Alkanes

• conditions for initiation


  UV light
• complete combustion


  when there is an excess of oxygen available
• complete combustion equation with alkane


  alkane + oxygen -> Carbon dioxide + water
• Incomplete combustion


  not enough oxygen available
• incomplete combustion equations with alkanes:

  • Alkane + oxygen -> carbon monoxide + water
  • Alkane + oxygen -> Carbon (soot) + water
• Why is carbon monoxide dangerous?

  it binds to haemoglobin in blood reducing the ability of the blood to carry oxygen
• Why do straight chain alkanes have higher boiling points than branched chain alkanes?

  the molecules have a larger surface ares so more surface contact is possible between the molecules, the london forces are stronger because there are more of them and more energy is needed to break these forces.
• Alkanes have induced dipole/ London forces between molecules. The molecules. Are non polar as the difference in electro negativity of C and H is very small. They do not have hydrogen bonding as the H atoms are not bonded to O,F or N. The London forces arise because of the movement of electrons which causes the formation of temporary dipoles.
• 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 sigma bond in alkane is a covalent bond with a direct overlap of the electron clouds of the bonding atoms
• The shape and angle of an alkane is tetrahedral with an angle of 109.5°
• Alkanes have London forces as intermolecular forces due to their nonpolar nature
• The boiling point of alkanes increases as the chain length increases because of more surface area and induced dipole-dipole interactions
• A branched molecule has a lower boiling point compared to an equivalent straight chain due to fewer surface areas and less induced dipole-dipole interactions
• Alkanes are insoluble in water because the hydrogen bonds in water are stronger than alkanes' London forces of attraction
• 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 in alkanes occurs in a limited supply of oxygen and produces water, carbon dioxide, and carbon monoxide
• The complete combustion equation of propane is C3H8 + 5O2 → 3CO2 + 4H2O
• Longer chain hydrocarbons are most likely to undergo incomplete combustion
• Carbon monoxide is toxic/poisonous
• Soot (carbon) can lead to 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. (in presence of UV light)
  • Propagation: Cl. + CH4 → HCl + .CH3
  • CH3 + Cl2 → CH3Cl + Cl.
  • Termination: .CH3 + Cl. → CH3Cl

  2Cl. → Cl2

  • CH3 + .CH3 → CH3CH3