14 - Alkenes

Question 1

What is the general formula of an alkene?

Answer 1

CnH2n

Question 2

Why is there no rotation about the double bond?

Answer 2

A double bond contains a Pi bond as well as a Sigma bond.

Question 3

What is geometrical isomerism?

Answer 3

A form of stereoisomerism. Only occurs around C=C double bonds. Also knows as E-Z isomerism.

Question 4

Describe the solubility of alkenes.

Answer 4

Not soluble in water due to being non-polar.

Question 5

Why are alkenes more reactive than alkanes?

Answer 5

C=C forms electron rich area, which can then be attacked by electrophiles.

Question 6

Describe the process of electrophilic addition.

Answer 6

Electrophile is attracted to double bond.

Electrophiles are positively charged, accept pair of electrons from double bond.

Bond breaks heterolytically.

Carbocation forms.

Negatively charged ion forms bond with carbocation.

Question 7

What is the positive inductive effect?

Answer 7

The more alkyl groups attached to a carbon atom, the more stable the carbocation formed. Therefore it is more likely to be the major product. (Because alkyl groups release electrons)

Question 8

What is the test for C=C double bond?

Answer 8

Add a few drops of bromine water. The solution will decolourise if there is a C=C double bond present.

Question 9

Give the nomenclature of an addition polymer.

Answer 9

Poly(monomer)

Question 10

What is feedstock recycling?

Answer 10

Heating the plastics to a temperature that the bonds break and monomers are formed.

Question 11

Define an alkene.

Answer 11

Unsaturated hydrocarbon

Question 12

Why are alkenes susceptible to electrophilic attack?

Answer 12

C=C bond is high electron density.

Question 13

What is the bond angle around a C=C bond?

Answer 13

120

Question 14

How is a haloalkane formed from an alkene?

Answer 14

Electrophilic addition

Question 15

What is heterolytic bond breaking?

Answer 15

Both electrons go to one atom and not the other, forming a positive and negative ion.

Question 16

How do alkenes react with halogens?

Answer 16

Halogen atoms add across the double bond

Question 17

What conditions are used to react an alkene with water.

Answer 17

Phosphoric acid
Steam

Question 18

What is the product of an alkene reaction with concentrated sulfuric acid?

Answer 18

Alcohol

Question 19

Describe the reaction of an alkene with concentrated sulphuric acid.

Answer 19

Sulfuric acid acts as electrophile to add H and O-SO3H. When water is added, sulfuric acid reforms and an alcohol forms.

Question 20

What are addition polymers made from?

Answer 20

Alkene monomers

Question 21

Describe how you would draw the repeating unit of a condensation polymer.

Answer 21

• Bonds stick through square brackets
• Square brackets
• n outside brackets bottom right

Question 22

What is PVC?

Answer 22

Poly(chloroethene)

Question 23

What is a typical use for non-modified PVC?

Answer 23

Drainpipes

Question 24

How do plasticisers change polymer properties?

Answer 24

Go between polymer chains to force them apart, which allows them to slide over each other, making the polymer less rigid.

Question 25

What is a typical use for plasticiser modified PVC?

Answer 25

Aprons

Question 26

Describe and explain the reactivity of addition polymers.

Answer 26

Unreactive

Strong, non-polar C-C and C-H bonds

Question 27

Explain how isomeric products are formed in electrophilic addition.

Answer 27

A product can be a pair of enantiomers.
Can be obtained via primary carbocation.

Question 28

Explain why the two products of electrophilic addition form in different amounts.

Answer 28

Major is a x carbocation
Minor is y carbocation.

Y carbocation is more stable.

Question 29

Explain why there is an attraction between a C=C double bond and Br2. (3)

Answer 29

C=C has high electron density
Br-Br is polarised
Delta positive Br attracted to C=C.

Question 30

What form should the repeating unit of an addition polymer.

Answer 30

Ethene with side chains