C13- Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons.
- contain atleast one c=c bond.
General formula= CnH2n. Doesn’t apply to alkenes that contain more than one double bond or are cyclic.
- contain both sigma and pi bonds.

Question 2

Explain what a pi bond is?

Answer 2

• a pi bond is formed by the sideways overlap of 2 p-orbitals.
• the bond locks the 2 carbons in position and prevents rotation.
• electron density of the pi bond is concentrated above and below the line joining the nuclei of bonding atoms.
• each carbon atom contributes one electron to the bond.

Question 3

Why do alkenes have a pi bond?

Answer 3

For each carbon atom of the double bond, 3 out of 4 electrons are used in 3 sigma bonds. So 1 electron is in a p-orbital.

Question 4

What is the shape around a double bond?why?

Answer 4

Trigonal planar:

• 3 regions of electron density around each carbon.
• the 3 regions repel each other as far as possible so the bond angle around each carbon is 120*.
• all atoms are in the same place.

Question 5

What are stereoisomers and the two types?

Answer 5

Stereoisomers have the same structural formula but different arrangement of the atoms in space.

1. E/Z isomerism- only occurs in compounds with a c=c bond.
2. Optical isomerism- can occur in much wider range of compounds.

Question 6

What causes stereo isomerism?

Answer 6

Stereoisomerism around double bonds arises because rotation about the double bond is restricted.
Reason for the rigidity is the position of the pi bonds electron density above and velow the plane of the sigma bond.

Question 7

Explain E/Z isomerism.

Answer 7

A molecule will have E/Z isomer if it has:

• a c=c double bond.
• different groups attached to each carbon atom of the double bond.

E isomers= different.
Z isomers= same.

Question 8

Explain cis-trans isomerism.

Answer 8

A special kind of E/Z isomerism where one of the groups attached to each carbon of the double bond has to be a hydrogen/the same.
Cis isomer= Z isomer.
Trans isomer= E isomer.

Question 9

What are the cahn-ingold-prelog rules?

Answer 9

• a set of rules that assign priorities to different groups in order to decide which isomer it is, based on atomic number.
• if groups of higher priority are on same side = Z isomer.
• if groups of higher priority are on different sides(diagonal) = E isomer.
  The higher the atomic number, the higher the priority.

Question 10

Reactivity of alkenes?

Answer 10

More reactive than alkanes due to the presence of the pi bond.

• as the pi bonds are on the outside of the double bond, they are more exposed and therefore break and undergo addition reactions easily.
• bond enthalpy of pi bond is also relatively low compared to sigma bonds, showing they are weaker.

Question 11

Addition reactions of alkenes.

Answer 11

Alkenes undergo addition reactions with:
- hydrogen (presence of nickel catalyst).
- halogens.
- hydrogen halides.
- steam (presence of acid catalyst).
Each involves the addition of a small molecule across the double bond, causing the pi bond to break and for new bonds to from.

Question 12

Hydrogenation of alkenes.

Answer 12

Alkene + hydrogen = alkane.

• passed over nickel catalyst.
• at 423K.

Question 13

Halogenation of alkenes.

Answer 13

Alkene + halogen = haloalkane.

- at room temp.

Question 14

How can you test for unsaturation?

Answer 14

The reaction of alkenes with bromine can be used to identify if there is a c=c bond present and the compound is unsaturated.
1. Add bromine water to alkene sample.
2. Orange colour will disappear. Compounds with c=c bond decolourise bromine water.
There will be no change is the organic compound is saturated.

Question 15

Addition reactions of alkenes with hydrogen halides.

Answer 15

Alkene + hydrogen halide (g) = haloalkane.
-at room temp.
If alkene = liquid: hydrogen halide is bubbled through it.
If alkene= gas: mix two gases together.

Question 16

Hydration of alkenes.

Answer 16

Alkene + steam = alcohol.

• phosphoric acid catalyst (H3PO4)
• used to make ethanol from ethene.

Question 17

Explain what electrophilic addition is.

Answer 17

The mechanism of addition reactions which alkenes undergo to form saturated compounds.
Involves the attack by an electrophile on a region of high electron density.
- the c=c bond in alkenes = region of high electron density due to presence on pi bonds.
- usually heterolytic fission.

Question 18

What is an electrophile and nucleophile?

Answer 18

Electrophile: an electron pair acceptor. They are attracted to electron-rich centres (usually a +ion or molecule with partial + charge).

Nucleophile: can donate an electron pair to form a new covalent bond. Attracted to electron deficient centre. (Negative ion/molecule)

Question 19

Explain the full mechanism for the electrophilic addition of butene and HBr.

