Alkenes 🏖️

• what are alkenes?

  unsaturated hydrocarbons with the general formula CnH2n

  they can be branched, polyunsaturated (more than one double bond) and cyclic
• what is the type of bonding that makes up the double bond

  a double bond is made up of a pi and sigma bond

  a pi bond is the sideways overlap of 2 pi bonds

  each carbon atom contributes one electron to the electron pair in the P orbital

  The pi bonds locks the carbon atoms in position and prevents rotation
• how does a c=c look?

  .

  
• what is the shape around a double bond?

  three regions of electron density around each carbon atom= 120’ and a trigonal planar shape
• What are sterioisomers?

  Molecules with the same structural formula but different arrangements of atoms in space.
• what is one main difference between optical and E/Z isomerism?

  E/z can only occur in alkenes because of their carbon carbon double bond but an optical isomer can occur in pretty much any molecule
• what are the two conditions that myst be satisfied for E/Z isomerism?

  has a c=c double bond and has a different group attached to each carbon atom of the double bond

  
• what is the E isomer and what is the Z isomer?

  • Z=sameZ
  • E= oppositE

  
• what is must be present if a molecule has cis/trans isomerism?

  one of the attached groups to carbon must be hydrogen

  • ciS= Z isomer= Same
  • trans= E iso
• how to determine if an alkene just containing one hydrogen is cis or trans?

  using the cahn-ingold-prolog rules

  • high priority groups in the same side= Z isomer
  • high priority groups in diagonal= E isomer
  • priority is based on atomic number
• what is the reactivity of an alkene relative to an alkane?

  They are more reactive as a pi bond is weaker than a sigma bond and therefore can be broken with less energy
• what is hydrogenation of alkenes? conditions

  • alkene + H2 forming an alkane
  • nickel catalyst at 423 K
  • all C=C double bonds react this way and break and attach to H
• what is the halogenation of an alkane? what is a common example in chemistry?


  Rapid addition forming haloalkane

  • requires room temp

  can be used to identify alkene- will causes orange colour of iodine to disappear
• What happens when an alkene react with hydrogen halide?

  addition reaction

  if alkene is unsymmetrical = two possible products

  • product that has hydrogen from hydrogen halide attached to the carbon atom already attached to the most hydrogen atoms = major product

  
• what happens during hydration of am alkene and what conditions does it require?

  • requires steam h20 (g) and a phosphoric acid catalyst (H3PO4)
  • is used in industry to produce ethanol from ethene
  • two possible products

  
• what is an electrophile?

  electron deficient atom that is attracted to a electron rich centre nand accepts an electron pair

  • electron pair acceptor
• mechanism for electrophilic addition between but2ene and hydrogen bromide?

  .

  
• what is the mechanism for the reaction between propene and bromine?

  .

  
• using markownikoff’s rule to determine whether the products from electrophilic addition is major or minor?

  • primary carbocation= positive charge on first carbon= smaller yield= minor product
  • secondary= positive charge is on cation that is attached to 2 other carbons= major product

  
• how can you determine the stability of a carbocation?

  eah alkyl groups donates electrons toward the carbon with the positive charge, the more alkyl groups the more stronger attraction to the positive charge and the stronger the spread of the charge= more stable ion

  • primary carbocation= least and the tertiary carbocation= most stable
• what are polymers?

  large molecules formed from thousands of repeating units called monomers
• what is addition polymerisation?

  when unsaturated alkenes are polymerised at high temp and pressure with a catalyst to produce long saturated chains with no double bonds
• what are some uses of poly(ethene)?

  most commonly used polymers: straws, toys, supermarket bags, shampoo bottlew

  low density and high density version

  • HDPE- linear chains= strength
  • LDPE- branched chains= little strength but flexible
• What is the use of poly(chloroethane) and what is it more commonly known as?

  Use: Insulation, packaging and to make a polymer that is flexible or rigid Common Name: PVC (Polyvinyl chloride)
• What are problems with disposing of waste polymers?

  • lack reactivity= good for storing food= used a lot and thrown away
  • Most= non-biodegradable
• how to prevent the waste polymers harming the environment?

  • sorting discarded polymers by type allowing them to be chopped into flakes, washed, dried, melted and cut into pellets that can be used again
  • dissolving PVC using solvents and collecting high grade PVC from the precipitates to be used again. This is instead of burning them due to lack of space in landfill which releases hydrogen chloride= pollutant and hazardous
  • polymers made of petroleum or natural gas= high energy value= hard to recycle= incinerated to produce heat= steam generate—> drives turbines to produce electricity
• what is feedstock recycling?

  Chemical and thermal processes that turn even unsourced and unwashed polymers back into their monomers gases or oils to be turned back into new polymers
• What is biodegradable polymers made up of and used for?

  • Made up of starch or cellulose and contains additives that alter their structure which allows them to be broken down by micro organisms into water, co2 and biological compounds
  • eg compostable polymers= degrade and leave no visible trace or toxic residues
  • Can be used in supermarket bags, plates and cups and packaging
• What are photodegradable polymers?

  Polymers that contain bonds that are weakened by absorbing light to start the degradation process

  this can also be done by adding light absorbing additives