Chapter 15 - Haloalkanes Flashcards
The chemistry of the haloalkanes, Organohalogen compounds in the environment.
What are nucleophiles?
Species that donate a lone pair of electrons.
Give three examples of nucleophiles
Hydroxide ions
Water molecules
Ammonia molecules
When a haloalkane reacts with a nucleophile, what is the name of the reaction mechanism?
Nucleophilic substitution
What is hydrolysis?
A chemical reaction involving water or aqueous hydroxide that causes the breaking of a bond in a molecule.
Describe the nucleophilic substitution reaction for the hydrolysis of a haloalkane
OH- approaches carbon bonded to halogen on opposite side to halogen
This minimises repulsion between nucleophile and halogen’s dipole
Lone pair of electrons on OH attracted to and donated to carbon atom
New bond formed betwen oxygen and carbon atom
Carbon-halogen bond breaks
Alcohol and halide ion formed
Describe the conditions for the hydrolysis of haloalkanes
Using aqueous sodium hydroxide and is heated under reflux.
What determines how fast a specific carbon-halogen bond will break?
Bond enthalpy
Which halogenoalkane will react the fastest and why?
Iodoalkanes because C-I bonds are the weakest so require less energy to break.
How can hydrolysis of haloalkanes be compared to one another?
Perform the reaction in the presence of aqueous silver nitrate and record the time taken to form a precipitate.
What would the results show?
Chloroalkanes react slowest (will form white PPT)
Iodoalkanes react fastest (will form yellow PPT)
Therefore rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.
How does primary, secondary or tertiary halogenoalkane status affect rate of hydrolysis?
Tertiary reacts the fastest because the intermediary carbocation that is formed is much more stable than a primary carbocation.
How do CFCs deplete the ozone layer?
Once in the stratosphere, the UV radiation provides sufficient energy for the carbon-halogen bond to break by homolytic fission forming radicals. This process is called photodissociation.
The carbon to halogen breaks because of the low bond enthalpy
What other radicals are responsible for ozone depletion?
Nitrogen oxide radicals which are formed naturally when lightening strikes or when an aircraft is travelling theogony the stratosphere
Describe the test for hydrolysis of haloalkanes:
1) Tube 1: Add 1 cm^3 of ethanol and two drops of 1-chlorobutane (will form white PPT)
Tube 2: Add 1 cm^3 of ethanol and two drops of 1-bromobutane (will form cream PPT)
Tube 3: Add 1 cm^3 of ethanol and two drops of 1-iodobutane (will form yellow PPT)
2) Stand the test tubes in a water bath at 60C
3) Place a test tube containing 0.1 mol dm^-3 silver nitrate in the water bath and allow all tubes to reach a constant temperature.
4) Add 1 cm^3 of the silver nitrate quickly to each of the test tubes. Immediately start a stop-clock.
5) Observe the test tubes for five miniutes and record the time taken for the precipitate to form.
Earth’s atmosphere is divided into five layers: what are they in order from the lowest to highest level?
troposphere tropopause stratosphere ozone layer ozone layer
List three sources of CFCs in the atmosphere, and their reasons why?
1) Refrigerants- Non toxic and non-flammable
2) Dry cleaning- good solvent for organic material
3) Aerosol propellants- because it is volatile
why is ozone depletion a major concern to humans
The depletion of ozone layer will allow more harmful UV-b radiation to penetrate the troposphere leading to increased incidence of skin cancer and genetic damage
show the reaction of nitrogen oxide(NO) radical reacting with ozone layer
NO• + O3»_space;> NO2• + O2
NO2• + O»_space;> NO• + O2
show the reaction of Cl radical reacting with ozone layer
Cl• + O3»_space;> ClO• + O2
ClO• + O»_space;> Cl• + O2
what is the Montreal protocol
Ban CFCs
Stop using CCl4
Stop using ‘halon’ fire extinguishers
What alternative can we use to CFC’s
HFCs
Supercritical CO2
e.g. CO2(l)
