3.4 Alkenes Flashcards
What is an alkene?
Unsaturated hydrocarbons with a C=C double bond
What is the general formula of an alkene?
CnH2n
Why is there no rotation about the C=C double bond?
Due to the pi orbital - electron density above and below the single bond, which holds the carbon atoms in place
Are they more or less reactive than alkanes? Why?
More reactive, due to high electron density of double bond and the fact that the pi-bond is slightly easier to break
What intermolecular forces of attraction do they have
Only van de Waals due to non-polar bonds
Are they soluble in water?
No, non-polar bonds (van de Waals < hydrogen bonds)
Name and describe the three different types of isomers alkenes can have
Chain isomers - Branched chains
Position isomers - C=C on different double bonds
Geometric E-Z isomers - Z is when 2 highest number atomic chains are on the same side of the double bond; E is when they’re on opposite sides
Write an equation for the complete combustion of pent-2-ene
CH3CH=CHCH2CH3 + 7.5 O2 —> 5CO2 + 5H2O
What is an electrophile
Electron deficient atoms / ions which accept a pair of electrons (here from the C=C double bond)
What is the most stable carbocation intermediate? Why?
Alkyl groups have a positive inductive effect, so the most stable carbocation is the one bonded to most other carbon atoms i.e. A tertiary carbon
Major products will be formed from which kind of carbocations?
Tertiary
What conditions are needed for the electrophilic addition of H2O to an alkene?
Acid catalyst, usually phosphoric acid
What are the products for the electrophilic addition of H2O to an alkene?
An alcohol
What conditions are need for the electrophilic addition of a hydrogen halide to an alkene?
Room temperature
What conditions are needed for the electrophilic addiction of a halogen molecule to an alkene
Room temperature and organic solvent
How does a molecule with a non-polar bond react as if it is an electrophile
C=C double bond with high electron density induces a temporary dipole in the halogen molecule —> δ+ atom attracted to double bond
How would turn the product (from sulphuric acid + ethene) into an alcohol and how does this show that sulphuric acid catalyses the addition of water to an alkene?
Add water
H2SO4 reforms, showing it catalyses the hydration of alkenes
What is an addition polymer?
Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
What are monomers? What form do they usually take?
Molecules which combine to make a monomer
Usually have a C=C double bond which breaks to leave a repeating pattern
Give three uses of poly(chloroethene) / PVC
Drainpipes
Vinyl
Aprons
Give two examples of plasticisers
Esters and phthalates
What are plasticisers?
Small molecules which get between polymer chains to force them apart and allow them to slide over one another
How do the physical properties of PVC change due to a plasticiser? What applications does this lead to?
PVC with a plasticiser become flexible, used for aprons
Without a plasticiser, PVC is rigid and used for drainpipes
Why do many things containing C-C and C-H bonds not decompose easily?
Bonds are non-polar so not attacked by enzymes
Why is a lack of biodegradability in compounds with C-C and C-H bonds a problem?
Disposal is very problematic
What is mechanical recycling?
Where plastics are separated into different types, washed, ground down, melted and re-moulded
What is mechanical recycling used for?
Soft drinks bottles —> fleeces
What is feedstock recycling?
Plastics heated to a temperature which breaks polymer bonds, leaving original monomers which can be made into new polymers
What is feedstock recycling used for?
Making totally new plastics
What is a problem with recycling?
Each time a thermosoftening plastic is melted down and remoulded, their properties degrade, so they can only be remoulded a limited number of times
