12 - Alkanes

• What are the main components of natural gas and crude oil?


  Alkanes
• Why has crude oil deposits been able to remain in the earth for many millions of years?


  The lack of reactivity of alkanes, which are amongst the most stable organic compounds.
• What are alkanes mainly used as?


  They are mainly used as fuels, exploiting their reaction with oxygen to generate heat.
• What is the general formula of alkanes?


  CnH₂n+₂
• Are alkanes saturated or unsaturated compounds?


  Saturated
• Alkanes


  Saturated compounds containing only carbon and hydrogen atoms joined together by single covalent bonds.
• What type of covalent bonds are in alkanes?


  Sigma bonds (σ-bonds)
• How many electron pairs surround a carbon atom in an alkane?

  4 electron pairs in 4 σ-bonds.
• What shape and bond angles are in an alkane and why?


  Repulsion between the electron pairs result in a 3D tetrahedral arrangement around each carbon atom. Each bond angle is approximately 109.5 degrees.
• What is the rotation around bonds in alkanes like and why?


  The σ-bonds act as axes around which atoms can rotate freely, so these shapes are not rigid.
• By what process do oil refineries separate crude oil into fractions?

  Fractional distillation in a distillation tower.
• What is the trend of boiling points as alkane chain length increases?


  As chain length increases, boiling point increases.
• What are London forces?


  Weak intermolecular forces that exist between all molecules.
• What is the trend of boiling points of isomers of alkanes with more branches?


  The more branches, the lower the boiling point.
• Why do boiling points increase as chain length increases in alkanes?


  London forces act between molecules that are in close surface contact. As chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules. The London forces between molecules will be greater and so more energy is required to overcome these forces.
• Why do isomers of alkanes with more branches have lower boiling points?


  There are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces. Another factor lies with the shape of the molecules. The branches get in the way and prevent the branched molecules getting as close together as straight chain molecules, decreasing the intermolecular forces further.

  
• Why are alkanes unreactive?


  1. C-C and C-H sigma bonds are strong

  2. C-C and C-H bonds are non-polar
• What do alkanes react with despite their lack of reactivity?

  Plentiful supply of oxygen
• What do all combustion reactions give out?


  Heat
• What is the reaction for complete combustion of alkanes?


  Alkane + excess oxygen → carbon dioxide + water
• How do we balance combustion reaction equations?


  Add the number of carbons in the alkane, in front of the carbon dioxide produced and then balance the rest. Fractions can be used when balancing organic equations?

  
• Why does incomplete combustion occur?


  Limited supply of oxygen
• What are the reactions of incomplete combustion of alkanes?


  1. alkane + oxygen → carbon monoxide + water

  
  

  2. alkane + oxygen → carbon + water
• What happens in incomplete combustion?


  When oxygen is limited, 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 or ever carbon itself as soot.
• How do alkanes react with halogens?


  In the presence of sunlight, alkanes react with halogens. The high energy UV radiation present in sunlight provides the initial energy for a reaction to take place.
• What type of reaction is halogenation of alkanes?


  Substitution as one of the hydrogen atoms in the alkane is replaced by one of the halogen atoms.
• What type of reaction mechanism is halogenation of alkanes?


  Radical substitution
• What are the three stages in the reaction mechanism of halogenation of alkanes?


  1. Initiation

  2. Propagation

  3. Termination
• What happens in the initiation stage of halogenation of alkanes?

  
  

  Covalent bond of the diatomic halogen is broken by homolytic fission, forming two reaction halogen radicals.

  Br-Br → Br∙ + Br∙
• What happens in the propagation stage of halogenation of alkanes?


  
  

  Two propagation steps:

  
  

  1. Halogen radical reacts with a C-H bond in the alkane, forming an alkyl radical, and a molecule of the hydrogen halide.

  I.e. CH₄ + Br∙ → ∙CH₃ + HBr

  
  

  2. Each alkyl radical reacts with another halogen molecule, forming the haloalkane, and a new halogen radical.

  
  

  I.e. ∙CH₃ + Br₂ → CH₃Br + Br∙

  
  

  This is a chain reaction.
• What happens in the termination stage of halogenation of alkanes?


  3 termination steps:

  
  

  Two halogen radicals react to form halogen molecule.

  Br∙ + Br∙ → Br₂

  
  

  Two alkyl radical react to form an alkane.

  ∙CH₃ + ∙CH₃ → C₂H₆

  
  

  Alkyl radical and halogen radical react to form a haloalkane.

  ∙CH₃ + Br∙ → CH₃Br
• How does further substitution occur in halogenation of alkanes?


  In propagation step 2 a haloalkane, e.g. CH₃Br, is formed. Another halogen radical can collide with it, substituting another hydrogen atom to form dihaloalkane, e.g. CH₂Br₂. Further substitution can continue until all hydrogen atoms have been substituted.

  
  

  The result is a mixture of CH₃Br, CH₂Br₂, CHBr₃, and CBr₄.