alkenes

• What are alkenes?

  they are unsaturated hydrocarbons which have a double bond and have the general formula CnH2n
• The nature of the double bond in the alkene?

  3 of the 4 electrons in the carbon are used to make sigma bonds, the other carbon bond forms a pi bond
• What is a pi bond?

  It is the sideways overlap of two p orbitals, the electron density of these pi bonds are above and below the line joining the nuclei of the bonding atom
• What is the shape of an alkene?

  trigonal planar with a bonding angle of 120 degrees
• What is a stereoisomer?

  it is when the compound has the same structural formula but different arrangement of atoms in space
• what is an e-isomer?

  it is when the functional group is on a diagonal to eachother
• what is a z-isomer?


  It is when an isomer is either on both the top branches or both the bottom branches
• What is the cahn-ingold-prelog nomenclature?

  It is a set of rules to see whether a compound is an e/z isomer, it is determined by the proton number of each compound
• how is the cahn nomenclature done?

  1. assign the atomic number to each of the elements
  2. Assign the priority- you need to find the first point of difference, the one with the higher atomic number is the higher priority
  3. Then alike to the previous e/z isomerism determine if it is an e or a z isomer
• Why are alkenes reactive?

  It is due to their pi bond, as the electron density is above and below the plane meaning it can easily be attacked by electrophiles and therefore means that it can undergo addition reactions relatively easily
• What addition reactions can alkenes undergo?

  they can undergo the addition of hydrogen(forms an alkane)

  addition of a halogen (forms an haloalkane)

  addition of hydrogen halides (forms a haloalkane)

  addition of steam in the presence of an acid catalyst (forms an alcohol)
• What is the test for unsaturated molecules?

  Add bromine water and it turns from orange to colourless
• What mechanism do alkenes usually undergo?

  They usually undergo electrophilic addition reactions
• Why are alkenes able to undergo this electrophilic addition?

  There is a double bond with a region of high electron density because of the existence of pi bonds

  The electron density of the pi electrons attracts electrophile which enables an electron to be lost and the opening up of the double bond
• Mechanism for electrophilic addition? (H-Br) (part 1)

  1. there is a difference in electronegativity meaning that the molecule becomes polar
  2. This means that the electron pair in the double bond becomes attracted to the pi bond causing it to break.
  3. A bond then forms between the hydrogen atom of H-Br and the carbon which was part of the double bond
• Mechanism for electrophilic addition (H-Br) Part 2?

  1. The H-Br bond then breaks by heterolytic fission with an electron pair going to the bromine causing it to form a negative ion
  2. A bromide ion and a carbocation are formed (a carbocation is a positively charged ion)
  3. The bromide ion then reacts with a the carbocation to form the addition product
• Carbocation stability?

  Carbocations stability are categorised by the number of alkyl groups attached to carbocation, as alkyl groups increase the stability of the molecule increases along side it too
• What are polymers?

  They are long repeating chains of small units known as monomers
• How can polymers be formed?

  Through the process of addition reactions or condensation reactions
• Things to remember for polymer drawing

  1. The n on both sides of the reaction
  2. The square brackets around the polymer
  3. The trailing bonds out of the polymer
• What are the conditions of the formation of addition polymers usually?

  High Pressure and High Temperature
• What are some environmental effects of disposal of polymers?

  Pollution, landfill space depletion, and harm to wildlife.
• What can burning of PVC (polymer) produce?

  Hydrogen chloride gas which is a very corrosive gas
• What are the methods used for recycling PVCs now?

  It is ground down, it can also be formed again by the precipitation of a solvent
• What is feedstock recycling?

  Chemical process of reclaiming monomers gases or oils from waste polymers. These materials can then be further used in the production of new polymers
• What are bioplastics?

  they are plastics made up of renewable and sustainable alternatives to oil based products, this conserves the environment and natural resources too
• What are photodegradable polymers?

  they are polymers that will dissolve and be weakened by light
• Combustion of polymers can produce HCl, how can this be removed?

  Pass it through an alkali to be neutrilised