TOPIC 13 Flashcards
(13 cards)
What are alkenes?
Alkenes are examples of unsaturated hydrocarbons which contain a double carbon to carbon bond in their structure
What are the bonds found in alkenes?
- 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
What is the shape of alkenes?
- 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
What are more reactive- alkenes or alkanes?
- 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
What is the hydrogenation of alkenes?
- 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
What is the halogenation of an alkene?
At rtp, an alkene has a halogen added across the double bond to produce a haloalkane
How can we test for unsaturation?
Bromine water added to a sample of alkene, bromine is added across the double bond. The orange colour becomes colourless showing decolorisation
What is the hydration of alkenes?
When alkenes react with steam under the presence of a phosphoric acid catalyst (H3PO4) they produce an alcohol
What is an electrophile?
Electron pair acceptor- a positive ion or molecule containing a partial positive charge
What is carbocation stability?
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
How can we reuse plastics?
- 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
How can we sort waste plastic?
- 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
What are biodegradable polymers?
- 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
