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4.1.2 Alkanes

Alkanes – saturated hydrocarbons containing single C-C and C-H bonds as σ-bonds with free rotation of the σ-bonds.

Shape of Alkanes – tetrahedral with bond angle 109°. Electron pairs repel. lone pairs repel more than bonded pairs.

Boiling points of alkanes –

  • longer chained alkanes have higher melting points due to the induced dipole-dipole interactions (London forces) between the molecules.
  • Longer chained alkanes have more electrons and more contact area for london forces to occur on.
  • This means it requires more energy to break the higher number of London forces in longer chained alkanes giving them a higher melting point.
  • Branched chains have lower melting points because they have a smaller surface contact area for London forces.

Reactions of alkanes 

Low reactivity – High bond enthalpy and very low polarity of σ bonds means that the alkanes react much less readily with many reagents.

Alkanes as fuels

Complete combustion – C3H8 + 5 O2  3 CO2 + 4 H2O           produces CO2 and H2O

Incomplete combustion – 2 CH4 + 3 O2  2 CO + 4 H2O           produces CO and H2O

Carbon monoxide is poisonous because it binds to haemoglobin in the bloodstream which prevents oxygen from being able to bind. Therefore, you can get oxygen deprivation.

Free radical substitution

In the presence of UV light, halogens react with alkanes to form haloalkanes. There is an intiiation, propogation and termination step. It works the same way as the breakdown of ozone except ozone is replaces by an alkane

(http://slideplayer.com/slide/2491571/9/images/5/Free+radical+substitution+mechanism.jpg)

Limitations of free radical substitution

Synthesis of a mixture of organic products –

Multiple substitutions can occur on each molecule. The best way to reduce the chance of this is to have an excess of your organic reactant. Then there is more chance of the organic reactant and not your already substituted product being substituted.

Also, the substitution can occur on any part of the chain so you may get an isomer of the product you are after. The desired product would then have to be removed from the mixture. There is no way to prevent this from happening.

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