What are some properties/features of alkanes?
- naturally occuring hydrocarbons formed from crude oil
- very unreactive - has non polar bonds
- they burn in oxygen in combustion reactions & produce halogenoalkanes in free radical substitution reactions
some of this is not needed just helpful to know!
What kind of bonds make up alkanes?
sigma bonds
They form a tetrahedral structure
What is a sigma bond?
A bond formed by the DIRECT overlap of orbitals between two bonding atoms
sigma bonds are lengthways
Why are alkanes unreactive?
Due to the high bond enthalpy - means the bond is strong & stable
a lot of energy is required to break the bonds
What affect does increasing no. of carbon atoms have on boiling points?
As the no. of carbon atoms increases boiling point increases - more e- so stronger london forces between molecules
How does branching affect the boiling point of alkanes?
As branching increases the boiling point decreases - less contact between the molecules, weaker london forces
What is the general equation for combustion?
- Complete - Alkane + oxygen > carbon dioxide + water
- Incomplete - Alkane + oxygen > carbon monoxide/carbon + water
May be asked to use combustion equation with moles/data to deduce empirical formula of a hydrocarbon
What are the issues with carbon monoxide?
Bonding between carbon monoxide is strong and irreversible so normal cell respiration is blocked - leads to death
Which types of fission can break a covalent bond?
- Homolytic fission
- Heterolytic fission
What is homolytic fission?
Each bonding atom receives one electron from the covalent bond, forming two radicals
What is a radical/free radical?
A species with an unpaired electron
What is free radical substitution?
When alkanes react with halogens in the presence of UV light to form halogenoalkanes and a hydrogen halide
What are the stages of free radical substitution reaction?
- Initiation - 2 halogen radicals
- 1st Propagation - halogen radical reacts with alkane, alkyl radical forms
- 2nd Propagation - alkyl radical forms with halogen
- Termination - 2 radicals react to form a molecule
after 2nd propagation a chain reaction happens & 1st propagation repeats
Why is there typically a low yield with free radical substitution?
- Further substitution can occur forming a mixture of products
- Different termination steps can take place forming a mixture of different products
- Substitution can occur in different positions creating position isomers
