3.3.4 Alkenes

Question 1

What is an alkene?

Answer 1

An unsaturated hydrocarbon

Question 2

What is the bonding in alkenes like?

Answer 2

Double C=C bond is a centre of high electron density

Question 3

What are electrophiles?

Answer 3

Electron-pair acceptors

Question 4

How are alcohols produced from alkenes? (4)

Answer 4

Electrophilic addition
Heat alkene with water and conc. sulphuric acid catalyst
Sulphuric acid acts as electrophile
Cold water hydrolyses intermediate to form alcohol

Question 5

What is the test for unsaturation? (3)

Answer 5

Bromine water
In the presence of double bonds, there is a colour change from orange (bromine) to colourless (dibromoalkane)
Electrophilic addition reaction

Question 6

How do major and minor products form in alkene electrophilic addition reactions? (3)

Answer 6

Unsymmetrical alkenes cause two products to form
The major product is made from the most stable carbocation
Carbocations are more stable if they have more alkyl groups attached because electrons are donated towards the positive charge

Question 7

What are the possible monomers in addition polymerisation? (2)

Answer 7

Alkenes

Substituted alkenes

Question 8

What are the properties of addition polymers?

Answer 8

Unreactive - non-polar carbon chain

Question 9

What determines the properties of polymers? (3)

Answer 9

Length - longer chains have stronger intermolecular forces
Branches - fewer branches cause stronger intermolecular forces
Plasticisers - pushes chains apart, reducing strength of intermolecular forces

Question 10

What makes polymers strong and rigid?

Answer 10

Long, straight chains

Question 11

What makes polymers more flexible?

Answer 11

Adding plasticisers

Question 12

What is PVC? (4)

Answer 12

Poly(chloroethene)
The polar C-Cl bond causes permanent dipole-dipole forces, making the material harder but more brittle
Rigid PVC is used for drain pipes and window frames
Plasticised PVC is more flexible and used for cable insulation, flooring tiles and clothing