4.2 - Alkenes Flashcards
What are Alkenes ?
They are a group of unsaturated hydrocarbons with the general formula CnH2n
What type of bonds are Alkenes composed of ?
The C=C carbon double bond consists of one sigma bind and one pi bond.
How is a sigma bond formed ?
A sigma bond is formed by the overlap of 2 atomic orbitals between thr nuclei of two bonded atoms
How is a pi bond formed ?
A pi bond is formed by the sideways overlap of two p-orbitals above and below the plane of a molecule
Draw a diagram showing sigma and pi bonds in an Alkene ?
How is the geometry of Alkenes different from Alkenes ?
- The ( pi ) π-bond locks the two carbon atoms in position and prevents them from rotating around the double bond.
- This makes the geometry of the alkenes different from that of the alkanes, where rotation is possible around every atom.
What is the shape and bond angle of Alkenes ?
- There are three bonding pairs surrounding each carbon atom in the C=C carbon double bond which repel meaning it has a trigonal planar shape
- The bond angle is 120o
Define the term ‘stereoisomers’ ?
Stereoisomers - Isomers that have the same structural formula but a different spatial arrangement of atoms
What are the two types of stereoisomers ?
- E/Z isomerism
- Optical isomerism
What are the conditions for E/Z isomerism ?
- Must contain a C=C carbon double bond that does not rotate/ has restricted rotation
- Must have different groups attached to each carbon atom of the double bond
What is cis/trans isomerism ?
A special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same
What determines whether atom/molecules attached to carbon atoms are high/ low priority ?
Atoms / molecules attached to each carbon atom in a double bond are given priority based on atomic number
Describe how you can identify whether an alkene is an E or Z stereoisomer ?
- — takes priority over — on the LHS/RHS due to higher atomic number
- — group takes priority over — group on the LHS/RHS
When are isomers Z or E isomers ?
- If groups of high priority are on the same side of the double bond, it is a Z isomer
- If groups of high priority are on different sides of double bond, it is an E isomer
When are isomers cis or trans isomers ?
- If the same groups are on the same side of the double bond, it is a cis isomer
- If the same groups are on different sides of the double bond, it is a trans isomer
Do you know how to identify E/Z isomerism ?
Yes
Do you know how to identify cis/trans isomerism ?
Yes
Explain the reactivity of Alkenes ?
- Alkenes are more reactive than Alkenes due to the presence of the pi bonds
- The electron density in the pi bond is concentrated above and below the plane of the sigma bond, making the pi electrons more exposed than sigma electrons
- This means the pi bond is ‘weaker’ than the sigma bond and can easily break
What is an addition reaction ?
A reaction where two or more reactants result in just one product
What reactants can Alkenes undergo addition reactions with ?
- Hydrogen
- Halogens
- Hydrogen halides
- Steam
What is the addition reaction with hydrogen called ? What are the conditions ?
- ‘Hydrogenation’
- Reagents = Hv2 (g)
- Conditions = Nickel catalyst, 423k
What is the addition reaction with halogens called ? What are the conditions ?
- ‘Halogenation’
- Reagents = Brv2, Clv2 or Iv2
- Conditions = Room temperature
What is the addition reaction with hydrogen halides called ? What are the conditions ?
- ‘’ / no specific name
- Reagents = HBr (g), HCl (g), HI (g)
- Conditions = Room temperature
What is the addition reaction with steam/ water called ? What are the conditions ?
- ‘Hydration’
- Reagents = Hv2O (g)/ steam
- Conditions = Phosphoric acid or concentrated sulphuric acid catalyst
Define the term ‘electrophile’ ?
An electron pair acceptor
Define the term ‘nucleophile’ ?
An electron pair donor
What is the mechanism called for addition reactions in Alkenes ?
Electrophilic addition
Draw a diagram portraying electrophilic addition between ethene and hydrogen bromide ?
Draw a diagram portraying electrophilic addition between ethene and bromine ?
