pellets & made to new products.
PVC Recycling:
• Solvents are used to dissolve PVC.
• It’s recovered by precipitation & solvent is used again.
Combustion:
• Some polymers are difficult to recycle.
• But, they have high stored energy value.
• They can be incinerated to produce heat.
• Generates steam to drive turbines & make electricity.
Feedstock Recycling:
• Reclaims monomers/gases/oil from waste polymers.
• Products used as raw materials to make new polymers.
• It can handle unsorted & unwashed polymers.
Biodegradable Polymers:
• Broken down by microorganisms into H₂O & CO₂.
Photodegradable Polymers:
• Used if plant-based polymers can’t be used.
• They contain bonds weakened by absorbing light.
• Light-absorbing additives can also be used.
What are the benefits of processing waste polymers?
Recycling:
• Conserves finite fossil fuels & reduces land fill waste.
• This reduces environmental impact of waste.
PVC Recycling:
• Disposal/recycling of PVC is hazardous.
• It’s due to high range of additives & chlorine content.
• Dumping PVC in landfills is not sustainable.
• Burning PVC release HCl (g), which is corrosive.
Combustion:
• Reduces dependence on finite resources for energy.
• Conserves fossil fuels & other finite resources.
Biodegradable Polymers:
• Renewable, sustainable choice to oil-based products.
• Protects environment & conserves oil.
Photodegradable Polymers:
• They leave no visible/toxic residues.
