Chemistry - Alkanes

• An alkane is a saturated hydrocarbon containing C-H bonds only
• The general formula of an alkane is CnH2n+2
• Alkane bonds are nonpolar because carbon and hydrogen have similar electronegativities
• The shape and angle of an alkane is tetrahedral with an angle of 109.5 degrees
• The sigma bond in alkane is a covalent bond with a direct overlap of the electron clouds of the bonding atoms
• Alkanes have London forces (induced dipole-dipole interaction) as intermolecular forces because the bonds are nonpolar
• The boiling point of alkanes increases as the chain length increases due to more surface area and more induced dipole-dipole interactions, requiring more energy to overcome the attraction
• A branched molecule has a lower boiling point compared to an equivalent straight chain because it has fewer surface areas and hence fewer induced dipole-dipole interactions
• Alkanes are insoluble in water because the hydrogen bonds in water are stronger than the London forces of attraction in alkanes
• Alkanes are very unreactive
• Alkanes undergo combustion and reaction with halogens
• Combustion is an oxidation reaction
• Complete combustion occurs with a plentiful supply of air and produces carbon dioxide and water
• The bunsen burner flame during complete combustion is blue
• Incomplete combustion occurs in a limited supply of oxygen and produces water, carbon dioxide, and carbon monoxide
• An equation for the complete combustion of propane is C3H8 + 5O2 → 3CO2 + 4H2O
• Longer chain hydrocarbons are most likely to undergo incomplete combustion
• The environmental impact of carbon monoxide is that it is toxic/poisonous
• The environmental impact of soot (carbon) includes asthma, cancer, and global dimming
• Halogenoalkanes are formed from alkanes through radical substitution
• Alkanes react with halogens in the presence of UV light
• The three stages of free radical substitution are:

  • Initiation: breaking halogen bond to form free radicals
  • Propagation: chain part of the reaction where products are formed but free radical remains
  • Termination: free radicals removed, stable products formed
• Equations for the reaction of CH4 with Cl2 to form CH3Cl:

  • Initiation: Cl2 → 2Cl (radical) (in presence of UV light)
  • Propagation: Cl (radical) + CH4 → HCl + CH3 (radical)
  • CH3 + Cl2 → CH3Cl + Cl (radical)
  • Termination: CH3 (radical) + Cl (radical) → CH3Cl

  2Cl (radical) → Cl2

  • CH3 + CH3 (radical) → C2H6