Alkanes Flashcards
(12 cards)
What is the bonding and shape of alkanes?
They are saturated hydrocarbons with general formula CnH2n+2.
Each C atom bonded to 4 others by single covalent bonds called a sigma bond.
Tetrahedral shape as has 4 bonding pairs and 0 lone with bond angle 109.5’
What are 3 types of alkanes?
- Straight chained (aliphatic) alkanes (e.g. butane)
- Branched chained (aliphatic) alkanes (e.g. 2-methylpropane)
- Ring or Cycloalkanes (alicyclic) (e.g. cyclobutane)
What are the boiling points of alkanes? And how is it effected?
Simple covalent molecules with weak induced dipole dipole forces between molecules.
- Effect of chain length: Longer chains, More points of contact between molecules, stronger induced dipole dipole forces so higher BP as more energy required to break.
- Effect of branching: More branching, less points of contact between molecules, weaker induced dipole dipole forcs, lower BP as less energy required to break.
Why are some alkanes unreactive?
Strong single covalent bonds between atoms
No polar bonds
Whats the combustion of alkanes produce?
With plentiful oxygen (complete combustion) - burn completely to produce CO2 and H2O
With limited oxygen (incomplete combustion) - burn to form CO (carbon monoxide) and H2O.
What are substitution reactions of alkanes?
Alkanes react with halogens in presence of ultraviolet radiation and at temp of 300’C.
For reaction to occur between alkanes and halogens, covalent bond must be broken homolytically to form free radicals.
Halogen rects with alkanes by substitution reaction where hydrogen atom in alkanes substituted by halogen atom (follows a radical substitution mechanism).
Alkane + Halogen —> Haloalkane + hydrogen halide
What is radical?
A species within an unpaired electron.
What is radical substitution mechanism? And what is the overall equation for it?
A reaction in which a species within an unpaired electron replaces an atom or group of atoms in a molecule.
Alkane + halogen —> haloalkane + hydrogen halide
What happens in step 1 of the radical substitution mechanism?
Initiation:
Halogen bond in halogen molecule broken by homolytic fission forming 2 halogen radicals. UV light/300’C provides energy for bond fission.
E.g.
Cl2 —> 2Cl’
What happens in step 2 of radical substitution mechanism?
Propagation: involves 2 steps.
1st step- alkane react with halogens radical forming alkane radical and hydrogen halide.
E.g. CH4 + Cl’ —> CH3’ + HCl
2nd step- alkane radical reacts with halogens molecule forming haloalkane and halogen radical.
E.g. CH3’ + Cl2 —> CH3Cl + Cl’
What happens in step 3 of radical substitution mechanism?
Termination:
When 2 radicals combine forming a new molecule. All radicals are removed.
Possible steps are -
- 2 halogen radicals ( e.g. 2Cl’ — > Cl2)
- 2 alkane radicals (e.g. 2CH3’ —> C2H6)
- 1 alkane radical and 1 halogen radical (e.g. CH3’ + Cl’ —> CH3Cl)
What are the problems with radical substitution reactions?
Lead to formation of a mixture of organic products. So once a new product formed it can undergo further reactions with halogens radical and different radicals may collide forming new products.
Halogen halide may react with further halogen radicals until all hydrogen atoms have been replaced resulting in a mixture of different structural isomers depending on which hydrogen atom in chain has been replaced by a halogen.
(E.g. Chloro methane react with chlorine radicals form mixture of CH3Cl, CH2Cl2, CHCl3 and CCl4)
