chapter 13 alkenes

• a double bond consists of a sigma bond, and a pi bond
• a pi bond is formed by the sideways overlap of two p-orbitals, which is above and below the plane of the carbon atoms
• shape around a carbon double bond is 120
• stereoisomers have the same structural formula but a different arrangement of atoms in space
• an E isomer is where the highest priority molecules on each carbon, or on opposite sides
• a Z isomer is an isomer where each carbon has the highest priority molecule on the same side
• a cis isomer is a Z isomer which has hydrogen and one molecule on ecah carbon
• a trans isomer is an E isomer which has one molecule and one hydrogen on each carbon
• using the cahn-ingold-prelog rules

  • assigning priority - the higher the atomic number, the higher the priority
  • if the two atoms are the same, you will need to find the first point of difference, the group with a higher atomic number is given the higher priority
• the pi bond is weaker than the sigma bond and is therefore broken more readily
• hydrogenation of alkenes

  alkene + hyrogen -nickel catalyst-> alkane

  high temperatures
• halogenation of alkenes

  alkene + halogen -> haloalkane

  at room temp
• you can use halogenation to test for unsaturation
• addition reactions of alkenes with hydrogen halides

  alkene + hydrogen halide -> haloalkane

  major/minor products

  gases mixed together/gas bubbled through
• hydration reactions of alkenes

  alkene + steam -phosphoric acid-> alcohol

  major and minor
• the high density of the pi electrons in a double bond attracts electrophiles, so any reactions with alkenes are electrophilic addition
• mechanism for but-2-ene and hydrogen bromide

  1. bromine is more electronegative than hydrogen
  2. the electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break
  3. a bond forms between the hydrogen atom of the H-Br molecule and a carbon atom from the double bond
  4. the H-Br bond breaks by heterolytic fission, with the electron pair going to Br
  5. Br- and a carbocation is formed. a carbocation contains a positively charged carbon atom
  6. in the final step the Br- ion reacts with the carbocation to form the addition product
• mechanism for propene and bromine

  1. pi electrons interact with the electrons in Br-Br, with and induced dipole in the atom
  2. the electron pair in the pi bond is attracted to the Br polar+ end, causing the double bond to break
  3. a bond is formed between the carbon atom from the double bond, and Br
  4. the Br-Br breaks by heterolytic fission, with the electron pair going to the Br polar-
  5. Br- and a carbocation is formed
  6. in the final stage, Br- ion reacts with a carocation to form the addition product
• markownikoffs rule states that the added reagent travels to the carbon that bears the double bond and that has the least number of Hydrogen (H2) atoms

  this means that a secondary carbocation will become the major product, and a primary will become the minor
• carbocation stability

  • primary is the least stable
  • tertiary is the most stable
• a repeat unit is the specific arrangement of atoms in the polymer molecule that repeats over and over again

  always written in square brackets, after you place a letter n
• monomers become polymers in high temperature and pressure
• poly(ethene) is used for bags, bottles and toys
• poly(chloroethene) is used for pipes, films and insulation
• poly(propene) is used for toys, packing and uPVC windows
• poly(styrene) is used for packaging materials, food trays and cups
• poly(tetrafluoroethene) (teflon) -non stick pans, clothing and shoes
• recycling polymers

  • sort
  • chop into flakes
  • wash
  • dry
  • melt
• PVC recycling is used as PVC is toxic, so PVC is grinded, and reused
• waste polymers can be incinerated to produce heat, generating steam to drive a turbine to produce electricity
• feedstock recycling describes the chemical and thermal processes that can reclaim products from waste polymers, to be used as raw materials
• biodegradable poymers are broken down by microogrganisms into water, co2 and biological compounds

  they leave no visible toxic residue
• photodegradable polymers contain bonds that are weakened by absorbing light to start the degradation