3.4 Alkenes

Question 1

What is an alkene?

Answer 1

Unsaturated hydrocarbons with a C=C double bond

Question 2

What is the general formula of an alkene?

Answer 2

Cn H2n

Question 3

Why is there no rotation about the C=C double bond?

Answer 3

Question 4

Are they more or less reactive than alkanes? why?

Answer 4

More reactive, due to high electron density of double bond and the fact the pi-bond is slightly easier to break

Question 5

What intermolecular forces of attraction do they have?

Answer 5

Only van der Waals due to non-polar bonds

Question 6

Are they soluble in water? why?

Answer 6

No, non-polar bonds (van der Waals’ < hydrogen bonding)

Question 7

Name and describe the three kinds of isomers alkenes can have

Answer 7

Question 8

Write an equation for the complete combustion of pent-2-ene

Answer 8

Question 9

What is an electrophile?

Answer 9

Electron deficient atoms/ions which accept a pair of electrons

Question 10

What is the most stable kind of carbonation intermediate? why?

Answer 10

Alkyl groups have positive inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms i.e A tertiary carbocation

Question 11

Major products will be formed from which kinds of carbocations?

Answer 11

Tertiary (or the most stable available)

Question 12

What conditions are needed for the electrophilic addition of H2O to an alkene?

Answer 12

Acid catalyst, usually phosphoric acid

Question 13

What are the products of the reaction of H2O and an alkene

Answer 13

An alcohol

Question 14

Draw a mechanism for the addition of water to ethene

Answer 14

Question 15

What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?

Answer 15

Room temp

Question 16

Draw a mechanism for the reaction of HBr and ethene

Answer 16

Question 17

What conditions are needed for the electrophilic addition of a halogen molecule to an alkene?

Answer 17

Room temperature and organic solvent

Question 18

How does a molecule with a non-polar bond react as if it is an electrophile?

Answer 18

Question 19

Draw a mechanism for the reaction between bromine and ethene.

Answer 19

Question 20

Draw a mechanism for the reaction of sulfuric acid with ethene.

Answer 20

Question 21

How would you turn the product into an alcohol and how does show that sulfuric acid catalyses the addition of water to an alkene?

Answer 21

Add water

H2SO4 reforms, showing it catalyses the hydration of alkenes

Question 22

What is an addition polymer?

Answer 22

Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule

Question 23

What are monomers? What form do they usually take?

Answer 23

Molecules which combine to form a polymer usually have a C=C bond which breaks to leave a repeating pattern

Question 24

Draw how you would represent the polymerisation of ethene.

Answer 24

Question 25

Give 3 uses of poly(chloroethene) / PVC

Answer 25

Drainpipes
Vinyl
Aprons

Question 26

Give two examples of plasticisers

Answer 26

esters and phthalates

Question 27

What are plasticisers?

Answer 27

Small molecules that get between polymer chains to force them apart and allow them to slide over one another

Question 28

How do the physical properties of PVC change due to a plasticiser? What applications does this lead to?

Answer 28

PVC with a plasticiser become flexible, used for aprons
Without a plasticiser, PVC is rigid, used for drainpipes

Question 29

Why do things containing mainly C-C and C-H bones not decompose easily?

Answer 29

Bonds and non-polar so are not attacked by enzymes

Question 30

Why is a lack of biodegradability in compounds with C-C or C-H bonds a problem?

Answer 30

Disposal is very problematic

Question 31

What is mechanical recycling?

Answer 31

Where plastics are separated into different types, washed, ground down, melted and re-moulded

Question 32

What is mechanical recycling used for?

Answer 32

Soft drink bottles, fleeces

Question 33

What is feedstock recycling?

Answer 33

Plastics heated to a temperature which break polymer bonds, leaving original monomers that can be made into new plastics

Question 34

What is feedstock recycling used for?

Answer 34

Making totally new plastics

Question 35

What is a problem with recycling?

Answer 35

Each time thermosoftening plastics are melted and remoulded, their properties degrade, so they can only be remoulded a limited number of times