6.2.7 - Alkenes

• what are alkenes?

  homologous series of unsaturated hydrocarbons with at least one C=C bond

  general formula C(n)H(2n)
• what is a cycloalkene?

  an alkene which contains a closed ring of carbon atoms

  C=C functional group

  general formula C(n)H(2n-2)
• trend in boiling point of alkenes

  increases as the number of carbon atoms increases because number of electrons increases so more London forces
• solubility of alkenes

  insoluble in water but soluble in organic solvents because they are non-polar
• bonding in alkenes

  double bond made of a pi and sigma bond
• how is a sigma bond formed and what is it's bond enthalpy like?

  formed when 2 orbitals overlap end-on in a straight line between the 2 atoms creating a region of high electron density so strong electrostatic attraction between nuclei and shared pair of electrons and so high bond enthalpy
• rotation in sigma bonds

  free rotation possible
• how is a pi bond formed and what is the electron density like?

  formed when two atomic orbitals overlap sideways

  electron density concentrated in two regions (above and below molecular axis) so weaker electrostatic force of attraction between nuclei and shared pair of electrons so low bond enthalpy
• rotation in pi bonds

  rotation is restricted due to regions of high electron density
• test for alkenes

  bromine water goes from brown-orange to colourless

  double bond breaks