3.4 Alkenes Flashcards
What is an alkene?
Unsaturated hydrocarbon with 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 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 the pi-bond is slightly easier to break
What intermolecular forces of attraction do they have.
Only VDW due to non-polar bonds
Are they soluble in water? Why?
No, non-polar bonds (VDW < Hydrogen bonding)
Name and describe the three kinds of isomers alkenes can have
Chain isomers (branched chains)
Position isomer (C=C on different carbon atom)
Geometric E-Z isomers (Z is when 2 highest atomic number chains are on the 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 1/2O2 —> 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 type of carbocation intermediate? Why?
Alkyl groups have a positive inductive effect, so the most stable carbonation is the one bonded to the most other atoms i.e. a tertiary carbocation
Major products will be formed from which kinds of carbocations?
Tertiary (or the most stable available)
What conditions are needed for the electrophilic addition of H2O to an alkene?
Acid catalyst, usually phosphoric acid
What are the product(s) of the reaction?
An alcohol
Draw a mechanism for the addition of water to ethene
Slide 30
What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?
Room temperature
Draw a mechanism for the reaction of HBr and ethene?
Slide 34
What conditions are needed for the electrophilic addition 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 a high electron density induces a temporary dipole in the halogen molecule —> 𝛿+ atom attracted to double bond
Draw a mechanism for the reaction between bromine and ethene
Slide 40
Draw a mechanism for the reaction of sulfuric acid with ethene
Slide 41
How would you turn the product (from sulferic acid + ethene) into an alcohol and how does this show that sulfuric acid catalysed 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 loss of any atom or molecule
What are monomers? What form do they usually take?
Molecules which combine to form a polymer
Usually have a C=C bond which breaks to leave a repeating pattern
Draw how you would represent the polymerisation of ethene
Slide 50
Give 3 uses of poly(chloroethene) / PVC
Drainpipes
Vinyl
Aprons
Give to examples of plasticisers
Esters and phthalates
What are plasticisers?
Small molecules that 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 plasticiser become flexible, used for aprons
Without a plasticiser, PVC is rigid, used for drainpipes
Why do things containing mainly C-C and C-H bonds not decompose easily?
Bonds are non-polar so are not attacked by enzymes
Why is a lack of biodegradability in compounds with C-C or C-H bonds a problem
Disposal is very problematic
What is mechanical recycling
Where plastics are seperated into different types, washed, ground down, melted and re-moulded
What is mechanical recycling used for?
Soft drink bottles —> fleeces
What is feedstock recycling?
Plastics heated to a temperature which break polymer bonds, Leaving original monomers that can be made into new plastics
What is feedstock recycling used for?
Making totally new plastics
What is a problem with recycling?
Each time thermosoftening plastics are melted and remoulded, their properties degrade, so they can only be remoulded a limited number of times
