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A-level Chemistry > 4.2 - Alkenes > Flashcards

4.2 - Alkenes Flashcards

1
Q

What are Alkenes ?

A

They are a group of unsaturated hydrocarbons with the general formula CnH2n

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2
Q

What type of bonds are Alkenes composed of ?

A

The C=C carbon double bond consists of one sigma bind and one pi bond.

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3
Q

How is a sigma bond formed ?

A

A sigma bond is formed by the overlap of 2 atomic orbitals between thr nuclei of two bonded atoms

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4
Q

How is a pi bond formed ?

A

A pi bond is formed by the sideways overlap of two p-orbitals above and below the plane of a molecule

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5
Q

Draw a diagram showing sigma and pi bonds in an Alkene ?

A
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6
Q

How is the geometry of Alkenes different from Alkenes ?

A
  • 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.
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7
Q

What is the shape and bond angle of Alkenes ?

A
  • 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
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8
Q

Define the term ‘stereoisomers’ ?

A

Stereoisomers - Isomers that have the same structural formula but a different spatial arrangement of atoms

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9
Q

What are the two types of stereoisomers ?

A
  • E/Z isomerism
  • Optical isomerism
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10
Q

What are the conditions for E/Z isomerism ?

A
  • 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
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11
Q

What is cis/trans isomerism ?

A

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

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12
Q

What determines whether atom/molecules attached to carbon atoms are high/ low priority ?

A

Atoms / molecules attached to each carbon atom in a double bond are given priority based on atomic number

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13
Q

Describe how you can identify whether an alkene is an E or Z stereoisomer ?

A
  • — takes priority over — on the LHS/RHS due to higher atomic number
  • — group takes priority over — group on the LHS/RHS
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14
Q

When are isomers Z or E isomers ?

A
  • 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
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15
Q

When are isomers cis or trans isomers ?

A
  • 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
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16
Q

Do you know how to identify E/Z isomerism ?

A

Yes

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17
Q

Do you know how to identify cis/trans isomerism ?

A

Yes

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18
Q

Explain the reactivity of Alkenes ?

A
  • 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
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19
Q

What is an addition reaction ?

A

A reaction where two or more reactants result in just one product

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20
Q

What reactants can Alkenes undergo addition reactions with ?

A
  • Hydrogen
  • Halogens
  • Hydrogen halides
  • Steam
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21
Q

What is the addition reaction with hydrogen called ? What are the conditions ?

A
  • ‘Hydrogenation’
  • Reagents = Hv2 (g)
  • Conditions = Nickel catalyst, 423k
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22
Q

What is the addition reaction with halogens called ? What are the conditions ?

A
  • ‘Halogenation’
  • Reagents = Brv2, Clv2 or Iv2
  • Conditions = Room temperature
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23
Q

What is the addition reaction with hydrogen halides called ? What are the conditions ?

A
  • ‘’ / no specific name
  • Reagents = HBr (g), HCl (g), HI (g)
  • Conditions = Room temperature
24
Q

What is the addition reaction with steam/ water called ? What are the conditions ?

A
  • ‘Hydration’
  • Reagents = Hv2O (g)/ steam
  • Conditions = Phosphoric acid or concentrated sulphuric acid catalyst
25
Q

Define the term ‘electrophile’ ?

A

An electron pair acceptor

26
Q

Define the term ‘nucleophile’ ?

A

An electron pair donor

27
Q

What is the mechanism called for addition reactions in Alkenes ?

A

Electrophilic addition

28
Q

Draw a diagram portraying electrophilic addition between ethene and hydrogen bromide ?

A
29
Q

Draw a diagram portraying electrophilic addition between ethene and bromine ?

A
30
Q

What does Markownikoff’s rule state ?

A

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

31
Q

What are the different carbocation intermediates ?

A
  • Primary carbocation
  • Secondary carbocation
  • Tertiary carbocation
32
Q

Draw different carbocation intermediates ?

A
33
Q

Why are primary carbocations least stable while tertiary carbocations are more stable ?

A
  • 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
34
Q

When will the major product be formed ? How do you explain this ?

A
  • 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
35
Q

Do you know how to identify the major and minor product ?

A

Yes

36
Q

Define the term ‘polymer’ ?

A

A long-chain molecule made from many smaller molecules called monomers chemically joined together

37
Q

Define the term ‘monomer’ ?

A

A small molecule that, when joined with many other monomer forms a polymer

38
Q

Define the term ‘addition polymerisation’ ?

A

When many unsaturated monomers join together to form a long-chain polymer and no other products

39
Q

Write a polymerisation equation for ethene ?

A
40
Q

Draw a diagram to represent the monomer, repeat unit and polymer of ethene ?

A
41
Q

Do you know how to draw monomers, repeat units and polymers ?

A

Yes

42
Q

Explain why it is so hard to dispose of waste polymers and it’s effect ?

A
  • Polymers lack of reactivity makes them hard to dispose of and many are non-biodegradable
  • This has serious environmental effects, ie. Killing marine life
43
Q

Name some sustainable methods of processing waste polymers ?

A
  • Recycling / PVC recycling
  • Using waste polymers as fuels
  • Feedstock recycling
44
Q

Explain the benefits of recycling waste polymers on sustainability ?

A
  • 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
45
Q

What is the problem when recycling PVC ?

A

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 )

46
Q

How is PVC recycled ?

A
  • Solvents are used to dissolve PVC, high-grade PVC is recovered by precipitation from solvent
  • The solvent and PVC are then reused
47
Q

Explain the benefits of using waste polymers as fuels on sustainability ?

A

Waste polymers can be incinerated to produce heat, generating steam which drives a turbine which generates electricity

48
Q

Explain the drawbacks of using waste polymers as fuels on sustainability ?

A

Produces COv2 and possibly toxic gases

49
Q

Explain the benefits of feedstock recycling of waste polymers/ waste polymers as organic feedstock on sustainability ?

A
  • 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
50
Q

Explain the benefits of the development of biodegradable polymers on the environment ?

A
  • 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
51
Q

Explain the benefits of the development of photodegradable polymers on the environment ?

A
  • They contain bonds that are weakened by absorbing light to start the degradation process
  • Light-absorbing additives may be used
  • Still under development
52
Q

What are the uses of poly(ethene) ?

A
  • Supermarket bags
  • Shampoo bottles
53
Q

What are the uses of poly(propene) ?

A
  • Packing crates
  • Fibre for ropes
54
Q

What are the uses of poly(phenylethene/styrene) ?

A
  • Packing foam
  • Food trays/ cups
55
Q

What are the uses of poly(chloroethene) / PVC ?

A
  • Window frames
  • Flooring
56
Q

What are the uses of poly(tetrafluoroethene) / PTFE / Teflon ?

A
  • Frying pans
  • Plumbing tape
57
Q

What are the uses of PMMA / Perspex ?

A
  • Perspex windows

A-level Chemistry (31 decks)

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