15.1 - 15.2 Flashcards
What are halogenoalkanes
- alkanes with 1 or more halogen attached to it
What do all haloalkanes have and why
- a polar bond as they have halogen attached to them
What are haloalkanes attacked by
- nucleophiles
Why do haloalkanes have polar bonds
- as halogens are more electronegative than carbon so they pull electrons towards themselves in a covalent bond
What is a nucleophile
- a electron pair donor that forms a new covalent bond
What are 4 examples of nucleophiles
1) NH3 (ammonia)
2) CN- (cyanide ions)
3) OH- (hydroxide ions)
4) H2O (water)
What do curly arrows show
- the movement of electron pairs
Where does the arrow start from and go to with nucleophiles
- starts from the lone pair on the nucleophile
- goes to the carbon atom
What reaction do haloalkanes undergo
- nucleophilic substitution
How do haloalkanes form alcohols
- nucleophilic substitution
REAGENT: warm NaOH - carried out under REFLUX
In nucleophilic substitution the what type of bond fission happens when the bond between the carbon and the halogen break
- heterolytic bond fission as both electron move to form the bond to the halogen
Is water a weak or strong nucleophile, and what affect does this have not he reaction
- weak so it is slower
What is the reactivity of haloalkanes
- they become more reactive as we go down the group and hydrolysed (breaking the C-HALOGEN bond)the fastest
What determines reactivity of haloalkanes
- bond strength
- bond enthalpy
NOT THE BOND POLARITY
How do we test for which haloalkane is which and what results should we get
1) Place chloroalkane, bromolakane and iodoalkane into 3 test tubes
2) add silver nitrate solution and ethanol (solvent) to each tube
RESULTS:
- iodoalkanes will form a yellow precipitate first as it has the weakest bond enthalpy so is more reactive
- bromoalkane will form a cream precipitate second
- chloroalkane will form a white precipitate last as it has the highest bond enthalpy and so is less reactive
What are CFCs
- chlorofluorocarbons
- they have all their hydrogens replaces by chlorine and fluorine
- stable molecules but are broken down by UV)
What do CFCs do
- break down ozone (O3) in the atmosphere
How are the C-Cl bond in CFCs broken and what does it form
- by UV radiation in the atmosphere
- which forms radicals
What catalyses the break down of ozone
- the radical formed when the C-Cl bonds are broken by UV radiation
Why is a C-Cl bond more likely to be broken in CFCs compared to a C-F bond
- as C-Cl bonds have a lower bond enthalpy so not a lot of energy is needed to break the bond
Write the free radical substitution reaction for CFCs
Initiation:
UV
CCl3F(g) ——> . CCl2F(g) + Cl .(g)
Propagation:
1) Cl .(g) + O3(g) —> O2(g) + ClO .(g)
2) ClO .(g) + O3(g) —> 2O2(g) + Cl .(g)
Termination:
Cl .(g) + Cl .(g) —> Cl2(g)
What is the overall equation in the breakdown of the ozone and what is the catalyst
2O3(g) —> 3O2 (g)
* Cl . Is the catalyst
What are other radicals that destroy ozone
- Nitrogen oxides
True or false:
CFCs are now banned
- True
Why do we need to ozone layer
- as it absorbs most harmful UV radiation that causes skin cancer
What are alternative to CFCs
- HFCs (hydrofluorocarbon)=> beneficial as it doesn’t have the chlorine which would from radicals to break down the ozone layer
How do we know that the alternative HFCs and other alternatives are working
- because the holes in the ozone layer are slowly becoming smaller
