Alkenes

Question 1

What are alkenes?

Answer 1

CnH2n
They have atleast one C=C double covalent bond.
They are unsaturated.

Question 2

How are alkenes named?

Answer 2

The carbon the double bond starts on is numbered in the name.
If it has two double bonds the suffix is diene.

Question 3

What is the structure of double bond in alkenes?

Answer 3

It is made of a sigma bond and a pi bond.

Question 4

What is a pi bond?

Answer 4

Sideways overlap of adjacent p-orbitals above and below the bonding C atoms.

Question 5

What is the strength of pi bonds?

Answer 5

Pi bonds are weaker than sigma because electron density is spread out above and below the nuclei.
This means the electrostatic attraction is weaker, and have a low bond enthalpy.

Question 6

What is the reactivity of alkenes?

Answer 6

There’s two pairs of electrons in the bond, so it has very high electron density - very reactive.
The pi bond also sticks out of the molecule, so is more likely to be attacked by electrophiles.

Question 7

What are electrophiles?

Answer 7

Electron pair acceptors.
They have a low electron density.

Question 8

What is the rotation of alkenes?

Answer 8

Can’t rotate because the p-orbitals have to stay in the same position to overlap and form a pi bond.
Double bonds are rigid.
Things can still rotate around the single bonds though.

Question 9

What are stereoisomers?

Answer 9

The same stuctural formula but different arrangement of atoms in space.
Alkenes have stereoisomers because there’s a lack of rotation around the C=C double bond.

Question 10

What is E/Z isomerism?

Answer 10

When the double bonded carbons atoms each have two different atoms or groups attached to them.

Question 11

What are priority groups?

Answer 11

If the two groups have their ‘higher priority groups’ on opposite sides, it’s an E isomer.
If they are on the same side, it’s a Z isomer.

Question 12

What are the CIP priority rules?

Answer 12

The highest priority group on each of the C=C is the one with the atom with the highest atomic number directly bonded to the Carbon atom.
If the atoms directly bonded to carbon are the same, look to the next atom along.

Question 13

What is Cis/Trans isomerism?

Answer 13

A type of E/Z isomerism where the two carbon atoms either side of the double bond have at least one group in common.
The cis isomer is the one with two identical groups on the same side of the C=C bond.
The trans isomer has the two identical groups on opposite sides of the C=C bond.

Question 14

What are electrophilic addition reactions?

Answer 14

The double bond in an alkene opens up and atoms are added to carbon atoms.
This happens because the double bond has plenty of electrons and is easily attacked by electrophiles.
The double bond is also nucleophilic - it is attracted to places with not enough electrons.

Question 15

How are alkanes produced from alkenes?

Answer 15

Hydrogenation:
Alkenes react with hydrogen gas to form alkanes.
At the temperature of 150 c with a nickel catalyst.

Question 16

How do alkenes react with halogens?

Answer 16

They form dihaloalkanes.
The halogens add across the double bond, and each carbon ends up bonded to a halogen atom.
It’s an electrophilic addition reaction.

Question 17

What is the test for unsaturation?

Answer 17

If the hydrocarbon is an alkene, the bromine water will go from orange to colourless.
This is because bromine is added across the double bond to form a colourless dibromoalkane, by electrophilic addition.

Question 18

How do you synthesise alcohols from alkenes?

Answer 18

Hydrating alkenes with steam at 300 c and 60-70 pressure.
The reaction needs catalysing by a strong acid such as phosphoric or sulfuric.
It can produce ethanol from ethene.

Question 19

What is the equation for synthesising alcohols?

Answer 19

H2C=CH2 (g) +H2O (g) <–> CH3CH2OH (g)
It is reversible and the reaction yield is low.

Question 20

How are haloalkanes synthesised from alkenes?

Answer 20

Alkenes undergo electrophilic addition reactions with hydrogen halides to form haloalkanes.

Question 21

What is Markownikoff’s rule?

Answer 21

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen attaches itself to the carbon with the greater number of hydrogens and smaller of carbons.

Question 22

What are the three possible carbocations?

Answer 22

Primary - 1 R group
Secondary - 2 R groups
Tertiary - 3 R groups
R is an alkyll group, an alkane with a hydrogen removed.

Question 23

How does the stability of carbocations affect reactions?

Answer 23

The more stable carbocations are more likely to form than less stable ones.
This is because the first step of the mechanism is more likely to lead to the formation of the most stable carbocation.
So more product will be formed via the more stable carbocation.
This can explain what the major product will be when a hydrogen halide reacts with an unsymmetrical alkene.

Question 24

What is the carbocation stability dependent on?

Answer 24

Each alkyl group donates and pushes electrons towards the positive charge of the carbocation.
The positive charge is spread over the alkyl groups.
The more alkyl groups attached to the carbocation, the more the charge is spread out, making the ion more stable.

Question 25

What is addition polymerisation?

Answer 25

When the double bonds in small alkenes (monomers) open up and join together to make long chains called polymers.

Question 26

What are common polymers?

Answer 26

PVC, polychloroethene, made from chloroethene, it is flexible or rigid - pipes.
Polypropene, from propene, for toys, crates, uPVC windows, fibre for ropes.
Polystyrene, from phenylethene, for packing material, or insulating.
PTFE, from tetrafluoroethene, for non-stick pans, clothing, cable insulation.

Question 27

What is the biodegradability of polymers?

Answer 27

They’re very unreactive.
Food doesn’t react with PTFE coating on pans.
Plastic windows don’t rot.
But this makes polymers difficult to dispose of.

Question 28

What is burying waste plastic?

Answer 28

Waste is taken to landfill, compacted, and covered with soil.
It is used when plastic is difficult to separate, not in sufficent quantities to make separation financially worthwhile, and too difficult to technically recycle.
But waste is generated too much so needs reducing.

Question 29

What is recycling plastics?

Answer 29

Some plastics can be melted and remoulded, while others can be cracked into monomers which can be used as an organic feedstock to make more plastics or other chemicals.

Question 30

What is burning waste plastic?

Answer 30

Wastes can be burned, and the heat used to generate electricity.
But it needs to be carefully controlled to reduce toxic gases.
Waste gases (e.g. HCl) are passed through scrubbers which can neutralise gases by allowing them to react with a base.

Question 31

What are biodegradable polymers?

Answer 31

Biodegradable polymers decompose quickly in certain conditions, because organisms can digest them.
They can be made from materials such as starch, and from the hydrocarbon isoprene.
So they can be produced from renewable raw materials or from oil fractions.

Question 32

What are the advantages of renewable raw materials?

Answer 32

They won’t run out.
Some plant-based polymers save energy compared to oil-based.
Biodegrading produces carbon dioxide, but plant-based will release the same CO2 absorbed when growing.

Question 33

What are the disadvantages of biodegradable polymers?

Answer 33

They still need the right conditions before they’ll decompose.
So biodegradable plastics still need collecting and separating from non-biodegradable.
They’re also more expensive than oil-based equivalents.

Question 34

What are the uses of biodegradable polymers?

Answer 34

E.g. plastic sheeting to protect plants from frost, made from polyethene with starch grains embedded in it.
In time the starch grains are broken down by microorganisms and the remaining polyethene crumbles to dust.

Question 35

What things can scientists do to reduce waste plastics?

Answer 35

Continue developing photodegradable polymers.
Develop better techniques for cracking polymers.
Find new ways to make polymers from plant-based substances.
Use processes with higher atom economy.
Develop new, more efficient ways of sorting and recycling polymers.