13 - Alkenes Flashcards
Alkenes general formula
CnH2n
How is a Pi bond formed
The sideways overlap of two p-orbitals, one from each C atom of the double bond where it’s election density is concentrated above and below the line of bonded atoms
Difference between sigma and pi bond
- sigma is between bonding atoms where pi is above and below
- head on overlap compared to sideways overlap
- pi has lower bind enthalpy and weaker
- sigma has e density head on but pi is sideways density
What is optical isomerism
Can occur in much wider range of compounds including alkanes with no functional groups
Why can some not have E/Z stereoisomerism
One of the carbon of the double bonds have the same two groups attached and so cannot show two different stereoisomers
Alkenes have
Addition reactions
I.e they only form one product
Hydrogenation
- needs nickel catalyst and 423 K
- forms an alkane
Halogénation
- no conditions
- Alkene with halogen forms bromo, chloro, iodo..
- can be confirmed with a bromination test to see if it de colourises and is unsaturated
Hydrogen Halides
- no conditions
- addition of a H and Cl, which can be major or minor
Hydration of Alkenes
- react with steam (gas) and H3PO4 catalyst
- can be major or minor
What are alkenes reactions known as..
Electrophilic reactions
- double bond of alkene is a high electron density because of pi electrons
- this attracts electrophiles (electron pair acceptor)
- this is usually positive charged…
Reaction mechanism of electrophilic reactions
- molecule of HBr or Cl2 is polar or becomes induced dipole in presence of alkene
- pi bond is attracted to positive side (H+) causing the double bond to break and form with electrophile
- H-Br bond breaks through heterolytic and Br becomes negatively charged
- as double bond broken, Br - and carbocation formed (+)
- negative Br reacts with C+ making another bond
What is a carbocation
Positive C ion
- can be primary - when c+ on the end of chain
- secondary - c+ in middle of chain
What must be shown when drawing electrophilic reactions
- charges on Br2 or HBr
- two arrows: double bond to H and bond to Br to make negative
- intermediate step with a Br- and carbocation
- arrow from electron pair to carbocation
Carbocation stability
- linked to the electron donating ability of alkyl groups
- each alkyl (CH3) donates and pushes electrons towards the positive charge of carbocation
- more alkyl groups attach to C+ the more the charge is spread out and more stable
General equation of polymerisation
n with monomer ——> repeating unit of polymer
How to show polymer
- square brackets
- long extended bonds
- n on corner
- double bond broken
Conditions of polymerisation
- High temperature and pressure
uses of polymers
- PVC (CH2CHCl) - pipes, films, sheets, windows
- polypropene (CH2CH(CH3)) - kids toys, packing crates, fibre for ropes
- polystyrene (CH2CH(benzene)) - packing materials, food trays and cups due to thermal insulating properties
- PFTE, Teflon (CF2CF2) - non stick pan coating, membrane for shoes and clothes and cable insulation
Environmental impacts
- often burnt and non-biodegradable
- can kill marine life and contribute to global warming
How are polymers recycled
- recycled and melted to form new things
- cannot recycle PVC due to high Cl content so use tech to dissolve
- waste polymers can be used as a source of fuel
- acts as a feedstock material
Bio plastics
Natural polymers made from plant starch, cellulose, plant oil and protein
Advantages of recycling polymers
- protects environment and conserves valuable oil reserves
- Leave no visible or toxic residue
- polylatic acid are becoming more common and plant starch bin liners
- like compostable plates and cups instead of polystyrene
- photodegradable - have bonds that weaken by absorbing light
Advantages and dis of polymer combustion
- source of energy production and fuel
- atmospheric pollutant and can also form HCl as acid rain
If toxic gas produced i.e HCl
Then can be neutralised with Alkali or carbonate
