Alkanes

• Alkanes are saturated hydrocarbons containing single C-C and C-H bonds as σ-bonds that freely rotate.
• The shape and bond angle around a C atom in an alkane is tetrahedral due to 4 bonding pairs that equally repel with a bond angle of 109.5 o.
• The trend in boiling points of alkanes is that as the carbon chain length increases (or the branching becomes less), the boiling points increase.
• Alkanes are very unreactive due to high bond enthalpy and very low polarity of the σ-bonds.
• Complete combustion is the reaction of a fuel in excess oxygen to form CO2 and H2O only.
• Incomplete combustion is the reaction of fuels in a limited supply of oxygen.
• Incomplete combustion is less efficient, requiring less energy per mole of fuel burned.
• CO, carbon monoxide, is toxic.
• Substitution in a reaction between an alkane and a halogen is when a halogen atom takes the place of a H atom in the alkane.
• The general word equation for the reaction of an alkane with a halogen is: Alkane + halogen -> haloalkane + hydrogen halide.
• The balanced equation for the reaction of methane with chlorine is: CH4 + Cl2 -> CH3Cl + HCl.
• Substitutions can occur at different positions along the C chain in free radical substitution.
• The final step in a free radical substitution mechanism is to add two radicals together.
• An intermediate in a mechanism is a short lived reactive species formed and used up.
• Structural formula is better to use than molecular formula when dealing with substitution of alkanes as the halogen needs to be bonded to a specific place in the molecule.
• In the first step of propagation, the alkane and halogen radical are put as reactants and the hydrogen halide is put as a product.
• In the second step of propagation, the halogen is put as a reactant and the haloalkane and the halogen radical are put as products.
• The intermediate in a propagation can be identified by removing a H atom from the C in the alkane and putting a radical dot in its place.
• Ethane can undergo 6 times of substitution.
• The initiation of bromine can be represented by the equation Br 2 -- > 2Br.
• The three steps in the mechanism of free radical substitution are Initiation, propagation and termination.
• In the first step of free radical substitution, the halogen undergoes homolytic fission using UV light to form 2 halogen radicals.
• Further substitutions can occur in free radical substitution, meaning more than one halide can be substituted.
• The balancing numbers of the halogen and hydrogen halide match the number of times the alkane was substituted.
• The balanced equation for the reaction of butane with bromine to make 2,3-dibromobutane is: C4H10 + 2Br2 -> CH3CHBrCHBrCH3 + 2HBr.
• Alkanes can be substituted multiple times due to all the C-H bonds.
• Describe two limitations of free radical substitution:

  
  

  Further substitutions can occur. Substitutions occur at different positions along the C chain
• What happens in the first step of free radical substitution?

  The halogen undergoes homolytic fission using UV light to form two halogen radicals
• Write a balanced equation for the reaction of methane with chlorine

  CH4 + Cl2 ---> CH3Cl + HCl
• What is sigma bonding?


  Overlap of orbitals directly between the bonding atoms
• Describe alkanes

  Saturated hydrocarbons containing single C-C and C-H bonds as sigma bonds that freely rotate