4C - Haloalkanes Flashcards
Mechanism for the hydrolysis of bromoethane?
What are CFCs and HCFCs used?
Industry e.g. for aerosoles and refridgerators
Apart from CFCs and HCFCs what else contributes to the depletion of the ozone layer
Nitrogen monoxide (NO) which naturally forms during lightning.
What type of bond fission is the hydrolysis of a haloalkane?
Heterolytic fission
Equations for how the ozone layer is formed?
O2 → O• + O•
O2 + O• → O3
Define electronegativity
A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
Regarding the depletion of the ozone layer due to CFCs. What role does the Cl radical take in this reaction?
It’s a reactive intermediate
Why do iodoalkanes have the fastest rate of hydrolysis?
They contain the most number of sheilding electrons, decreases the nuclear attraction and the energy required to break the C-I bond is the least
Give 3 examples of nucleophiles
H2O
OH-
NH3
Write an equation to show the photodissociation of CFCs in the stratosphere.
Conditions required?
What type of bond fission is this?
CF2Cl2 → CFCl• + Cl•
Condition = UV light
Homolytic fission
Equation to show the depletion of the ozone layer
O3 → O2 + O•
What does a curly arrow show in a mechanism?
The movement of an electron pair
Out of radical substitution, electrophilic addition and nucleophilic substitution:
Which has the greated atom economy?
Nucleophilic substitution = 100% atom economy (produces only one product.
- Radical substitution - Too many possible isomers and byproducts.
- Electrophilic addition - Produce isomers.
What does it stand for?
CFCs and HCFCs?
CFCs = Chlorofluorocarbons
HCFCs = Hydrochlorofluorocarbons
3 Ways of synthesising a haloalkane?
- Free radical substitution - Alkane + Halogen
- Electrophilic addition - Alkene + Halogen or Alkene + Hydrogenhalide
- Nucleophilic substitution - Alcohol + hydrogenhalide
Write equations showing how NO contributes to the depletion of the ozone layer.
Overall equation?
Propagation:
NO• + O3 → NO2• + O2
NO2• + O → NO• + O2
Overall equation:
O3 + O → 2O2
The rates of hydrolysis of chloroethane, bromoethane and iodoethane are different.
- Describe how you would monitor the reaction rates.
- Explain why the different haloalkanes react at different rates.
(8 marks)
- Add 3 drops of each haloalkane into a test tube. Lable them accordingly.
- Add 1cm3 of ethanol to each of these 3 test tubes. The ethanol acts as a co-solvent.
- Get 3 new test tubes. Add 1cm3 of aqueous AgNO3 to each of them. Place these 3 test tubes into a hot water bath. Hot water bath ensures that a constant temperature is maintained throughout experiment.
- Add one test tube of the aqueous AgNO3 to each of the test tubes containing the haloalkanes and ethanol.
- Place the 3 test tubes into the hot water bath. Heat from hot water bath will provide heat to speed up the rate of the reaction.
- Use stop watch to time how long it takes for a coloured precipitate to form (this is the silver halide precipitate).
-Equation for hydrolysis (X = halogen):
CH3CH2X + H2O → CH3CH2OH + X-
-Equation for precipitation reaction:
Ag+ + X- → AgX
Iodoethane reacted the fasted, chloroethane reacted the slowest. The strength of the C-X bond affects the reactivity of the haloalkane. C-I bond is the weakest of the 3 C-X bonds and so iodoethane had the fastest rate of reaction.
What does the ozone layer protect us from?
UV radiation - over exposure increases chance of developing skin cancer
Define nucleophile
An atom (or group of atoms) which is attracted to an electron deficient center or atom, where it donates a pair of electrons to form a new covalent bond.
Where is the ozone layer found?
Stratosphere
Preperation of an alchohol by hydrolysis of a haloalkane:
- Name reagent used
- Conditions?
Reagent = Aqueous NaOH
Conditions = Heat under reflux
Radical mechanism for the depletion of the ozone layer due to CFCs
Conditions?
Overall equation?
Conditions = UV light
Initiation:
CF2Cl2 → CFCl• + Cl•
Propagation:
Cl• + O3 → ClO• + O2
ClO• + O → Cl• + O2
Overall equation:
O3 + O → 2O2
Why do organic reactions often require heating under reflux?
Organic compounds have a simple molecular structure so they ahve low boiling points. In order to keep the contents of the flask from boiling dry you need to heat the mixture under reflux.
Order of rate of hydrolysis of chloro-, bromo- and iodoalkanes?
Iodoalkanes = Fastest
Bromoalkanes
Chloroalkanes = Slowest
Why do you think haloalkanes are generally more reactive than alkanes?
Why is this not the case for fluoroalkanes?
Carbon-Halogen bond is weaker than the C-H bond which means that they require less energy to break so they can react more easily.
C-F bond is stronger than the C-H bond so the C-F bond requires more energy to break, therefore fluoroalkanes are less reactive than alkanes.
In CFCs why does the C-Cl bond break instead of the C-F bond?
The C-F bond is stronger than the C-Cl bond
