Chapter 15 - Haloalkanes Flashcards
What are nucleophiles ?
Species that donate a lone pair of electrons
What are nucleophiles likely to have ?
- A negative charge
- A lone pair of electrons
Give three examples of nucleophiles
- OH -
- NH3
- H2O
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 hydroxide ions that causes the breaking of a bond in a molecule
Describe the nucleophilic substitution reaction for the hydrolysis of a haloalkane
- OH- approaches the carbon bonded to the halogen on the opposite side to the halogen
- This minimises repulsion between the 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 (NaOH) and is heated under reflux
What determines how fast a specific carbon-halogen bond will break ?
Bond enthalpy
Which haloalkane will react the fastest and why ?
Iodoalkanes because the C-I bonds are the weakest so require the least amount of energy to break
How can hydrolysis of haloalkanes be compared to one another ?
Perform the reaction in the presence of aqueous silver nitrate (AgNO3) and record the time taken to form a precipitate
What would the result of hydrolysis of haloalkanes
show ?
- Chloroalkanes react slowest
- Iodoalkanes react fastest
- Therefore rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.
How does the primary, secondary or tertiary haloalkanes status affect rate of hydrolysis?
Tertiary reacts the fastest because the intermediary carbocation that is formed is much more stable than a primary carbocation
What are organohalogen compounds ?
These are molecules that contain at least one halogen atom joined to a carbon chain
What are organohalogen compounds used in ?
- General solvents
- Dry cleaning solvents
- Making polymers
- Flame retardants
- Refrigerants
- Pesticides
Where is the ozone layer found ?
10km to 40km above the earths surface
What is ozone (formula) ?
O3
What is UV-B ?
Biologically damaging UV radiation
What health conditions are UV-B linked to ?
Sun burn, eye cataracts, skin cancer
What does UV - B cause ?
It causes depletion of the ozone layer
What happens to the ozone in the stratosphere ?
In the stratosphere, the ozone is continually being formed and broken down by the action of UV radiation
Explain the breakdown of oxygen molecules into radicals
- Oxygen is broken down into oxygen radicals by UV radiation
- O2 —> 2O
What is the next step after oxygen has been broken down ?
- O2 + O —> O3, this is reversible
- In this steady state, the rate of formation of ozone is the same as the rate at which it is broken down
What has upset this equilibrium ?
The use of chlorofluorocarbons (CFCs)
Until recently, where were CFCs used ?
- They were used in refrigerants, AC units and as aerosol propellants
- This is because they are stable because of the strength of their carbon-halogen bonds within their molecules
What happens when CFCs reach the stratosphere ?
Once in the stratosphere, UV radiation provides sufficient energy to break a carbon halogen bond in CFCs by homolytic fission to form halogen radicals
What is this process called ?
This process is called photodissociation
What else can be responsible for the depletion of the ozone layer ?
NO, nitrogen oxide
