alkanes & haloalkanes

• complete combustion of hydrocarbons produces carbon dioxide and water
• incomplete combustion in a limited supply of combustion produces carbon monoxide or carbon and water
• water is always produced in incomplete combustion reactions as it is always assumed that there is enough oxygen to oxidise hydrogen
• 2,2,4-trimethylpentane is described as the perfect fuel as it has an octane rating of 100
• the more branched the hydrocarbon the slower and more controlled the burn
• alkanes are unreactive compounds because their bonds are non-polar electronegativities of carbon and hydrogen are very similar
• other reagents are not attracted to alkanes as the chain is non-polar
• a reaction can occur between alkanes and halogens in the presence of ultra violet light which is a photochemical reaction
• alkanes react with halogens in the presence of uv light and undergo a free radical substitution reaction
• 3 stages of free radical substitution reaction:

  1. initiation
  2. propagation
  3. termination
• initiation:

  • chlorine molecule -> 2 chlorine free radicals
  • brings about homolytic fission as the covalent bond breaks evenly (each atom gets one electron)
• a free radical is an atom or a molecule that contains an unpaired electron
• propagation:

  • use up a free radical and also generate a free radical
  • Cl• + CH4 -> HCl + CH3•
  • the H atom is the first thing the free radical bumps into and it pulls off an H
• must always draw out 2 steps of propagation reactions
• termination:

  • 2 free radicals combine to form a stable molecule
  • Cl• +CH3• -> CH3Cl
  • can be any free radicals from any propagation reaction
• when a CFC molecule absorbs UV light a carbon-chlorine bond breaks and one of the electrons goes with the released chlorine atom while the other stays with the remainder of the CFC molecule
• a small number of chlorine radicals can destroy lots of ozone molecules
• initiation step of ozone depletion:

  • CCl3F -> •CCl2F + Cl•
• propagation step of ozone depletion:

  • Cl• + O3 -> •ClO + O2
  • •ClO + O3 -> •Cl + 2O2
• overall reaction of ozone depletion:

  • 2O3 -> 3O2
• nitric oxide is another ozone depleting free radicals which originates from contrails
• step 1 of nitric oxide ozone depletion:

  • O3 + NO -> NO2 + O2
• step 2 nitric oxide ozone depletion:

  • NO2 + O -> NO + O2
• overall reaction for nitric oxide ozone depletion:

  • O3 + O -> 2O2
• volatile liquids that do not mix with water because they cannot form hydrogen bonds with water as they can only form induced dipole-dipole forces
• primary structure of a haloalkane is when a halogen is attached to a carbon that is attached to 1 other carbon
• secondary structure of a haloalkane is when a halogen is attached to a carbon that is attached to 2 other carbons
• tertiary structure of a haloalkane is when a halogen is attached to a carbon that is attached to 3 other carbons
• boiling points of haloalkanes increases from Cl to I because I has more electrons which means it has stronger induced dipole forces which require a larger amount of energy to overcome
• in the hydrolysis of haloalkanes the C-I bond substitutes faster than the C-Br or C-Cl bond as it is the weakest, because it is the longest bond due to the large atomic radius of the iodine atom
• boiling points of alkanes increase as the length of the carbon chain increases. this is because the longer chains have more surface area contact which increases the number of induced dipole forces so more energy is required to overcome these forces
• the more branched a compound is the fewer surface area interactions that are between molecules this is because the molecules cannot fit together neatly. branched molecules have weaker induced dipole forces and therefore a low boiling point
• incomplete combustion can produce carbon monoxide and carbon, carbon monoxide and carbon dioxide or a mix of all three products and water is always produced
• a sigma bond is formed between two carbon atoms the direct overlap of the electron clouds of the two atoms
• a pi bond is formed by the electrons in the adjacent p-orbitals overlapping above and below the carbon atoms, it can only be made after a sigma bond has formed
• a pi bond holds the atoms in position by restricting rotation around the double bond
• the pi bond is the reactive part of the molecule as it has a high electron density around it
• stereoisomers have the same structural formula but different arrangements of atoms in space
• cis trans isomerism is a type of E/Z isomerism where two substituent groups attached to each carbon atom in the double bond are the same
• an electrophile is an electron pair acceptor