alkenes

• sigma bond: overlap of orbitals directly between bonding atoms

  
• pi bond:

  • sideways overlap of adjacent p-orbitals
  • above and below plane of molecule
  • restrict rotation of the pi bond
  • formed by two 2p-orbitals to form the second bond

  
• alkynes:

  • triple carbon bond
  • sigma + two pi bonds
  • one pi bond -> above and below
  • second pi bond -> front and back
  • alkenes and alkynes will be attacked by electrophiles because of electron-rich pi bonds
• addition reactions (3): hydrogenation, hydration, halogenation
• conditions for hydrogenation (type of electrophilic addition): 150 degrees + nickel catalyst (good at breaking the bond between H atom)
• hydrogenation: hydrogen adds across the double bond and an alkane is formed
• conditions for hydration (steam) (reversible): high temperature (300C) and high pressure (60 atm), phosphoric acid catalyst (H3PO)
• addtition reactions page
• stereoisomers: have the same structural formula but a different arrangement of atoms in space
• atoms cant rotate around the C--C bond and is rigid
• E isomerism: same groups opposite the double bond

  
• Z isomerism: same groups on the same side of the double bond

  
• when 4 different groups around the double bond, calculate the atomic number of the 1st element bonded to the C=C, the atom w the highest atomic number is given higher priority (so treat them like its the one with the same group)

  if the atomic numbers are same, look at the next atom down the chain
• electrophillic addition: electrophiles are electron pair acceptors so they are attracted to the double bond.
• addition of bromine: test for alkene that decolourises bromine water, changing it from brown-orange to colourless
• bromine is the electrophile and adds to the alkene forming a dibromoalkane
• what are the steps of addition of hydrogen halides?

  adcs

  • HBr is polarised
  • an electron pair in the double bond is attracted to the positive dipole on the hydrogen and forms a bond. this breaks the H-Br bond.
  • a carbocation intermediate is formed and Br- is attracted to C+
  • bromoethane is formed
• what are the steps for addition of bromine?

  a

  • Br2 is polarised as the electrons in the double bond repels electrons in Br2
  • an electron pair in the double is attracted to the positive dipole on the bromine and forms a bond. this breaks the Br-Br bond
  • a carbocation intermediate is formed and Br- is attracted to C+
  • colourless 1,2-dibromoethane is formed
• when unsymmetrical alkenes produce 2 different products: the more alkyl groups bonded to the carbocation, the more stable the intermediate is. this is because the alkyl groups push electron towards the positive carbocation stabilising it. the more stable the carbocation, the more likely it will form
• markownikoff's rule: te major product when we add a hydrogen halide to an unsymmetrical alkene is where the hydrogen adds to the carbon with the most number of hydrogens attached already
• limitations of free radical substitution:

  1. hard to control - radical substitutions can lead to further substitutions along the carbon chain
  2. variation of different products - low atom economy