Alkenes

Question 1

What are Alkenes?

Answer 1

⦾ Unsaturated HCs
⦾ (+)1 C=C - area of high e- density
⦾ Reactive due to C=C
⦾ CnH2n

Question 2

How can you identify Alkenes?

Answer 2

⦾ Shake Alkene with orange bromine water

⦾ Turns solution orange-brown to colourless (+ve test_

Question 3

What are electrophiles?

Answer 3

⦾ Electron acceptors
⦾ Electron defficient
⦾ Attracted to areas of high e- density
⦾ +vely charged

Question 4

Give 3 examples of electrophiles?

Answer 4

⦾ HBr
⦾ Br2
⦾ H2SO4

Question 5

What generally happens in an electrophillic addition reaction?
e.g. CH2CH2 + X-Y → CH2XCH2Y

Answer 5

1 ⦾ C=C repels e-s in X-Y
2 ⦾ Polaries bond in X-Y
3 ⦾ 2 e- from C=C attack 𝛿+ X creating new bond between C-1 and X
4 ⦾ X-Y bond breaks and e-s from bond taken by Y to form -ve ion with lone e- pair
5 ⦾ C-2 left with +ve charge (bond with C=C that broke took e-s to form bond with X) - carbocation intermediate
6 ⦾ :Y- acts as nucleophile
7 ⦾ It attacks +ve carbocation, donating its lone e- pair + forming bond with C-2
8 ⦾ Overall, X-Y has been added to alkene across C=C to form saturated compound.

Question 6

What happens if you draw the mechanism involving an unsymmetrical alkene?

Answer 6

⦾ 2 different carbocation form

⦾ More stable carbocation more likely to form (make sure to draw right one)

Question 7

What is the product for the electrophilic addition reaction for:
Heat: Ethene + Water + conc. sulfuric acid

Answer 7

⦾ Ethanol

⦾ Overall: CH2=CH2 + H2O + conc.H2SO4 → C2H5OH

Question 8

Explain the reaction of:

CH2=CH2 + H2O + conc.H2SO4 → C2H5OH?

Answer 8

1 ⦾ Conc. H2SO4 reacts with ethene in nucleophillic addition - forms hydrogen sulfate
2 ⦾ Add cold water + warm product - hydrolysed to form ethanol
⦾ H2SO4 not used up - catalyst

Question 9

Explain (fully) the mechanism for the electrophillic addition reaction below:
CH2=CH2 + H2O + conc.H2SO4 → C2H5OH?

Answer 9

1 ⦾ C=C attacks 𝛿+H on H2SO4
2 ⦾ Bond formed between Cs and H
3 ⦾ e-s from O-H bond taken by O to form lone pair
4 ⦾ 2nd C left with +ve charge as e- lost from double bond
5 ⦾ -ve ion created acts as nucleophile + attacks carbocation creating new intermediate
6 ⦾ Water then added to produce sulfuric acid + ethanol

Question 10

Explain how alkenes are able to decolourise bromine water?
What does it form?
Via which mechanism?

Answer 10

⦾ Bromine added across double bond
⦾ Form colourless dibromoalkane
⦾ Electrophillic Addition

Question 11

What is the equation for the reaction of bromine water decolourising ethene?

Answer 11

H2C=CH2 + Br2 → CH2BrCH2Br

Question 12

Explain (fully) the mechanism for the reaction between bromine water and ethene?

Answer 12

1 ⦾ C=C repels e- in Br2, polarising 𝛿+Br-𝛿+Br - induced dipole
2 ⦾ e- pair in C=C attracts 𝛿+Br and forms bond with it
3 ⦾ Repels e- in Br-Br further until bond breaks + 𝛿+Br bonds with it
4 ⦾ You get +vely charged carbocation intermediate
5 ⦾ Br- attracted
6 ⦾ Bonds to other C
⦾ Forms: 1,2-dibromoethane

Question 13

What is the mechanism for the reaction between alkenes and hydrogen halides?

Answer 13

⦾ Electrophilic Addition

Question 14

What does the reaction between alkenes and hydrogen halides form?

Answer 14

⦾ Hydrogen halides

Question 15

Draw the mechanism for the reaction between ethene and hydrogen bromide?

