3.3.4 Alkenes Flashcards Preview

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Flashcards in 3.3.4 Alkenes Deck (51):
1

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

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

(High conc. of electrons )

2

Why is there restricted rotation around double bond in alkenes?

  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

3

What is the only intermolecular forces between alkene molecules?

Van der Waals forces

4

Alkenes are ____ in water

insoluble

5

Alkenes are ____ reactive than alkanes

more

6

Why are alkenes more reactive than alkanes?

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

7

Why do electrophilic addition reactions occur?

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

8

What are electrophiles?

electron-pair acceptors

9

Where are electrophiles attracted to?

Attracted to areas where there's lots of electrons

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

10

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

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

(happens via electrophilic addition)

11

Electrophilic Addition 

Name the reactant + bonds & the products

  • Halogen
    • X---X = dihalogenoalkane
  • Hydrogen Halides 
    • H---X = halogenoalkane
  • Sulfuric Acid
    • H2SO4 = alcohol

12

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

You get 2 possible products

13

Adding Hydrogen Halides to Unsymmetrical Alkenes

What does the amount of each product formed depend on?

How stable carbocation formed is = known as carbocation intermediate

14

Why are carbocations with more alkyl groups more stable?

∵ alkyl groups feed electrons towards positive charge

15

Ethene + Br2

Balanced Equation (structural formula)

16

Ethene + Br2

Mechanism

17

Propene + HBr

Mechanism

18

Ethene + H2SO4

Mechanism

19

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

Alkyl hydrogen sulfates

20

How can you convert alkyl hydrogen sulfates into alcohols?

By adding water & warming reaction mixture

21

What are polymers?

Long chain molecules formed when monomers join together

22

What are addition polymers formed from?

alkenes and substituted alkenes

23

Why can alkenes make up addition polymers?

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

24

Draw poly(phenylethene) from phenylethene

25

Why are addition polymers very unreactive?

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

26

How are the polyalkene chains held together?

 

Van der Waals forces

(∵ polyalkene chains = non-polar)

27

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

stronger Van der Waals forces between chains will be

28

Name 2 properties of polyalkenes made from long, straight chains

strong & rigid

29

Name 2 properties of polyalkenes made from short, branched chains

weaker & more flexible

30

Why are addition polymers not biodegradable?

  1. ∵ Chain = saturated alkane molecule
    • = unreactive molecules 
  2. Means they cannot be attacked by biological agents (e.g. enzymes)

31

How can you modify the properties of polymers?

By adding plasticiser to polymer = more them more flexible

32

Why does adding plasticiser to polymer make it more flexible?

  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

Draw the repeating unit of the polymer formed from ethene, C2H4

34

35

Draw chloroethene becoming poly(chloroethene)

36

What is epoxyethane used as?

An intermediate chemical from which many useful products can be made

37

How is epoxyethane made?

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

38

Why is making epoxyethane hazardous?

  1. Epoxyethane = poisonous and carcinogenic
  2. It's highly flammable/explosive when in contact with air

39

Why is epoxyethane highly reactive?

  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

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

Plasticisers or detergents

41

What is formed when epoxyethane reacts with water?

ethane-1,2-diol

42

What is ethane-1,2-diol used as?

antifreeze

43

Write equation for when epoxyethane reacts with water

C2H4O + H2O → C2H6O2

44

Write mechanism for when epoxyethane reacts with water

45

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

Rigid PVC:

  • Drainpipes
  • Window frames

Plasticised PVC:

  • Electrical cable insulation 
  • Flooring tiles
  • Clothing

46

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)

  • 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

Write equation for how epoxyethane is formed

2 CH2CH2 + O2 → 2 CH2CH2O

48

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)

49

Name the conditions for making expoxyethane

  • 250-300°C
  • 15 atm
  • Silver catalyst

50

Write the skeletal formula for epoxyethane

51

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

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