Chapter 13 - Alkenes

• Properties of Alkenes:

  • Alkenes are unsaturated hydrocarbons
  • C(n)H(2n)
  • Alkenes can be branched, contain more than one double bond, or be cyclic
  • Alkenes with more than one double bond or are cyclic do not have the normal formula
• Composition of C=C

  • One sigma bond, one pi bond
  • Formed from the sideways overlap of 2p atomic orbitals
  • The Pi electron density is concentrated above and below sigma bond
  • The Pi bond locks two carbon atoms in position, preventing rotation around the double bond

  
• Stereoisomerism:

  • Stereoisomers have the same molecular and structural formula
  • But a different arrangement of atoms in space
• Features of Stereoisomerism:

  • E/Z isomerism Occurs in compounds with a C=C double bond because restricted rotation
  • Two different atoms/ groups of atoms on each carbon of the C=C
• Requirements for cis-trans isomerism:

  • C=C double bond
  • Each C must be attached to two different groups
  • One of attached groups on each C atom of the double bond must be the same
• E/Z isomerism:

  • If groups on higher priority are on the same side of double bond, compound is Z isomer
  • If groups of higher priority are diagonally placed across the double bond, compound is E isomer

  
• Reactions of Alkenes:

  • Alkenes are more reactive than Alkanes because of the pi bond
  • The pi electrons being on the outside of the double bond are more exposed than the electrons in the sigma bond
  • The pi bond breaks more readily so Alkenes undergo addition reactions
• Electrophiles:

  • An atom (or group of) that is attracted to an electron rich centre and accepts an electron pair, usually a positive ion or molecule, containing an atom with a partial positive charge
• Electrophillic addition:

  Ethene + Bromine —> Dibromoethane

  
• Markownikoff’s Rule

  • When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen halide attaches itself to the C atom of the alkene with the greater number of hydrogens and smaller number of carbons.

  
• Carbocations:

  • Classified by the number of Alkyl groups attached to the positive charged C atom
  • Alkyl groups donate and push electrons towards the positive charge of the Carbocation
  • The more alkyl groups attached to the positively charged C atom, the more charge is spread out and the more stable the carbocation is