Alkenes Flashcards
Why are alkenes more reactive than alkanes?
Carbon-carbon double bond. A region of high electron density so attracts electrophiles easily
How does a carbon-carbon double bond form?
Sideways overlap of a spare un-bonded singly filled p-orbital present on each carbon atom in the bond. This overlap produces a cloud of electron density above and below the molecule, forming a p bond
What bond angle does the carbon-carbon double bond have?
Planar, 120 degrees
What is structural isomerism?
Molecules with the same molecular formula but different structural formula
What is stereoisomerism?
Molecules with the same structural formula but different arrangement of atoms in space
What is geometric isomerism?
When there is a C=C bond and two different groups are bonded to each C atom in C=C. Occurs as a result of restricted rotation
What is a Z isomer?
The same groups are on the same side
What is an E isomer?
The same groups are on either side
When is cis or trans labelling revelant?
When there are at least two of the same groups present
What is cis?
The same groups are on either side
What is trans?
The same groups are on either side
What are the 4 addition reactions in alkenes?
- Hydrogenation - addition of hydrogen - used in the manufacture of margarine - requires H2 and Ni as a catalyst
- Halogenation - addition of a halogen - used in test-tube lab test for the presence of an alkene - requires Br2 or Cl2
- Addition of an acid - used in organic synthesis when making a haloalkane - requires HBr or HCl
- Hydration - addition of water - used to make industrial ethanol for fuel and solvent purposes - requires steam. temp above or equal to 100 degrees celcius and catalyst H2SO4
What is an electrophile?
Electron pair acceptor
What are the two steps for the electrophilic addition reaction for alkenes?
- A pair of electrons from C=C bond forms a covalent bond with the slightly positive X atom on the electrophile. X-Y bond breaks
- This forms a Y- ion with a lone pair and the addition of the X atom onto the alkene results in a positively charged carbon atom ‘carbocation intermediate’. The lone pair of electrons on the Y- ions form a covalent bond with the carbocation
What two things can alkenes react with in electrophilic addition?
A hydrogen halide or with bromine water or H20
Where does the major product form in unsymmetrical alkanes?
Via the more stable cation
What increases stability of a carbocation?
Presence of directly bonded alkyl groups as these groups are electron releasing relative to the hydrogen atoms, help to stabilise the positive charge on the carbocation. This is called the positive inductive effect
What is addition polymerisation?
Joining together many small molecules to form a very large polymer which are unreactive
Give 3 points about PVC (polychloroethene)
- Pure PVC tends to be hard and rigid due to permanent dipole-dipole forces between chains
- Plasticisers are added to the PVC to reduce the effectiveness of these attractions so plastic is more flexible.
- Most common plasticisers are phthalates
What are the 4 problems with plastics?
- Not quickly broken down my microbes. Non-biodegradable due to strong non-polar C-C bonds
- Buried plastics in landfill may take hundreds of years to decompose
- Although plastics are recycled in the UK - a large proportion are exported and dumped
- Tons of plastic found in the ocean, killing habitats and wildlife
What are 3 ways we can reduce the problems associated with plastics?
- Polymers can be melted down and remoulded by heating the polymer and breaking the London forces between polymer molecules
- Some plastics are separated from others, washed, melted and remoulded to make something else
- Some plastics are heated to a high temp that will break the polymer bonds and produce the monomers again. These can be used to make new plastics
