chapter 12 (alkanes) Flashcards
what is the shape of alkanes ?
Each carbon atom has four valence electrons and will form four sigma bonds in an alkane
Therefore there are no lone pairs present
The shape will be tetrahedral around each carbon atom
bond angle of 109.5 degrees
what affects the boiling point of alkanes ?
chain length:
- tempory induced dipole-dipole interactions are small for alkanes with short carbon chains
- boiling point increases with chain length size
Branching:
- branched alaknes exhibit lower boiling points than unbranched alkanes with the same number of carbons
- it is more difficult for short and bulky molecules to lie close together, meaning branched alkanes have weaker london forces due to less surace area
what is free radical substitution ?
- alkanes undergo free-radical substitution in which a H atom gets substituted by a Cl or Br
what happens in the intiation step ?
of free radical substitution
- halogen double bond is broken by UV light
- this produces 2 radicals in a homolytic fission reaction
what happens in the propagation step?
of free radical substitution
- free radicals are very reactrive and will attack unreactive alkanes
- A C-H bond breaks homolytically
- An alkyl radical is produced
- the alkyl radical can attack another halogen to form a halogenalkane, this regenerates a halogen free radical
- the free raducal can repeat the cycle
what happens in the termination step?
of free radical substitution
- the termination step is when the chain reaction terminates due to 2 free radicals reacting together
- this forms an uncreactive molecule
what are some limitations of free radical substitution ?
Impurities:
- there are a nuumber of possible products in the termination step, meaning if you are trying to form a specific molecule, the formation of other molecules are impurities
further substitution:
- if there is an excess of halogen, when reacted with methane under U-V light can cause further substitution
