13. Alkenes

• general formula: CnH2n
• alkenes are held together by london forces
• Alkenes are not soluble in water as they cannot hydrogen bond
• molecular orbitals result when atomic orbitals overlap and hence bonds form between atoms
• sigma bond: single covalent bond:

  • formed by a pair of electrons, in an orbital, in a molecule with electron density concentrated between two nuclei
  • can be formed by:
  • overlap of two s-orbitals
  • overlap of one s-orbital and one p-orbital
  • overlap of two p-orbitals
  • sigma bonds are the head-on overlap of atomic orbitals
• Pi bond:

  • found in double/triple bonds
  • formed by sideways overlap of two adjacent atomic p-orbitals
  • electron density is concentrated in two regions - above and below the plane of the molecule
  • rotation around a double bond is restricted because of these regions of electron density
• stereoisomer: the same molecular, structural formula, but different spacial orientation
• Z isomer/Cis-isomer:

  • two bigger groups on the same side
  • two groups are the same
• E isomer/trans-isomer:

  • bigger groups on different sides
• E/Z Isomerism occurs when:

  • there is restricted rotation around a bond
  • different groups are attached to the carbon's atoms at each end of the double bond
• the pi bond electron density is above and below the plane of the molecule which makes its electrons more exposed. therefore alkenes are more reactive than alkanes
• the pi bond has a bond enthalpy of 265 kJ mol-1 so less energy is needed to break the pi component of the double bond
• Combustion of alkenes:

  • hydrocarbons willl burn in the presence of oxygen to produce carbon dioxide and water
  • the alkenes burn with a sootier flame than alkanes as there are fewer hydrogens per carbon
• Addition of Hydrogen:

  • unsaturated hydrocarbon --> saturated hydrocarbon
  • catalyst: Nickel
  • temperature: 150 degrees
• Addition of halogens:

  • alkene --> haloalkane
  • bromine water is decolourised
• Addition of Hydrogen Halides:

  • alkene --> monohaloalkane
  • temperature: room temp.
• Addition of Steam:

  • alkene --> alcohol
  • catalyst: Phosphoric Acid
  • temp: high enough for H2O to be steam
• Electrophile: a chemical species which is attracted to regions of electron density. it accepts a pair of electrons
• nucleophile: a chemical species which is attracted to an electron deficient region. it donates a pair of electrons
• Electrophilic addition:

  • reaction has 100% atom economy
  • the small molecule that reacts with the alkene is an electrophile
• double bonds are nucleophiles because they are electron rich
• carbocation - a reactive species containing a carbon atom which has a positive charge. it is an intermediate (formed in one step, used in the next)
• bromine molecules are polarised as they approach the electron rich double bond
• inductive effect of methyl groups: they push electrons from one another towards the central carbon
• stability of carbocation: depends on the number of alkyl groups attached to the carbon. Alkyl groups are electron donating and they reduce the positive charge on the carbon
• Saturated addition polymers:

  • large molecules
  • formed when thousands of monomers join
  • formed from the addition reaction between alkene monomers
  • the c=c bond must break so c-c bonds can form between neighbouring monomers
• Environmental concerns of polymers:

  • addition polymers are very unreactive due to the strong, non-polar C-C bond
  • they are non-biodegradable so they cant be broken down by living organisms
  • they are hard to dispose of
• Recycling Polymers:

  • reduces the use of fossil fuels (to create more alkenes)
  • reduces the amount of waste in landfill
  • polymers must be sorted in advance
  • polymers are cut into flakes and melted to make a new plastic product
• Polymers as fuel:

  • burning polymers releases energy which produces steam that turns a turbine
  • cannot be used with polymers containing Cl
  • produces greenhouse gases which contribute to global warming
• Feedstock recycling:

  • polymer is converted back into monomer
  • the raw material can be reused as reactants to make new polymers
  • feedstock can be unsorted and unwashed, but it takes lots of time and energy
• to recycle polymers containing chlorine, the polymer must undergo chemical treatment
• biodegradable polymers:

  • bioplastics are made from monomers such as plant starch or cellulose
  • they can be broken down by living organisms into biological compounds
  • e.g. compostable bin bag liners
• photodegradable polymers:

  • bonds are weakened when they absorb light