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AQA A Level Chemistry > 3.4 Alkenes > Flashcards

3.4 Alkenes Flashcards

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1
Q

Why does the C=C double covalent bond in alkenes have a high electron density?

A

∵ they’re 2 pairs of electrons in the C=C double bond

(High conc. of electrons )

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2
Q

Why is there restricted rotation around double bond in alkenes?

A
  1. C-C single bond & p-orbital (contains single electron) on each carbon
  2. These 2 orbitals overlap & form an orbital with a cloud of electron density above and below the single bond
  3. Called π orbital & its presence = bond can’t rotate
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3
Q

What is the only intermolecular forces between alkene molecules?

A

Van der Waals forces

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4
Q

Alkenes are ____ in water

A

insoluble

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5
Q

Alkenes are ____ reactive than alkanes

A

more

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6
Q

Why are alkenes more reactive than alkanes?

A

∵ C=C forms electron-rich area = can be easily attacked by electrophiles

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7
Q

Why do electrophilic addition reactions occur?

A

∵ double bond has lots of electrons = easily attacked by electrophiles

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8
Q

What are electrophiles?

A

electron-pair acceptors

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9
Q

Where are electrophiles attracted to?

A

Attracted to areas where there’s lots of electrons

e.g. Positively charged ions (H+ NO2+) and polar molecules (δ+)

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10
Q

When you shake an alkene with orange bromine water, why does the solution decolourises?

A

∵ Bromine is added across C=C double bond to form colourless dibromoalkane

(happens via electrophilic addition)

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11
Q

Electrophilic Addition

Name the reactant + bonds & the products

A
  • Halogen
    • X—X = dihalogenoalkane
  • Hydrogen Halides
    • H—X = halogenoalkane
  • Sulfuric Acid
    • H2SO<span>4</span> = alcohol
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12
Q

What happens when you add a hydrogen halide to an unsymmetrical alkene?

A

You get 2 possible products

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13
Q

Adding Hydrogen Halides to Unsymmetrical Alkenes

What does the amount of each product formed depend on?

A

How stable carbocation formed is = known as carbocation intermediate

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14
Q

Why are carbocations with more alkyl groups more stable?

A

∵ alkyl groups feed electrons towards positive charge

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15
Q

Ethene + Br2

Balanced Equation (structural formula)

A
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16
Q

Ethene + Br2

Mechanism

A
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17
Q

Propene + HBr

Mechanism

A
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18
Q

Ethene + H2SO4

Mechanism

A
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19
Q

What do alkenes form initally when reacted with cold concentrated sulfuric acid?

A

Alkyl hydrogen sulfates

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20
Q

How can you convert alkyl hydrogen sulfates into alcohols?

A

By adding water & warming reaction mixture

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21
Q

What are polymers?

A

Long chain molecules formed when monomers join together

22
Q

What are addition polymers formed from?

A

alkenes and substituted alkenes

23
Q

Why can alkenes make up addition polymers?

A

Can act as monomers and form polymers ∵ double bond can open up & join together to make long chains

24
Q

Draw poly(phenylethene) from phenylethene

A
25
Q

Why are addition polymers very unreactive?

A
  • Polyalkene chains = saturated molecules
  • Main carbon chain of polyalkene = non-polar
26
Q

How are the polyalkene chains held together?

A

Van der Waals forces

(∵ polyalkene chains = non-polar)

27
Q

Longer the polymer chains & closer together they can get = …

A

stronger Van der Waals forces between chains will be

28
Q

Name 2 properties of polyalkenes made from long, straight chains

A

strong & rigid

29
Q

Name 2 properties of polyalkenes made from short, branched chains

A

weaker & more flexible

30
Q

Why are addition polymers not biodegradable?

A
  1. ∵ Chain = saturated alkane molecule
    • have non-polar C-C bonds
  2. Means they cannot be attacked by biological agents (e.g. enzymes)/nucleophiles
31
Q

How can you modify the properties of polymers?

A

By adding plasticiser to polymer = more them more flexible

32
Q

Why does adding plasticiser to polymer make it more flexible?

A
  1. ∵ plasticiser molecules get between polymer chains & push them apart
  2. = reduces strength of intermolecular forces between chains
  3. ∴ chains slide around more = making polymer easy to bend
33
Q

Draw the repeating unit of the polymer formed from ethene, C2H<span>4</span>

A
34
Q
A
35
Q

Draw chloroethene becoming poly(chloroethene)

A
36
Q

What is epoxyethane used as?

A

An intermediate chemical from which many useful products can be made

37
Q

How is epoxyethane made?

A
  1. Made by partially oxidising ethene
  2. Mixture of ethene and oxygen is passed over catalyst = silver
38
Q

Why is making epoxyethane hazardous?

A
  1. Epoxyethane = poisonous and carcinogenic
  2. It’s highly flammable/explosive when in contact with air
39
Q

Why is epoxyethane highly reactive?

A
  1. Bonding pairs in ring of atoms in molecule = forced very close together
  2. ∴ bond angles = 60° rather than 109.5° i.e. C–O–C bond angle much smaller than 109.5°
  3. Bonding pairs repel each other
  4. = ring tends to spring open (molecule has ‘ring strain’ due to the repulsion between bonding pairs - atomic orbitals overlap) as system becomes more stable if ring is broken
40
Q

What are the products produced from reacting epoxyethane with alcohol used as?

A

Plasticisers or detergents

41
Q

What is formed when epoxyethane reacts with water?

A

ethane-1,2-diol

42
Q

What is ethane-1,2-diol used as?

A

antifreeze

43
Q

Write the equation for when epoxyethane reacts with water

A

C2H4O + H2O → C2H6O2

44
Q

Draw the mechanism for when epoxyethane reacts with water

A
45
Q

What is poly(chloroethene), PVC, used for?

A

Rigid PVC:

  • Drainpipes
  • Window frames

Plasticised PVC:

  • Electrical cable insulation
  • Flooring tiles
  • Clothing
46
Q

Electrophilic Addition

(CH3)2C=CHCH3 + HBr → (CH3)2CBrCH2CH3

The minor product in this reaction mixture is 2-bromo-3-methylbutane. Explain why this bromoalkane is formed as a minor product. (3)

A
  • Reaction goes via intermediate carboncations
  • Formed via tertiary carboncation which is more stable (than secondary carboncation)
  • Due to more alkyl groups release electron density towards the positive carbon
47
Q

Write equation for how epoxyethane is formed

A

2 CH2CH2 + O2 → 2 CH2CH2O

48
Q

2-methyl but-2-ene reacts with concentrated sulfuric acid to form 2 different products. Draw the structure of the minor product of this reaction. (1)

A
49
Q

Name the conditions for making expoxyethane

A
  • 250-300°C
  • 15 atm
  • Silver catalyst
50
Q

Write the skeletal formula for epoxyethane

A
51
Q

Explain why alkenes undergo electrophilic addition reactions with bromine (2)

A
  • C=C is area of high electron density
  • (this) induces a dipole in the bromine molecule

AQA A Level Chemistry (36 decks)

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