TOPIC 13

Question 1

What are alkenes?

Answer 1

Alkenes are examples of unsaturated hydrocarbons which contain a double carbon to carbon bond in their structure

Question 2

What are the bonds found in alkenes?

Answer 2

• each carbon atom has 4 electrons in its outer shell
• 3 of these are use in three sigma bonds (head on overlap of 2 orbitals and a sigma bond is positioned on the line directly between the 2 bonding atoms)
• Sigma bonds are formed from the horizontal overlap of 2 s orbitals
• one electron is used in a pi bond (found in the p orbital) which is the sideways overlap of 2 p orbitals one from each carbon atom of the double bond
• Pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms (directly above the bonding atoms)
• Locks the 2 carbon atoms in position and prevents them from rotating around the double bond
• P orbitals have a dumbbell shape while s orbitals has a spherical shape

Question 3

What is the shape of alkenes?

Answer 3

• alkenes have a trigonal planar shape- there are 3 regions of electron density around each of the carbon atoms
• 3 regions repel each other as far as possible so the bond angle is 120 degrees

Question 4

What are more reactive- alkenes or alkanes?

Answer 4

• double bonds in alkenes has a high electron density and contain pi bonds which are more exposed therefore more open to attack from electrophiles
• Pi bond has a low bond enthaply
• Alkanes have only sigma bonds like C-C and C-H which are non polar so not reactive

Question 5

What is the hydrogenation of alkenes?

Answer 5

• alkenes undergo many addition reactions, for example with hydrogen in the presence of a nickel catalyst, hydrogen halides, halogens, steam in the presence of an acid catalyst
• Result in the addition of a small molecule across the double bond, causing the pi bond to break and new bonds to form

Hydrogenation is when an alkene turns into an alkane with a nickel catalyst at 423K- hydrogen is added across the double bond

Question 6

What is the halogenation of an alkene?

Answer 6

At rtp, an alkene has a halogen added across the double bond to produce a haloalkane

Question 7

How can we test for unsaturation?

Answer 7

Bromine water added to a sample of alkene, bromine is added across the double bond. The orange colour becomes colourless showing decolorisation

Question 8

What is the hydration of alkenes?

Answer 8

When alkenes react with steam under the presence of a phosphoric acid catalyst (H3PO4) they produce an alcohol

Question 9

What is an electrophile?

Answer 9

Electron pair acceptor- a positive ion or molecule containing a partial positive charge

Question 10

What is carbocation stability?

Answer 10

Each alkyl group will spread the positive charge therefore making it more stable- fewer bonds to hydrogen means its more stable
Primary- carbon is bonded to 2 hydrogens
Secondary- carbon is bonded to 2 R groups and one hydrogen
Tertiary- carbon is bonded to 3 R groups

Question 11

How can we reuse plastics?

Answer 11

• some plastics like polypropene can be recycled and reused by melting and remoulding them
• Some plastics can be cracked into monomers and these can be used as organic feedstock to make more plastics or other chemicals
• Can be handled into unsorted and unwashed polymers

Question 12

How can we sort waste plastic?

Answer 12

• burn it and generate electricity (incineration)- and therefore lowers our reliance on fossil fuels
• Some waste polymers produce toxic gases when burnt, such as PVC producing HCl when burnt which has to be removed
• Waste gases from combustion are passed through scrubbers which can neutralise gases such as HCl by allowing them to react with a base, though this is expensive it is nescessary to ensure combustion is safe

Question 13

What are biodegradable polymers?

Answer 13

• can decompose quickly because oraganisms can decompose them into CO2 and water
• Can be made of renewable raw materials such as starch
• But at the moment they’re more expensive
• They’ll need to be in a big compost heap to ensure there is enough moisture and oxygen for organisms to decompose them
• we would need to separate the biodegradable polymers from non biodegradable polymers
• Scientists are also developing photodegradable polymers as sunlight breaks down the polymers
• However this may also be disadvantageous as smaller pieces that have been fragmented would accumulate in the environment without being broken down further- this may be ingested by animals and they build up in the food chain
• If photodegradable plastic is buried in a landfill site, it is unlikely to be exposed to sufficient sunlight to break up into smaller pieces