Introduction to Alkanes (OCR AS Chemistry)

• 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 C_nH_2n+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

• 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

Tetrahedral 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

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