Haloalkanes Flashcards
Write the word equation for the preparation of Chloroethane from an alkane
Ethane + Chlorine→chloroethane
What are the conditions under which chloroethane is made from an alkane?
- Room temperature and pressure
- in the presence of UV radiation
What is the equation for the formation of chloroethane from an alkane?
CH3CH3 + Cl2→CH3CH2Cl + HCl
What is the mechanism for the formation of chloroethane from an alkane?
Free radical substitution
What are the reagents needed to make chloroethane from an alkene?
Ethene and HCl
What are the conditions needed to make chloroethane from an alkene?
Room temperature and pressure
What is the mechanism by which chloroethane can be made from an alkene?
Electrophilic addition
What is the chemical equation for the reaction of ethene and HCl to produce chloroethane?
H2C=CH2 + HCl →CH3CH2Cl
What is the atom economy for the reaction needed to make chloroethane from an alkene?
100%
Why are haloalkanes generally more reactive than alkanes?
- The strength of Carbon-Halogen bonds is lower than Carbon-hydrogen bonds
- So the Carbon-halogen bonds break more easily and allow the haloalkanes to react
What is the exception to haloalkanes being more reactive than alkanes? Why?
Fluroalkanes
- Carbon-fluorine bonds have a higher bond enthalpy than Carbon-Hydrogen bonds
- so require more energy to break
Are Carbon-Halogen bonds polar?
Yes
Why are Carbon-Halogen bonds polar?
Halogens have a higher electronegativity than Carbon
Define hydrolysis
A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule
What are the reagents needed to prepare an alcohol from a haloalkane by hydrolysis?
Haloalkane and NaOH(aq) or water
What are the conditions to prepare an alcohol from a haloalkane by hydrolysis?
Heat under reflux
What is the mechanism to prepare an alcohol from a haloalkane by hydrolysis?
Nucleophilic substitution
Give an equation for the reaction of bromoethane and water to produce an alcohol
CH3CH2Br + H2O —> CH3CH2OH + HBr
Define nucleophile
A species capable of donating a pair of electrons
Give 3 examples of nucleophiles
NH3(aq), H2O, OH-(aq)
What do most nucleophiles have in common?
They have lone pairs
How do you test for the rate of hydrolysis of haloalkanes?
Place the test tube with haloalkane in a hot water bath, add ethanol and then AgNO3 (aq)
Why is the experiment to test the rate of hydrolysis of haloalkanes conducted in a hot water bath?
To speed up the rate of reaction
Why does the experiment to test the rate of hydrolysis of haloalkanes use ethanol?
Ethanol acts as a cosolvent as both water and the haloalkane dissolve in ethanol so the reactants can mix
What are the two roles of AgNO3 (aq) in the experiment to test the rate of hydrolysis of haloalkanes?
- provides H2O to act as the reactant in hydrolysis -Ag+ from AgNO3 trap halide ions released by the reaction, to form a preciptate
- rate of precipitate production allows the rate of reaction to be calculated
Why is water used as the nucleophile in the experiment to test the rate of hydrolysis of haloalkanes rather than AgNO3 (aq)?
- if OH- ions were present they would react with Ag+ to form AgOH
- this would form a brown precipitate
- Ag+ ions would be removed from reacting with halides
- any silver halide precipitate formed would be masked
How is the rate of hydrolysis of haloalkanes compared?
The time taken for a visible quantity of silver halide to be formed is compared
What colour precipitate is formed in the hydrolysis of chloroethane?
White
What colour precipitate is formed in the hydrolysis of bromoethane?
Cream
What colour precipitate is formed in the hydrolysis of iodoethane?
Yellow
What are the relative rates of the hydrolysis of chloroethane, bromoethane, and iodoethane?
- iodoethane is the fastest
- then bromoethane
- chloroethane is the slowest
Why do the rates of hydrolysis of haloalkanes differ?
The C-Cl bond is the strongest, then the C-Br bond, then the C-I is the weakest
What were CFCs used for?
- coolants (in fridges)
- propellant (in aerosols)
- fire extinguishers
- blow moulding (making plastics)
What properties of CFCs made them good propellants/fire extinguishers/coolants?
They are:
- inert
- non-toxic
- non-flammable
- non-irritant
Give the equations to show the formation of ozone in the atmosphere
O2 → O• + O• under UV light
O2 + O• → O3
Why is the ozone layer important?
Protects organisms from high energy UV rays as O3 readily absorbs UV radiation
What are the risks to people without the ozone layer?
Increased risk of genetic damage and skin cancer due to UV radiation exposure
Why is there no overall change in concentration of ozone in the atmosphere (without human intervention)?
-ozone is constantly forming and breaking down at the same rate
Why do CFCs persist in the lower atmosphere?
C-F and C-Cl bonds are strong and so the molecules do not break down easily
Give the equation for the breakdown of ozone naturally within the atmosphere
Under the action of UV radiation O3→ O2 + O•
Show, using equations, how Cl radicals catalyse the breakdown of ozone
Cl• + O2→ ClO• + O2
ClO•+ O→Cl• + O2
Give the overall equation of the breakdown of ozone by Chlorine radicals
O3 + O→2O2
Give two substances that can catalyse the breakdown of ozone
CF2Cl2
NO
Where does Nitrogen monoxide come from in the atmosphere?
Lightning strikes and high flying aircrafts
What type of catalysts are NO and CFCs?
Homogeneous
Show, using equations, how NO radicals can catalyse the breakdown of ozone
NO• + O3→ NO2• + O2
NO2• + O —> NO• + O2
Define homogeneous catalyst
A catalyst which is in the same phase as the reactants of the reaction it catalyses
