Haloalkanes
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What mechanism is used to form haloalkanes
Free radical substitution
Conditions for radical substitution
UV light
What does free radical substitution produce
A haloalkane and a hydrogen halide
What are the 4 stages of free radical substitution?
Initiation 1st propagation 2nd propagation Termination
What is a free radical?
A very reactive species with an impaired molecule
Describe initiation
X-X bonds are weaker than C-H bonds so are broken first UV light causes covalent bond to break between halogen molecules, creating 2 halogen radicals X2 —> 2X•
Equation for CH4 in first propagation stage of free radical substitution
CH4+X• —> •CH3 + HX
Equation for CH4 in second propagation stage of free radical substitution
• CH3 + X2 —> CH3X + X•
There is a chain of first and second propagations. When does this chain end?
Once you run out of halogen molecules or alkanes
What is termination?
The remaining unstable species combining to make neutral molecules
What are the 3 possible termination steps?
X•+X• —> X2 • CH3 + X• —> CH3X • CH3 + •CH3 —> CH3CH3
What is the alkyl radical from CH4?
• CH3
What happens if an excess of the halogen is added to a free radical substitution?
Further substitution is possible, leading to the formation of CH2X2, CHX3, or CCl4
How can you reduce the chance of further substitution?
Using an excess of the alkanes
What do curly arrows represent?
The movement of an electron pair
Where must curly arrows always start?
At a bond or a lone electron pair
Define nucleophile
A species that donates a lone pair of electrons
Define Electrophile
Accepts a lone pair of electrons
What a carbon - halogen bonds?
Polar The halogen has a greater electronegativity so is delta negative The C is delta positive
Why is the C readily attacked by a nucleophile?
The C is delta positive
Describe the 3 steps of nucleophilic substitution
Lone pair of nucleophile forms a Covalent bond with delta positive C atom Electrons on C-halogen bond move onto halogen atom, breaking the C-X bond, and forms a halide ion
Order C- halogen bonds from strongest to weakest Comment on the reactivity
C-F C-Cl C- Br C- I Reactivity increases as you go down the group
Give reagents and conditions for nucleophilic addition to form alcohols
Reagents: NaOH Conditions: warm, aqueous
Are haloalkanes polar?
Yes