OCR A Level Chemistry

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4.2.1 Introduction to Alkanes

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Bonding in Alkanes

  • Alkanes are a homologous series made up of saturated hydrocarbons (containing only hydrogen and carbon) which are joined by sigma (σ) bonds 
  • They have a general formula of CnH2n+2
  • The electronegativities of carbon and hydrogen are very similar so the C-H sigma (σ) bonds are non-polar
  • As a result of this, alkanes are nonpolar molecules and have no partial positive or negative charges (δ+ and δ- respectively)
    • Alkanes therefore do not react with polar reagents
      • They have no electron-deficient areas to attract nucleophiles
      • And also lack electron-rich areas to attract electrophiles
  • Given that each carbon has four valance electrons, each carbon atom forms four covalent bonds 
  • This results in a tetrahedral bonding arrangement with a bond angle of 109.5 o
  • Alkanes are non-polar and therefore only exhibit weak temporary induced dipole-dipole forces between the molecules
  • As the bond length of the alkane molecule increases the boiling point also increases 

Homologous Series of Alkanes

Shape of Alkanes

  • Each carbon atom has four valence electrons and will form four sigma bonds in an alkane
    • Therefore there are no lone pairs present
    • The shape will be tetrahedral around each carbon atom

4-2-1-tetrahedral-alkaneTetrahedral shape of methane

  • Remember different types of electron pairs have different repulsive forces
    • Lone pairs of electrons have stronger repulsive forces than bonding pairs 
    • The order of repulsion is therefore: lone pair – lone pair > lone pair – bond pair > bond pair – bond pair
  • As alkanes only have bonding pairs of electrons there is equal repulsion between all four bonding electron pairs

Boiling Points of Alkanes

The boiling point of alkanes varies with two factors

  • Chain length
  • Branching

Chain length 

  • Temporary induced dipole-dipole forces will be very small for an alkane such as methane but will increase as the size of the molecules increase
  • Therefore, the boiling points of the alkanes increases with the molecular size, due to the increased temporary induced dipole-dipole forces

Branching

  • Branched alkanes normally exhibit lower boiling points than unbranched alkanes containing the same number of carbon atoms
  • The more branched the chain, the lower the boiling point tends to be
  • Temporary induced dipole-dipole forces can only operate over very short distances between one molecule and its neighbouring molecules
  • It is more difficult for short, bulky molecules to lie close together (compact) compared with long, thin molecules
  • The unbranched alkanes have greater van der Waals’ forces of attraction because of their greater surface areas

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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.