3.3.4 Alkenes

Question 1

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

Answer 1

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

(High conc. of electrons )

Question 2

Why is there restricted rotation around double bond in alkenes?

Answer 2

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

Question 3

What is the only intermolecular forces between alkene molecules?

Answer 3

Van der Waals forces

Question 4

Alkenes are ____ in water

Answer 4

insoluble

Question 5

Alkenes are ____ reactive than alkanes

Answer 5

more

Question 6

Why are alkenes more reactive than alkanes?

Answer 6

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

Question 7

Why do electrophilic addition reactions occur?

Answer 7

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

Question 8

What are electrophiles?

Answer 8

electron-pair acceptors

Question 9

Where are electrophiles attracted to?

Answer 9

Attracted to areas where there’s lots of electrons

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

Question 10

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

Answer 10

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

(happens via electrophilic addition)

Question 11

Electrophilic Addition

Name the reactant + bonds & the products

Answer 11

• Halogen
  
  • X—X = dihalogenoalkane
• Hydrogen Halides
  
  • H—X = halogenoalkane
• Sulfuric Acid
  
  • H₂SO_{<span>4</span>} = alcohol

Question 12

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

Answer 12

You get 2 possible products

Question 13

Adding Hydrogen Halides to Unsymmetrical Alkenes

What does the amount of each product formed depend on?

Answer 13

How stable carbocation formed is = known as carbocation intermediate

Question 14

Why are carbocations with more alkyl groups more stable?

Answer 14

∵ alkyl groups feed electrons towards positive charge

Question 15

Ethene + Br₂

Balanced Equation (structural formula)

Answer 15

Question 16

Ethene + Br₂

Mechanism

Answer 16

Question 17

Propene + HBr

Mechanism

Answer 17

Question 18

Ethene + H₂SO₄

Mechanism

Answer 18

Question 19

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

Answer 19

Alkyl hydrogen sulfates

Question 20

How can you convert alkyl hydrogen sulfates into alcohols?

Answer 20

By adding water & warming reaction mixture

Question 21

What are polymers?

Answer 21

Long chain molecules formed when monomers join together

Question 22

What are addition polymers formed from?

Answer 22

alkenes and substituted alkenes

Question 23

Why can alkenes make up addition polymers?

Answer 23

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

Question 24

Draw poly(phenylethene) from phenylethene

Answer 24

Question 25

Why are addition polymers very unreactive?

Answer 25

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

Question 26

How are the polyalkene chains held together?

Answer 26

Van der Waals forces | (∵ polyalkene chains = non-polar)

Question 27

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

Answer 27

stronger Van der Waals forces between chains will be

Question 28

Name 2 properties of polyalkenes made from long, straight chains

Answer 28

strong & rigid

Question 29

Name 2 properties of polyalkenes made from short, branched chains

Answer 29

weaker & more flexible

Question 30

Why are addition polymers not biodegradable?

Answer 30

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

Question 31

How can you modify the properties of polymers?

Answer 31

By adding plasticiser to polymer = more them more flexible

Question 32

Why does adding plasticiser to polymer make it more flexible?

Answer 32

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

Question 33

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

Answer 33

Question 34

Answer 34

Question 35

Draw chloroethene becoming poly(chloroethene)

Answer 35

Question 36

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

Answer 36

Rigid PVC: * Drainpipes * Window frames Plasticised PVC: * Electrical cable insulation * Flooring tiles * Clothing

Question 37

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)

Answer 37

* 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

Question 38

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)

Answer 38

Question 39

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

Answer 39

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