4.1.8/9 : Properties and addition reactions of alkenes

Question 1

What is meant by unsaturated?

Answer 1

Contain at least one C=C bond

Question 2

What is the general formula of an alkene?

Answer 2

CnH2n

Question 3

What bonding is present in alkenes and how are they formed?

Answer 3

C-H covalent bonds = sigma bonds
C=C double covalent bond = sigma bond and pi bond

A sigma bond is formed by the direct overlap of the orbitals of the two C atoms.

A pi bond is formed by the electrons in the adjacent p-orbitals overlapping above and below the C atoms - pi bonds can only be formed after the sigma bond has been formed

Question 4

Why are alkene molecules described as ‘planar’?

Answer 4

They have a flat shape in the region of the double bond.

Question 5

Why is the pi bond the most reactive part of an alkene molecule?

Answer 5

• it has a high electron density around it
• it has a lower bond enthalpy than a sigma bond does

Question 6

What is the shape and bond angle of an alkene?

Answer 6

Shape : trigonal planar
Bond angle : 120 degrees

Question 7

Why in reality is the bond angle in an alkene actually less than 120 degrees?

Answer 7

The double bond is more electron dense than the single bonds so causes more repulsion moving the single bond electron pairs slightly closer together.

Question 8

What is meant by the mean bond enthalpy?

Answer 8

The average energy required for one mole of a given bond to undergo homolytic fission in the gaseous state (the energy required for a bond to break).

Question 9

What is an electrophile?

Answer 9

An electron pair acceptor - e.g. positive ions and molecules with a slight positive charge

Question 10

What is hydrogenation, what conditions are needed?

Answer 10

Hydrogenation is when hydrogen is added across a C=C bond (it is a reduction reaction)

Conditions : 150 degrees temperature, nickel catalyst (powder), high pressure

Question 11

What is halogenation and what conditions are needed?

Answer 11

Halogenation is an electrophilic addition reaction where a halogen is added across a C=C bond.

Conditions : room temperature and pressure

Question 12

How can halogenation reactions be used to test for saturation?

Answer 12

When bromine/iodine solution is mixed with a saturated compound there is no reaction. However when mixed with an unsaturated compound, decolorisation happens because an addition reaction occurs.

Question 13

What is hydration and what conditions are needed?

Answer 13

Hydration is an electrophilic addition reaction between a gaseous alkene and steam - it is used in industry to make alcohols.

Conditions : high temperature, high pressure, phosphoric acid catalyst

Question 14

What is stated by Markovnikov’s rule?

Answer 14

When H-X is added to an unsymmetrical alkene, the H becomes attached to the C with the most H atoms to start with. This is because carbocations that have alkyl groups attached are more stable than those with H groups attached. More alkyl groups attached to carbocation = more stable = major

Question 15

What happens in polymerisation reactions?

Answer 15

Monomers have their pi bonds broken. The electrons from each pi bond make a sigma bond with a neighbouring C atom on a different monomer. This connects the monomers, forming a saturated polymer.

Question 16

What are some uses of polymers?

Answer 16

Plastic bags and bottles, electrical cable insulation, non stick coatings on cooking pans

Question 17

What are some uses of polymers?

Answer 17

Plastic bags and bottles, electrical cable insulation, non stick coatings on cooking pans

Question 18

Why are most polymers not biodegradable?

Answer 18

They are saturated organic compounds so are very stable

Question 19

Why should we not be reliant on addition polymer plastics?

Answer 19

Addition polymers are made from compounds made available from the processing of crude oil . Crude oil is a non-renewable resource which is being used up faster than the natural processes in the environment can make more.

Question 20

What are some ways that plastic can be disposed of?

Answer 20

Landfill, combustion (with electrical generation), reusing, recycling, using them as organic feedstock

Question 21

Why are the large holes dug for landfills lined?

Answer 21

To stop contaminants seeping into the water table

Question 22

Why does the rate of decomposition of biodegradable materials decrease when they are disposed of in landfills?

Answer 22

After rubbish is buried, conditions change from normal atmospheric to anaerobic and there is often a limited supply of water which decreases the rate of decomposition

Question 23

What does biodegradeable mean?

Answer 23

Biodegradable materials are affected by the action of microorganisms and environmental conditions, leading to decomposition

Question 24

What are the advantages and disadvantages of disposing of plastics through combustion?

Answer 24

❌ plastics contain lots of carbon so their combustion releases carbon dioxide which is a greenhouse gas and contributes to climate change
❌ other toxic gases like HCl can also be made - however these can be removed by using gas scrubbers, where a base like CaO neutralises the acidic gas
✅ as plastics have high calorific value, they can be burnt in power stations and the chemical energy transferred can be used to drive turbines and generate electricity

Question 25

What are some issues with recycling old plastics?

Answer 25

❌ plastics must be sorted into different types which can be expensive as either labour intensive or requires special technology ❌plastics could still be contaminated from previous use

Question 26

How does organic feedstock work?

Answer 26

Chemical reactions are used to break plastic polymers into small organic molecules which can be used in other industrial reactions

Question 27

What is an example of a biodegradable polymer and what is an issue with mixing them with other addition polymers to reduce waste?

Answer 27

Plant starch - when mixed with addition polymers, the starch part breaks down over time making the polymer chains smaller however the small pieces of addition polymers can still cause hazards to the ecosystem

Question 28

What is a bioplastic?

Answer 28

A bioplastic is a material made from a renewable source which is biodegradable

Question 29

How do photodegradable plastics work to reduce waste?

Answer 29

Photodegradable plastics break down chemically using wavelengths similar to light. Bonds within the structure of the addition polymers are weakened by the absorption of light so exposure to light causes the polymer to break down and biodegrade. However photodegradable plastics in landfill might not be exposed to sufficient light to biodegrade.

Question 30

Describe what is meant by the term ‘pi bond’

Answer 30

A pi bond is a region of space occupied by a bond pair of electrons, above and below the plane of the molecule, formed by overlap of two adjacent p-orbitals

Question 31

Which are more reactive, alkenes or alkanes?

Answer 31

Alkenes are more reactive as they have a double C=C bond which has a higher bond enthalpy and greater electron density than the single bonds in alkanes

Question 32

What takes place in an addition reaction?

Answer 32

A small molecule is added to an alkene causing the double C=C bond to break and a saturated compound is produced as a product of the reaction

Question 33

Why can alkenes be attached by electrophiles?

Answer 33

C=C double bond in alkene is a region of high electron density. Electrophiles are attracted to this region of high electron density.

Question 34

What is a polymer?

Answer 34

A large molecule (macromolecule) made up of many repeat units/monomers

Question 35

How do alkenes form polymers?

Answer 35

Lots of smaller alkene molecules add together to create a large saturated compound by addition polymerisation