Chapter 12 - Alkanes Flashcards
Why are alkanes so unreactive?
Because they don’t have a functional group, which are areas of an organic compound that are susceptible to reactivity or attack.
As a result, all covalent sigma bonding between C-C and C-H are all NON-POLAR.
Also, the C-C and C-H sigma bonds are strong.
Why are organic compounds with functional groups susceptible to reactions?
The bond between Carbon and the 1st atom of the functional group is polar (because of a change of Electronegativity values). Therefore, they are a lot easier to break.
Alkanes can participate in what
Combustion and halogenation.
When can alkanes undergo complete combustion
Only if there is a sufficient supply of oxygen. As a result, carbon dioxide, water and heat and light are produced.
What is the pattern for complete combustion
For each CH2 added, you need 1 and a half supply of oxygen.
When can alkanes undergo incomplete combustion
when there is a limited supply of oxygen. it produces: carbon monoxide, water and carbon and heat.
why would there be a limited supply of oxygen
The reaction is happening in a closed space or a badly ventilated space.
How are alkanes extracted from crude oil?
By fractional distillation and at oil refineries.
- The crude oil is heated.
- A temperature gradient is established within the column.
- Hydrocarbon vapours evaporate and rise up the column.
- The vapours reach a height in the column where the temperature is less than its boiling point.
- Vapours cool and condense into a liquid and are collected in separate tanks.
As branching increases, how does this affect the molecule’s boiling point?
Increased branching means there are less contact points for London forces to act upon. The total strength of attraction between alkane molecules decreases. Hence, the amount of energy needed to break the forces also decreases.
As you increase the number of carbon atoms, how does this affect the boiling point?
If you increase the number of carbon atoms, you increase the number of contact points for London forces to act upon and therefore the strength of attraction between alkane molecules also increases. Therefore, the amount of energy and heat needed to break these forces also increases.
What is a halogenation reaction
It can undergo these reactions in the presence of UV radiation because it can give it the energy to start the reaction. It happens through radical substitution.
Radical substitution stages
I : Initiation
P: Propagation
T : Termination
Explain the difference between a halogen and a halide
A halogen is a molecule or a element from group 7, but a halide is an ion of a molecule from group 7.
Explain initiation
Halogen molecule absorbs the UV radiation (you need to write this above the arrow).
Halogen molecule undergoes homolytic fission, to produce 2 highly reactive halogen radicals.
Explain propagation
This is a 2 step chain reaction.
1. Halogen radical attacks Alkane H atom.
Producing an alkane radical and a hydrogen halide compound.
2. Alkane radical attacks another halogen molecule, producing a haloalkane compound and a new halogen radical.