What does Markownikoff’s rule state ?
When an unsymmetrical Alkenes reacts with a hydrogen halide, the hydrogen of the hydrogen halide attaches itself to the carbon with the most H atoms already attached
What are the different carbocation intermediates ?
- Primary carbocation
- Secondary carbocation
- Tertiary carbocation
Draw different carbocation intermediates ?
Why are primary carbocations least stable while tertiary carbocations are more stable ?
- Primary carbocations are less stable as there are fewer alkyl groups to ‘push’ electron density onto +C, so less stabilising of charge
- Tertiary carbocations are more stable as there are more alkyl groups to ‘push’ electron density onto +C, stabilising the charge
When will the major product be formed ? How do you explain this ?
- The major product will be the one that is formed via the more stable carbocation intermediate
- ——— is the major product as it forms via a ——— carbocation, which is more stable than a ——— carbocation
Do you know how to identify the major and minor product ?
Yes
Define the term ‘polymer’ ?
A long-chain molecule made from many smaller molecules called monomers chemically joined together
Define the term ‘monomer’ ?
A small molecule that, when joined with many other monomer forms a polymer
Define the term ‘addition polymerisation’ ?
When many unsaturated monomers join together to form a long-chain polymer and no other products
Write a polymerisation equation for ethene ?
Draw a diagram to represent the monomer, repeat unit and polymer of ethene ?
Do you know how to draw monomers, repeat units and polymers ?
Yes
Explain why it is so hard to dispose of waste polymers and it’s effect ?
- Polymers lack of reactivity makes them hard to dispose of and many are non-biodegradable
- This has serious environmental effects, ie. Killing marine life
Name some sustainable methods of processing waste polymers ?
- Recycling / PVC recycling
- Using waste polymers as fuels
- Feedstock recycling
Explain the benefits of recycling waste polymers on sustainability ?
- Recycling polymers reduce the negative environmental impact by conserving finite fossil fuels and decreasing the amount of waste going to landfill
- Discarded polymers have to be sorted by type, chopped into flakes, washed, dried melted, cut into pellets and used to make new products
What is the problem when recycling PVC ?
Recycling PVC is hazardous due to high chloride content which means that when PVC is burnt it releases pollutants ( eg. HCL gas which is corrosive )
How is PVC recycled ?
- Solvents are used to dissolve PVC, high-grade PVC is recovered by precipitation from solvent
- The solvent and PVC are then reused
Explain the benefits of using waste polymers as fuels on sustainability ?
Waste polymers can be incinerated to produce heat, generating steam which drives a turbine which generates electricity
Explain the drawbacks of using waste polymers as fuels on sustainability ?
Produces COv2 and possibly toxic gases
Explain the benefits of feedstock recycling of waste polymers/ waste polymers as organic feedstock on sustainability ?
- It is a chemical/ thermal processes that reclaim monomers/ gas or oil from waste polymer
- These can be used as raw materials for production of new polymers
- Feedstock recycling can handle unsorted/ unwashed polymers
Explain the benefits of the development of biodegradable polymers on the environment ?
- Biodegradable polymer are broken down by microorganisms into Hv2O and COv2 and biological compounds
- They are made from starch and cellulose ( plant-based polymers )
- They contain additives that aid their breakdown
Explain the benefits of the development of photodegradable polymers on the environment ?
- They contain bonds that are weakened by absorbing light to start the degradation process
- Light-absorbing additives may be used
- Still under development
What are the uses of poly(ethene) ?
- Supermarket bags
- Shampoo bottles
What are the uses of poly(propene) ?
- Packing crates
- Fibre for ropes
What are the uses of poly(phenylethene/styrene) ?
- Packing foam
- Food trays/ cups
What are the uses of poly(chloroethene) / PVC ?
- Window frames
- Flooring
What are the uses of poly(tetrafluoroethene) / PTFE / Teflon ?
- Frying pans
- Plumbing tape
What are the uses of PMMA / Perspex ?
- Perspex windows
