Haloalkanes Flashcards
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
Give reagents and conditions for nucleophilic addition to form nitriles
How does this affect the C chain
KCN
Aqueous ethanol
(CN- adds an extra carbon to the chain)
Name CH3CH(CN)CH3
2-methyl propane nitrile
Give reagents and conditions for nucleophilic addition to form primary amines
Excess NH3
Why is excess NH3 used to form primary amines
To avoid further substitution
How can RNH2 act as a nucleophile?
There is a lone pair on N
How to get a high yield of quaternary ammonium salt
Use a large excess of haloalkane
What are quaternary ammonium salts used for?
Hair products
What is the basic structure of a quaternary ammonium salt?
A N+ surrounded by 4 carbon chains
3 rules for forming quaternary ammonium salts
Any C chains on the N at the start of he reaction will remain on the N
All Hs directly bonded to the N are removed
A halide ion from the haloalkane will also be produced
Give the reagents and conditions for elimination
NaOH
hot, in ethanol
What is the role of NaOH or OH- ions in elimination?
As a base (they accept H+)
What are the 3 stages of elimination?
1: OH- removes a H atom from an adjacent C to the halogen atom
2: the e- in the C-H bond move into the C-C bond to create a C=C bond
3: e- in the C-halogen bond move onto the halogen atom, breaking the bond and creating a halide ion
When naming an alkene what must you always check for?
E-Z isomerism
If the alkene product of elimination shows geometrical isomerism, which isomers will be formed?
Both E and Z
Do primary haloalkanes favour elimination or substitution?
Substitution
Do secondary haloalkanes favour elimination or substitution?
Both
Do tertiary haloalkanes favour elimination or substitution?
Elimination
How does the strength of the base affect the chances of elimination?
As the strength of the base increases the chances of elimination increase
What is a CFC
A chlorofluorocarbon
Do CFCs usually contain Hydrogen
No
What state are shirt chain CFCs usually in?
What are long chain ones used for?
Gaseous, used in refrigerators
Dry cleaning and degreasing solvents
Why is O3 important?
Absorbs UV radiation which causes cancer
Why are CFCs bad?
In UV light Cl• are produced which decompose O3
What is the overall equation to show ozone break down?
2O3 —> 3O2
Why can we say Cl• is a catalyst?
It is not destroyed and is regenerated
Give the 2 propagation steps of Cl• decomposing O3
Cl• + O3 —> ClO• + O2
ClO• + O3 —> 2O2 + Cl•
Why are CFC alternatives safer?
They do not contain Cl atoms so no Cl• can be produced
Or
They will break down before they reach the ozone layer
Why is the ozone layer important
It absorbs harmful UV radiation which can cause cancer by damaging DNA in skins cells
Why can we say Cl• is a catalyst
It is regenerated
