4.1.8/9 : Properties and addition reactions of alkenes Flashcards
What is meant by unsaturated?
Contain at least one C=C bond
What is the general formula of an alkene?
CnH2n
What bonding is present in alkenes and how are they formed?
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
Why are alkene molecules described as ‘planar’?
They have a flat shape in the region of the double bond.
Why is the pi bond the most reactive part of an alkene molecule?
- it has a high electron density around it
- it has a lower bond enthalpy than a sigma bond does
What is the shape and bond angle of an alkene?
Shape : trigonal planar
Bond angle : 120 degrees
Why in reality is the bond angle in an alkene actually less than 120 degrees?
The double bond is more electron dense than the single bonds so causes more repulsion moving the single bond electron pairs slightly closer together.
What is meant by the mean bond enthalpy?
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).
What is an electrophile?
An electron pair acceptor - e.g. positive ions and molecules with a slight positive charge
What is hydrogenation, what conditions are needed?
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
What is halogenation and what conditions are needed?
Halogenation is an electrophilic addition reaction where a halogen is added across a C=C bond.
Conditions : room temperature and pressure
How can halogenation reactions be used to test for saturation?
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.
What is hydration and what conditions are needed?
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
What is stated by Markovnikov’s rule?
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
What happens in polymerisation reactions?
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.
What are some uses of polymers?
Plastic bags and bottles, electrical cable insulation, non stick coatings on cooking pans
What are some uses of polymers?
Plastic bags and bottles, electrical cable insulation, non stick coatings on cooking pans
Why are most polymers not biodegradable?
They are saturated organic compounds so are very stable
Why should we not be reliant on addition polymer plastics?
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.
What are some ways that plastic can be disposed of?
Landfill, combustion (with electrical generation), reusing, recycling, using them as organic feedstock
Why are the large holes dug for landfills lined?
To stop contaminants seeping into the water table
Why does the rate of decomposition of biodegradable materials decrease when they are disposed of in landfills?
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
What does biodegradeable mean?
Biodegradable materials are affected by the action of microorganisms and environmental conditions, leading to decomposition
What are the advantages and disadvantages of disposing of plastics through combustion?
❌ 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
What are some issues with recycling old plastics?
❌ 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
How does organic feedstock work?
Chemical reactions are used to break plastic polymers into small organic molecules which can be used in other industrial reactions
What is an example of a biodegradable polymer and what is an issue with mixing them with other addition polymers to reduce waste?
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
What is a bioplastic?
A bioplastic is a material made from a renewable source which is biodegradable
How do photodegradable plastics work to reduce waste?
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.
Describe what is meant by the term ‘pi bond’
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
Which are more reactive, alkenes or alkanes?
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
What takes place in an addition reaction?
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
Why can alkenes be attached by electrophiles?
C=C double bond in alkene is a region of high electron density. Electrophiles are attracted to this region of high electron density.
What is a polymer?
A large molecule (macromolecule) made up of many repeat units/monomers
How do alkenes form polymers?
Lots of smaller alkene molecules add together to create a large saturated compound by addition polymerisation
