Alkanes
- Alkanes are a homologous series made up of saturated hydrocarbons, containing only carbon and hydrogen atoms joined by sigma bonds
- Carbon atoms have four electrons in their outer shell. Therefore, each carbon atom can form four covalent bonds, resulting in tetrahedral geometry
- Alkanes are not polar and only weak London forces of attraction occur, as carbon and hydrogen have similar electronegativities.
- Boiling point of alkanes increases with chain length, as there is a greater surface area and number of electrons for stronger London forces.
- Shorter carbon chain alkanes and more branched alkanes have a smaller surface area for contact between molecules and weaker London forces, resulting in lower boiling points
- C-C and C-H σ bonds are strong and non-polar therefore alkanes are not reactive. The electronegativities of C and H are so similar that the C-H bonds are considered non-polar
Combustion of Alkanes
- Alkanes can be used as a fuel source
- In complete combustion, the alkane burns with a clean blue flame. Water vapour and carbon dioxide are formed (greenhouse gases). E.g. C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)
- In incomplete combustion, the alkane burns with a dirty yellow flame. It can produce carbon, carbon monoxide and unburned hydrocarbons as products.
- CH4 (g) + O2 (g) → C (s) + 2H2O (g)
- CH4 (g) + 1.5O2 (g) → CO (g) + 2H2O (g)
Chlorination of Alkanes
- Alkanes react with the halogens, specifically chlorine and bromine, in the presence of UV light to form haloalkanes
- Methane reacts with chlorine to form chloromethane and hydrogen chloride:
- CH4 + Cl2 → CH3Cl + HCl
- This reaction is a free radical substitution with the steps:
- Initiation- free radicals are formed when exposed to UV
- Cl2 → 2Cl•
- Propagation- free radicals are used up and created in a chain reaction
- Cl• + CH4 → •CH3 + HCl
- •CH3 + Cl2 → CH3Cl + Cl•
- Initiation- free radicals are formed when exposed to UV
- Termination- free radicals are removed.
- 2Cl• → Cl2
- 2•CH3 → C2H6
- •CH3 + Cl• → CH3Cl
- Radical substitution reactions have limitations:
- Further substitution could occur if another chloride radical collides with the product of propagation
- If the carbon chain is longer, a mixture of products forms as substitution occurs at different positions along the chain