2. Alkanes
For each homologous series, we will focus mainly on:
1. Physical properties (trend)
2. Chemical properties (reactions and mechanism)
3. Chemical Properties
- Alkanes are chemically inert since:
1. all the carbons are saturated (have four bonds)
2. there are no electron rich/ poor regions due to the
similar electronegativities of C and H.
- Alkanes can only undergo reactions under high temp
(combustion) or with very reactive reagent (radicals)
4. Chemical Properties
- The mechanism for the substitution of alkanes to give
alkyl halides is free radical substitution
- Not ‘substitution’, ‘free radical reaction’ or ‘free
substitution’ or ‘FRS’
5. Writing
Free Radical Mechanism
When writing mechanisms:
1. It is a good practice to write the name of the
mechanism whether or not the question asks for it.
2. You just need to show the series of equations/ arrows.
There is no need for descriptions i.e. the chlorine
molecule undergoes homolytic cleavage to give two
chlorine atoms/ radicals.
9. Writing
Free Radical Mechanism
Error 4: Wrong halogen.
Under exam conditions, students sometimes use the wrong
halogen (i.e. use Cl2 when qns asks for Br2)
This happens when they regurgitate what they have
memorised blindly
10. Writing
Free Radical Mechanism
Error 4: Wrong halogen.
Under exam conditions, students sometimes use the wrong
halogen (i.e. use Cl2 when qns asks for Br2)
This happens when they regurgitate what they have
memorised blindly
