Chapter 15 - Haloalkanes Flashcards
(20 cards)
What are haloalkanes?
Compounds containing the elements carbon, hydrogen and at least one halogen
What are nucleophiles?
Species that donate a lone pair of electrons to form a new covalent bond
What happens when a haloalkane reacts with a nucleophile?
Nucleophile replaces the halogen in a substitution reaction to form a new compound with a different functional group
What is hydrolysis?
Chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule
What is the mechanism like for hydrolysis? Describe (look at notes section):
- Nucleophile, OH- approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
- This direction minimises the repulsion between the nucleophile and the halogen atom
- A lone pair of electrons in the hydroxide ion is attracted and donated to the carbon atom
- A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
- The carbon-halogen bond breaks by heterocyclic fission
- The new organic product is an alcohol and a halide ion
What is another way haloalkanes can be converted to alcohols?
Using sodium hydroxide heated under reflux as the reaction is very slow at room temperature
1-bromobutane + sodium hydroxide -> butan-1-ol + sodium bromide
What affects the rate of hydrolysis?
- The strength of the carbon-halogen bond in the haloalkane (bond enthralls)
Higher bond enthalpy = more energy required to break bond = react slower - Depending on if it is primary, secondary or tertiary alcohol:
tertiary fastest
What is the order of reactivity of hakoalkanes?
- Iodoalkanes react faster than bromoalkanes
- bromoalkanes react faster than chloroalkanes
- fluoroalkanes are unreactive as a large quantity of energy is required to break the C-F bond
How can we test for each halogen in haloalkanes?
1) Set up three test tubes: one with ethanol and two drops of 1-chlorobutane, one with ethanol and two drops of 1-bromobutane and one with ethanol and two drops of 1-iodobutane
2) Stand the test tubes in a water bath at 60C
3) Place a test tube containing 0.1moldm-3 of silver nitrate in the water bath and allow all tubes to reach a constant temperature
4) Add some of the silver nitrate quickly to each of the test tubes and immediate start a stop clock
1-chlorobutane = white precipitate forms very slowly
1-bromobitane = a cream precipitate forms slower than with 1-iodobutane but faster that 1-chlorobutane
1-iodobutane = a yellow precipitate forms rapidly
What is an organohalogen?
Contain at least one halogen atom joined to a carbon chain
What are the uses of organohalogen compounds?
- general solvents (CHCl3)
- dry cleaning solvents (C2,H2Cl2, C2HCl3)
- making polymers (C2H3Cl, C2F4)
- flame retardants (CF3Br)
- refrigerants (F2CCl2)
Why are organohalogen compounds concerning?
They’re rarely found in nature as they are not broken down naturally in the environment
What is the ozone layer?
Found at the outer edge of the stratosphere at a height about 10-40km above the Earth’s surface
Why is the ozone layer important?
A small fraction of the gases of the ozone layer is ozone which is enough to absorb most of the biologically damaging ultraviolet radiation form the sun
Why is ultraviolet radiation dangerous?
- sunburn
- increased genetic damage
- risk of skin cancer in humans
What is the process of ozone being formed and broken down?
1) High energy UV breaks oxygen molecules into oxygen radicals
O2 -> 2O
2) Ozone forms and then breaks down
O2 + O <> O3
How has human activity disrupted the ozone layer process?
The use of CFCs in refrigerators:
- CFCs remain stable (strong C-F bonds) until they reach the stratosphere
- here they break down from energy provided by UV radiation forming chlorine radical which are thought to be catalyse the breakdown of the ozone layer
What is the process of the depletion of the ozone layer?
Initiation: as radiation initiates the break down is it called photodissociation
CF2Cl2 -> CF2Cl. + Cl.
Propagation: Chlorine radical formed is very reactive and can react in a cycle over and over again in a chain reaction
Step 1: Cl. + O3 -> ClO. + O2
Step 2 ClO. + O -> Cl. + O2
Overall equation:
O3 + O -> 2O2
Are CFCs responsible for ozone depletion, if not why and give an example?
No as other radicals also catalyse the breakdown the ozone layer
Nitrogen oxide radical which are formed naturally during lightning strikes cause the breakdown of ozone by a mechanism similar to the chlorine radicals:
Propagation 1: NO. + O3 -> NO2. + O2
Propagation 2: NO2. + O -> NO. + O2
Overall equation:
O3 + O -> 2O2
Roughly how many molecules of ozone can 1 molecule of CFC promote the breakdown of?
100 000 molecules
