13 Alkenes

Question 1

What are the 2 types of bonds in alkenes?

Answer 1

Sigma and pi bonds

Question 2

Why are there 2 bonds in alkene double bonds?

Answer 2

3 of 4 electrons in the outer shell are used for sigma bonds
The fourth electron occupies a p orbital

Question 3

What is a pi bond?

Answer 3

• Formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond
• Each C atom contributes 1 electron to the pair
• Electron density is above and below the line joining the nuclei of the bonding atoms

Question 4

Why can atoms in the double bond not rotate?

Answer 4

Pi bond locks the 2 carbon atoms in position and prevents them from rotating around the double bond

Question 5

What is the shape around a double bond?

Answer 5

• 3 regions of electron density
• Repel as far apart as possible
• Trigonal planar shape (120)
• All atoms are in the same plane

Question 6

What is structural isomerism?

Answer 6

Molecules with the same molecular formula but a different structural formula

Question 7

What is stereoisomerism?

Answer 7

Molecules with the same structural formula but a different arrangement of atoms in space

Question 8

Why is there stereoisomerism around double bonds?

Answer 8

• The rotation about the double bond is restricted because of the electron density of the pi bond above and below the plane of the sigma bond
• The groups attached to each carbon atom are therefore fixed relative to each other

Question 9

What are the conditions for E/Z isomerism?

Answer 9

1. Contains a C=C double bond
2. Each carbon in the double bond must be bonded to different groups

Question 10

What is the test for unsaturation?

Answer 10

Addition reaction of alkene and bromine
Reacts with alkenes (orange to colourless)
Alkanes no reaction

Question 11

What is the difference in reactivity between alkenes and alkanes and why?

Answer 11

Alkenes more reactive than alkanes
Have a pi bond where the pi electrons are outside of the bond so more exposed

Question 12

What are the conditions for the hydrogenation of alkenes?

Answer 12

Nickel and 423K (150C)

Question 13

Write an equation for propane and hydrogen

Answer 13

Propene + hydrogen –> Propane

Question 14

Write an equation for alkenes with bromine

Answer 14

Ethene + bromine –> dibromoethane

Question 15

Write an equation for the reaction of propene with hydrogen chloride

Answer 15

Propene + hydrogen chloride (HCl) –> haloalkane (chloropropane)

Question 16

What are the 2 possible products of alkenes and hydrogen halides?

Answer 16

1-chloropropane –> minor product
2-chloropropane –> major product

Question 17

What are the conditions for hydration of alkenes?

Answer 17

High temp (reacting with steam)
Phosphoric acid
Used to make ethanol from ethene

Question 18

Write an equation of propene and steam

Answer 18

Propene + steam –> propan-1-ol or propan-2-ol

Question 19

What is an electrophile?

Answer 19

An atom or molecule with a (full or partial) positive charge that can accept a pair of electrons
- Electron-pair acceptor

Question 20

Draw the mechanism of the addition reaction of but-2-ene and hydrogen bromide and describe

Answer 20

• H in HBr is slightly positive due to permanent dipole
  Arrow from double bond to H and from single bond to Br (heterolytic fission)
• Br has a lone pair is bromide, C is positive
• Arrow from lone pair to positive C
• Br bonds to C
  Alkenes –> Carbocation intermediate and Br- –> Alkane

Question 21

What is a carbocation intermediate?

Answer 21

Has 3 bonds and a positive charge on the carbon

Question 22

Is the HBr an electrophile or not and explain why?

Answer 22

Yes as accepts a pair of electrons from the pi bond

Question 23

When do minor and major products form?

Answer 23

Unsymmetrical alkene

Question 24

What happens to the H and Br on the major product?

Answer 24

Br attached to the C with less H
H has attached to the C with more H

Question 25

Why does the major product form?

Answer 25

More stable carbocation intermediate forms major product (secondary carbocation more stable than primary)

Question 26

Why are secondary carbocations more stable than primary?

Answer 26

sigma bond electrons stabilise the positive charge, more sigma donation happening so more product formed

Question 27

What is a polymer?

Answer 27

A large molecule made from many small molecules called monomers

Question 28

What do unsaturated alkene molecules undergo?

Answer 28

Addition polymerisation
Produce a long, saturated chain

Question 29

What is a repeat unit?

Answer 29

The specific arrangement of atoms in the polymer that repeats over and over again, and is always drawn in square brackets

Question 30

What are the 5 ways of disposing of polymers?

Answer 30

Recycling
PVC recycling
Using waste polymers as fuel
Feedstock recycling
Biodegradable/
photodegradable polymers