Module 4 Chapter 13
13.1 Properties of alkenes
13.2
13.3 Reactions of alkenes
13.5 Polymerisation
13.4 Electrophilic addition
Structure
General formula 
Unsaturated hydrocarbons
When naming must specify where the C=C is
Nature of the double bond
There are 4 available electrons for bonding
3 are used in three σ-bonds leaving 1 left (on each carbon atom)
Electron is in the p-orbital- a π bond forms- sideways overlap of the p-orbitals, one from each carbon atom of the double bond- each carbon contributes 1 electron
The π-bond locks the two carbon atoms, preventing rotation
Shape around the C=C
Shape around double bond is trigonal planar
3 regions of electron density around each carbon atom
3 regions repel each other as far as possible
Bond angle 120°
All atoms in same plane
Stereoisomers
Same structural formula but a different arrangement of the atoms in space
E/Z isomers
For E/Z isomerism molecule must
Contain C=C bond
Have 2 different groups attached to each carbon of the C=C
Higher priority= higher atomic number
Alkenes are a lot more reactive than alkanes due to the π-bond
Cis/Trans
Conditions
Molecule must contain a C=C bond
Each carbon in the C=C must be connected to two different groups
Trans if identical group diagonally opposite
One group on each carbon must be identical
Cis if identical group on same side
Z if the 2 high priority are on the same side
E if the 2 high priority are diagonally opposite
Atomic number of 1st point of difference of the group off the same carbon atom
π-bond electron density is concentrated above and below σ-bonds so are exposed and readily breaks
Addition reactions
Hydrogenation
Halogenation
Hydrogen halides
Hydration (steam)
Alkene + Hydrogen + Nickel Catalyst -> Alkane
e.g. Propene + Hydrogen -> Propane
All C=C bonds break- have to balance number of hydrogens to account for molecules with multiple C=C
Alkene + Chlorine/Bromine -> Di(bromo/chloro)alkane
e.g. Propene + Bromine -> Dibromopropane
Test
If a molecule is unsaturated (contains C=C), when Bromine water is added and shaken it goes from orange to colourless
Alkene + Hydrogen halide (gas)-> Haloalkane
e.g. Propene + HCl -> Chloropropane
If the alkane is unsymmetrical there are two possible products depending on which side of the C=C the hydrogen is added
Alkenes react with steam in the presence of a phosphoric acid catalyst (H3PO4)
Alkene + Steam + Phosphoric acid catalyst -> Alcohol
Take part in electrophilic addition as the C=C is a region of high electron density
e.g. Propene + Steam -> Propanol
Used in industry to produce ethanol from ethene
If unsymmetrical alkene then 2 possible products
Addition Polymers
High electron density attracts electrophiles (electron pair acceptors)- usually a positive ion with a partial positive charge
Mechanism
HBr and
But-2-ene
1)Bromine more electronegative than hydrogen so HBr contains a partial charge (Hδ + and Brδ -)
2) Electron pair in π-bond is attracted to partially +ve hydrogen- breaks C=C
3) Bond forms between hydrogen in HBr and Carbon atom in the double bond
4) HBr bond breaks heterolytically - electron pain to bromine atom
5) Bromine ion (Br-) and a carbocation (contains positive carbon) formed
6) Br- reacts wit carbocation to form the addition product
Mechanism Br and Propene
1) As Br approaches π-bond the π-electrons interact with the Bromine electrons causing polarisation of Br (induced dipole). The electrons repel causing them to move closer to the Br atom further away from the alkene- the Br becomes δ - and the Br closer to the alkene becomes δ +
Steps 2-6 same as above but Brδ + acts as Hδ +
click to edit
Markownikoff's rule
Favoured product= major Other product= minor
Hydrogen of the hydrogen halide attaches itself in preference to the carbon with the most hydrogens and the fewest alkyl groups
Carbocations stability
More alkyl groups connected to positive carbon, more stable the molecule is because of the electron donating ability of alkyl groups- they push electrons towards the positive charge of the carbocation so the more alkyl groups, the more the charge is spread out so the more stable the carbocation is
If the non-hydrogen attaches to the 1st carbon in the C=C = primary, if it attaches to the 2nd= secondary
Polymers made of thousands of repeated units of smaller molecules (monomers)
Alkenes can undergo addition polymerisation at high temp and pressure- catalyst required
Name- poly(alkene)
Poly(ethene)- used in supermarket bags, shampoo bottles and children's toys
Poly(Chloroethene)- used for pipes, flooring, bottles and fabric treatment
Poly(propene)- used in packing crates, guttering and fibre for ropes
Poly(styrene)- Packing material, food trays and cups (thermal insulator)
Poly(tetrafluoroethene)- Coating for non-stick pans, permeable membrane for clothing and shoes
May be asked to find the monomer- look for the repeating units
Environmental concerns
Disposing of waste- Not reactive so are non-biodegradable and have serious environmental effects (e.g. suffocating marine life)
Recycling- Polymers made from crude oil- recycling conserves fossil fuels and reduces amount of polymer materials in landfill sites
However, polymer waster must be sorted into different polymers, mixed polymers cannot be recyled
PVC recycling- PVC contains chlorine- hazardous to dispose- produces chlorine gas when burnt
Can be recycled (grind it down and remoulded) or dissolved in solvent and recovered from solvent
Using them as fuel-made from crude oil- high store of energy- heat used to drive turbines and produce electricity
Feedstock- carried out on unsorted, unwashed polymers- treat waste polymers to reclaim the monomers
Bioplastics-Made from plant materials- starch, cellulose, plants, oils or proteins- renewable and they break down in the environment
Biodegradable polymerBroken down by organisms in the environment - no toxic residue
Photodegradable polymers- Oil-based - contain bonds that are weakened by absorbing light or light absorbing additive can be used