Answer 15

⦾ pg. 218

⦾ C2H4 + HBr → C2H5Br

Question 16

What happens if a hydrogen halide is added to an unsymmetrical alkene?

Answer 16

⦾ 2 possible products

Question 17

Draw the mechanism for the reaction between propene and hydrogen bromide and the 2 possible products?

Answer 17

⦾ pg. 218
⦾ Br could be added to 1st or 2nd C
⦾ 1-bromopropane or 2-bromopropane

Question 18

What are the 3 possible carbocation intermediates?

Answer 18

⦾ Primary carbocation (1 X R)
⦾ Secondary carbocation (2 X R)
⦾ Tertiary carbocation (3 X R)

Question 19

Which of the 3 carbocations is the most stable?

Answer 19

⦾ Tertiary

Question 20

Why is the tertiary carbocation the most stable?

Answer 20

⦾ Because the alkyl groups feed e-s towards the +ve charge

Question 21

How can you show an alkyl group is donating electrons to the +ve charge?

Answer 21

⦾ Draw arrow on bond that points to where e-s are being donated
⦾ pg. 219

Question 22

What can the product that there’s most of also be called?

Answer 22

⦾ Major product

⦾ (minor product)

Question 23

Draw out the mechanism for the reaction between Hydrogen bromide and propene?

Answer 23

⦾ H2C=CHCH3 + HBr → CH3CHBrCH3
⦾ H2C=CHCH3 + HBr → CH2BrCH2CH3
⦾ Secondary carbocation more stable - 2 alkyl groups

Question 24

Draw out the mechanism for the reaction between Hydrogen bromide and 2-methylbut-2-ene?

Answer 24

⦾ pg. 220

Question 25

What can also affect how much of the major product forms in the end?

Answer 25

⦾ bigger difference in stability of carbocations

Question 26

Define polymer?

Answer 26

⦾ Natural e.g. DNA | ⦾ Synthetic e.g. polyethene

Question 27

How are addition polymers formed?

Answer 27

⦾ Alkenes where the double bond is broken to form repeating unit.

Question 28

What should you remember when drawing a monomer in polymer form?

Answer 28

⦾ -(-x-x-)-n

Question 29

What does n stand for?

Answer 29

⦾ N = no. of repeating units in a polymer

Question 30

What should you do if you're asked for the formula of a polymer?

Answer 30

⦾ Include brackets and n

Question 31

What should you do if you're asked for the repeating unit?

Answer 31

⦾ Just bit inside bracket

Question 32

How do you find the monomer used to form an addition polymer?

Answer 32

⦾ Take repeating unit ⦾ Remove side/trailing bonds ⦾ Add double bond

Question 33

How do you name addition polymers?

Answer 33

⦾ Find monomer | ⦾ poly(X)

Question 34

How would you name the addition polymer for | but-2-ene?

Answer 34

⦾ poly(but-2-ene)

Question 35

What will altering the reaction conditions for the production of plastic polymer chains do?

Answer 35

⦾ Give different properties

Question 36

What conditions produce branched chain polymers?

Answer 36

⦾ Weak IMFs between branched chain polymers ⦾ High pressures ⦾ High temps

Question 37

What conditions produce straight polymers?

Answer 37

⦾ Strong IMFs ⦾ Lower pressures ⦾ Lower temps

Question 38

What are the features of polymers?

Answer 38

⦾ Unreactive HC chains ⦾ Lots of strong ⦾ Non-polar ⦾ Covalent bonds

Question 39

``` Give an everyday use of polymers? Which poly(X)? ```

Answer 39

⦾ Shopping bags | ⦾ poly(ethene)

Question 40

What is a disadvantage of addition polymers (for plastic in particular)?

Answer 40

⦾ Unreactive nature of bonds ⦾ Therefore not biodegradable ⦾ Cannot be broken down

Question 41

What is PVC?

Answer 41

⦾ Poly(chloroethene) ⦾ Addition polymer ⦾ Waterproof properties

Question 42

How does PVC gain its waterproof property?

Answer 42

⦾ Addition of plasticisers during reaction

Question 43

How does plasticiser make a polymer bendier?

Answer 43

⦾ Gets between polymer chains + pushes apart | ⦾ Reduces strength on IMFs

Question 44

What are the uses of PVC?

Answer 44

⦾ Electrical cable insulation ⦾ Floor tiles ⦾ Clothing