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

CnH2n

Question 3

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

Answer 3

Due to the π (pi) orbital - electron density above and below the single bond, which holds the carbon atoms in place

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

Chain isomers (branched chains)
Position isomers (C=C on different carbon atom)
Geometric E-Z isomers (Z is when 2 highest atomic number chains are on the same side of the double bond; E is when they're on opposite sides)

Question 8

Write an equation for the complete combustion of

pent-2-ene.

Answer 8

CH3CH=CHCH2CH3 + 7½O2 → 5CO2 + 5H2O

Question 9

What is an electrophile?

Answer 9

Electron deficient atoms/ions which accept a pair of electrons (here, from the C=C double bond)

Question 10

What is the most stable type
of carbocation intermediate?
why?

Answer 10

Alkyl groups have a positive inductive effect, so the most stable carbonation 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

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 product(s) of the reaction for the electrophilic addition of H2O to an alkene?

Answer 13

An alcohol

Question 14

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

Answer 14

Room temperature

Question 15

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

Answer 15

Room temperature and organic solvent

Question 16

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

Answer 16

C=C double bond with a high electron density induces a temporary dipole in the halogen molecule → δ+ atom attracted to double bond

Question 17

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 17

Add water

H₂SO₄ reforms, showing it catalyses the hydration of alkenes

Question 18

What is an addition polymer?

Answer 18

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

Question 19

What are monomers? What form do they usually take?

Answer 19

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

Question 20

Give 3 uses of

poly(chloroethene) / PVC

Answer 20

Drainpipes, Vinyl, Aprons

Question 21

Give two examples of plasticisers

Answer 21

esters and phthalates

Question 22

What are plasticisers?

Answer 22

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

Question 23

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

Answer 23

PVC with a plasticiser become flexible, used for aprons

Without a plasticiser, PVC is rigid, used for drainpipes

Question 24

Why do things containing mainly C-C and C-H bonds

not decompose easily?

Answer 24

Bonds are non-polar so are not attacked by enzymes

Question 25

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

Answer 25

Disposal is very problematic

Question 26

What is mechanical recycling?

Answer 26

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

Question 27

What is mechanical recycling used for?

Answer 27

Soft drinks bottles → fleeces

Question 28

What is feedstock recycling?

Answer 28

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

Question 29

What is feedstock recycling used for?

Answer 29

Making totally new plastics

Question 30

What is a problem with recycling?

Answer 30

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