Answer 19

1. Hydrogen bromide is polar (Br more electronegative than H). Contains dipole.
2. Electron pair in pi bond is attracted to partially positive hydrogen atom so double bond breaks.
3. Bond(single) forms between hydrogen of HBr molecule and C from old double bond.
4. HBr bond breaks by heterolytic fission, electron pair goes to bromine atom.
5. Br- and a carbocation (contains + charged carbon atom) formed.
6. Br- ion and carbocation react to form addition product.

Question 20

What is markownikoff’s rule?

Answer 20

When electrophilic addition occurs, a primary or secondary carbocation can be formed.

Question 21

Carbocation stability?

Answer 21

-tertiary carbocations are most stable, primary are the least.
Carbocation stability is linked to electron-donating ability of alkyl groups:
-each alkyl group donates and pushes electrons towards +charge of carbocation.
-more alkyl groups= +charge spreads out more and more, therefore more stable.

Addition of a hydrogen halide to an unsymmetrical alkene forms the major product via the carbocation.

Question 22

Addition polymerisation of alkenes?

Answer 22

Unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains containing no double bonds.
Industrial polymerisation is carried out at high temperatures and pressures using catalysts.
-addition polymers have high molecular masses.

Question 23

How do you draw curly arrows?

Answer 23

• start the arrow at the bond or loan pair.
• draw head of the arrow at the atom to which the electron pair transfers.

When asked to describe a mechanism, it is easier to draw it out even in the real exam.

Question 24

What are the 5 main addition polymers and their uses?

Answer 24

1. Poly(ethene)- one of most commonly used. Supermarket bags, plastic straws, toys.
2. Poly(chloroethene) / PVC - can make a polymer that is flexible or rigid. Pipes, films and sheeting, bottles, flooring, insulation.
3. Poly(propene)- toys, packing, guttering, rope fibres, upvc windows.
4. Poly(styrene)- packaging material, food trays and cups. Due to thermal insulating properties.
5. Poly(tetrafluoroethene)- coating for non-stick pans, cable insulation.

Question 25

Disposing of waste polymers.

Answer 25

Polymers are readily available, cheap to purchase, and convenient for throwing away as compared to alternatives.

• lack of reactivity makes polymers suitable for storing food and chemicals safely. But are hard to dispose.
• many alkene based polymers are non biodegradable.
• growing amount of polymer waste has environmental effects= killing marine life.

Question 26

Recycling polymers.

Answer 26

Recycling reduces environmental impact by:

• conserving finite fossil fuels.
• decreasing waste going to landfill.
  1. Polymers are sorted.
  2. Chopped into flakes.
  3. Washed, dried and melted.
  4. Cut into pellets and used by manufacturers to make new products.

Question 27

PVC recycling.

Answer 27

Disposal and recycling of pvc is hazardous due to high chlorine content and the range of additives present.
- dumping in landfill isn’t sustainable.
- burning releases hydrogen chloride (corrosive gas) and other pollutants.
Recycling involves:
1. Using solvents to dissolve polymer.
2. High grade pvc recovered by precipitation from the solvent.
3. Solvent re used.

Question 28

Using waste polymers as fuels.

Answer 28

Have high stored energy value as they are derived from petroleum or natural gas.
- Can be incinerated to produce heat, generating steam to drive a turbine producing electricity.

Question 29

Feedstock recycling.

Answer 29

Describes the chemical and thermal processes that can reclaim monomers/gases/oils from waste polymers.

• materials produced can be used as raw materials for production of new polymers.
• major advantage = can handle unsorted/unwashed polymers.

Question 30

What are bioplastics ?

Answer 30

Produced by plant starch, cellulose, oils and proteins.

• offer a renewable alternative to oil-bases products.
• protects environment and conserves valuable oil reserves.

Question 31

Biodegradable polymers.

Answer 31

• are broken down by microorganisms into water, co2 and biological compounds.
• usually made from starch/ cellulose or contain additives that microorganisms can break down.
• supermarket bags made from plant starch can be used as bin liners for food waste so they can be composted together.
• compostable plates/cups made from sugar cane fibre are replacing poly(styrene).

Question 32

What are compostable polymers?

Answer 32

Biodegradable polymers that degrade and leave no visible or toxic residues.
-Those based on poly (lactic acid) are becoming more common as alternative to alkene based polymers.

Question 33

Photodegradable polymers.

Answer 33

Where the use of plant based polymers is not possible, photodegradable oil-based polymers are being developed.

• these polymers contain bonds that are weakened by absorbing light to start the degradation.
• alternatively, light absorbing additives are used.