10.1 Reactions of Alkanes
Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds. There are no partial charges on alkane molecules that might initiate reactions. The effect is that alkanes only undergo very few reactions.
(1) Combustion of alkanes
Alkanes can undergo combustion, producing CO2(g) and H2O(g)
When asked to create a combustion equation for a particular fuel, do the following steps:
- Write the fuel’s molecular formula
- Add excess O2(g)
- Produce CO2(g) and H2O(g)
- Balance C, H and O in that order.
- REMEMBER TO INCLUDE ALL THE STATES!
General formula: alkane + O2(g) → CO2(g) + H2O(g)
Example: C6H14(l) + 9½O2(g) → 6CO2(g) + 7H2O(g) (halves are okay!)
(2) Substitution of alkanes
Alkanes can also undergo substitution, in which one of the hydrogen atoms is replaced with a halogen (e.g. F, Cl, Br, or I).
General formula: alkane + X2 → chloroalkane
Example: CH3CH3(g) + Cl2(g) + UV light → CH3CH2Cl(g) + HCl(g) (note that HCl is a gas!)
10.2 Reactions of alkenes
(1) Addition of alkenes
Alkenes can under addition reactions with halogens, hydrogen gas or water.
The first reaction happens at room temperature. If you have a gaseous alkene like ethene, you can bubble it through either pure liquid bromine or a solution of bromine in an organic solvent like tetrachloromethane. The reddish-brown bromine is decolourised as it reacts with the alkene.
(2) Addition polymerisation of alkenes
Chemguide is an excellent revision resource that goes a little further than VCE. Read the relevant Chemguide pages below.
10.3 Oxidising ethanol to ethanoic acid
You will need to memorise the following ways to oxidise an alkanol into a carboxylic acid.
For more information, visit this Chemguide page.
10.4: Making Esters
10.5: Organic Reaction Pathways
10.6: Fractional distillation
Fractional distillation can be used to separate compounds with different boiling points. It is commonly used in the separation of the compounds contained within crude oil.
When hydrochloric acid is added to propene, two products can be produced: 1-chloropropane and 2-chloropropane. Only the 1-chloropropane can be made into a carboxylic acid. We must therefore separate the 1-chloropropane from the 2-chloropropane by fractional distillation.
When reacting alkenes with 3 or more carbons (such as propene) with hydrochloric acid, we must write “HCl and fractional distillation” on the arrow.
Read: Heinemann Chemistry 2 Chapter 10