alkenes

• Alkenes have the general formula CnH2n, where n represents the number of carbon atoms.
• What is an electrophile?

  Electron deficient species (with an incomplete octect configuration) and are attacked by electron-riched species
• For every mechanism, what must I write?


  Polarity

  Lone pair

  Arrow

  Name

  Speed
• What is the distinguishing test for alkenes?

  Bromine test: the rapid decolourisation of Br indicates the presence of C=C double bond.
• What is Markovnikov's Rule?

  The majot product is the one in which the H atoms adds to the carbon of the double bond having the greater number of hydrogen atoms bonded to it and the halogen (X) is added to the other carbon
• What is Saytzeff's Rule?

  The most substituted alkene (the alkene with the the most number of alkyl groups attached to the C=C bond) will be the most stable product =major product
• When is Saytzeff's rule applied?

  To find out major product when H2O or HX is eliminated/removed from alcohol
• When is Markovnikov's Rule applied?

  To predict major product during electrophilic addition. (eg. adding H2O to alkenes)
• What are the reagents and conditions for reduction of alkenes?


  H2 (g), Ni (catalyst), heat

  H2(g), Pt room temp and pressure
• What are the reagents and conditions for hydration?

  Reagents: Water Conditions: H3PO4 concentrated phosphoric acid as catalyst, 300 C, 65 atm
• What are the reagents and conditions for hydration? (lab)

  Stage 1: concentrated H2SO4, cold

  Stage 2: water, warm the mixture
• What are the reagents and conditions of electrophilic addition of HX?

  Reagents: Gaseous HX

  Conditions: Room temperature
• What is the mechanism for electrophilic addition?

  • Electrophile is attracted to C=C due to electron density
  • Electrophiles are positive and accept a pair of electrons from the double bond
  • A positive ion is formed ( carbocation ) 
  • A negatively charged ion forms a bond with the carbocation
• Why can a product be optically inactive?

  A racemic mixture is formed.

  The carbocation intermediate is trigonal planar about the C+ which allows an equal probability of attack by the nucleophile from either side of the plane, resulting in a 50:50 mixture of a pair of enantiomers being formed.
• What are the reagents and conditions for the elimination of OH from halogenoalkanes to form alkenes?

  Reagents: excess conc H2SO4

  conc H3PO4

  Al2O3

  Conditions: Heat under reflux
• What are the reagents and conditions for the elimination of HX from halogenoalkanes?

  Reagents: NaOH (alc) or KOH in ethanol

  Conditions: Heat under reflux
• Why does the boiling point increase as carbon atom increase?

  • As size of electron cloud increases, the ease of distortion of e cloud increases.
  • This results in stronger instaneous dipole-induced dipole (id-id) attractions between alkene molecules.
  • Hence more energy is required to overcome stronger id-id attractions between the molecules.
• Why do cis isomers have higher boiling point than trans isomers?

  • More energy is needed to overcome stronger permanent dipole-permanent dipole (pd-pd) attractions between molecules of the polar cis isomer
  • than the energy needed to overcome the weaker instantaneous dipole-induced dipole (id-id) attractions between molecules od the non-polar trans isomer.