Chemical reactions of alkanes

• Alkanes do not react with most common reagents
• The reasons for their lack of reactivity:

  1. C-C and C-H sigma bonds are strong
  2. C-C bonds are non-polar
  3. the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar
• Despite low reactivity, all alkanes react with a plentiful supply of oxygen to produce carbon dioxide and water - combustion
• All combustion processes give out heat, and alkanes are used as fuels because they are readily available, easy to transport and burn in a plentiful supply of oxygen without releasing toxin products
• methane is a greenhouse gas with a global warming potential of 22 (22x the warming effect of carbon dioxide in the atmosphere).
• vast quantities of methane are stored in ice-saturated arctic tundra of siberia and northern canada. Global warming is very likely to thaw the tundra, releasing this methane into the atmosphere and greatly increasing global warming
• The methane locked in the frozen earth of the tundra was formed by decomposition of organic material in the absence of oxygen - same chemistry lies behind production of biogas, methane made as a fuel from organic waste or plant material
• In a plentiful supply of oxygen, alkanes burn completely to produce carbon dioxide and water
• Equation for balancing the complete combustion of any alkane:

  CxHy + (x+y/4)O2 = xCO2 + y/2 H2O
• In a limited supply of oxygen, there is not enough oxygen for complete combustion
• As you descend the homologous group each alkane molecule (-CH2) needs an extra 1 and 1/2 O2 molecules for complete combustion
• When oxygen is limited during combustion, the hydrogen atoms in the alkane are always oxidised to water, but combustion of the carbon may be incomplete, forming the toxic gas carbon monoxide, CO or even carbon itself as soot
• complete combustion: CO2 is formed

  C7H16 (l) + 11O2 (g) = 7CO2 (g) + 8H2O (l)
• incomplete combustion: CO is formed:

  C7H16 (l) + 7 1/2 O2 (g) = 7CO (g) + 8H2O (l)
• incomplete combustion: C is formed:

  C7H16 (l) + 4O2 (g) = 7C (s) + 8H2O (l)
• Carbon monoxide is a colourless, odourless and highly toxic gas. It combines irreversibly with haemoglobin in red blood cells to form a cherry-pink compound called carboxyhaemoglobin - prevents haemoglobin from transporting oxygen round the body
• Severe CO poisoning will turn victims lips this bright colour
• In the presence of sunlight, alkanes react with halogens. The high-energy ultraviolet present in sunlight provides the initial energy for a reaction to take place - substitution reaction
• A chemical reaction can often be represented as a series of steps - reaction mechanism -showing how electrons are thought to move during the reaction
• The reaction mechanism for the bromination of methane is radical substitution
• Free radical substitution takes place in 3 stages:

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

  in the initiation stage, the reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission - each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals.

  The energy for this bond fission is provided by UV radiation
• A radical is a very reactive species with an unpaired electron
• Step 2: Propagation:

  in the propagation stage, the reaction propagates through 2 propagation steps (a chain reaction)

  1. a bromine radical reacts with a C-H bond in the methane, forming a methyl radical and a molecule of hydrogen bromide
  2. each methyl radical reacts with another bromine molecule, forming the organic product bromomethane, CH3Br and a new bromine radical
• The new bromine radical then reacts with another CH4 molecule as in the first propagation step, and the two steps continue to cycle through in a chain reaction
• In theory, the propagation steps could continue until all the reactants have been used up. In practice propagation is terminated whenever two radicals collide
• Step 3: Termination:

  1. 2 radicals collide, forming a molecule with all electrons paired
  2. number of termination steps with different radicals in the reaction mixture
• When 2 radicals collide, both radicals are removed from the reaction mixture, stopping the reaction
• Limitations of radical sub in organic synth:

  although radical sub gives us a way of making haloalkanes this reaction has problems that limit its importance for synthesis of just one organic compound
• Further substitution: e.g. if bromomethane is formed in the second propagation step, another bromine radical can collide with it, substituting a further H atom to form dibromoethane. Further subs can continue until all H atoms have been substituted
• Substitution at different positions in a carbon chain:

  • for methane, all 4 hydrogen atoms are bonded to the same carbon atom, so only one monobromo compound, CH3Br is possible
  • with ethane, similarly, only one monosubstituted product, C2H5Br is possible
  • if the carbon chain is longer, we will get a mixture of monosubstituted isomers by substitution at different positions in the carbon chain
  • e.g. pentane could form three monosubstituted isomers
  • with further sub - even more possibilities