15. Haloalkanes Flashcards
What is an example of nucleophilic substitution of haloalkanes?
Hydrolysis
What happens during hydrolysis?
The halogen is replaced by an -OH group
What condition is required to start hydrolysis of haloalkane?
Heat under reflux
What solutions are used to measure the rate of hydrolysis of haloalkanes by water?
- aqueous silver nitrate (AgNO3)
- ethanol
What are the factors that affect the rate of hydrolysis? Which one is the main factor?
1. bond strength between carbon and halogen (outer shell)
2. polarity
What is the product when haloalkane is heated under reflux and reacts with sodium/potassium hydroxide (NaOH/KOH) ?
Alcohol (+metal halide)
What is the product when haloalkane is heated under reflux and reacts with sodium/potassium cyanide (NaCN/KCN) in ethanol?
Nitrile (+metal halide)
What is the functional group of nitrile? How to name nitrile?
C≡N. Add -nitrile after the alkane, eg. propanenitrile.
how many bonds does nitrogen form?
3
What is the product when haloalkane is heated under reflux and reacts with ammonia (NH3) in ethanol?
Amine (+hydrogen halide)
What is the functional group of amine? How to name amine?
NH2. Replace the -ane with -yl and add -amine after it, eg. propylamine.
What does nucleophile mean?
An electron pair donor / a species that is often negatively charged and seeks out positively charged area.
What are the examples of nucleophiles?
- hydroxide ions (:OH-)
- water (H2O:)
- ammonia (:NH3)
- cyanide ions (:CN-)
- ethyl (CH2CH3)
Which carbon–halogen bond is the strongest? How is its rate of hydrolysis? Why?
C-Cl. It reacts the slowest because chlorine’s outer shell is the closest to the nucleus. This means that C-Cl bonds have the lowest bond enthalpy. Permanent dipole increases intermolecular forces, so the melting point is higher.
What conditions and solutions are required to produce alcohol when hydrolysing haloalkane?
- NaOH/KOH (sodium/potassium hydroxide)
- reflux
What conditions and solutions are required to produce nitrile when hydrolysing haloalkane?
- NaCN/KCN (sodium/potassium cyanide)
- ethanol
- reflux
What conditions and solutions are required to produce amine when hydrolysing haloalkane?
- NH3 (ammonia)
- ethanol
- reflux
What are organohalogen molecules?
Compounds that contain at least one halogen joined to a carbon.
What are organohalogen molecules used as?
- solvent
- making polymers
- dry cleaning agents
- flame retardants (阻燃劑)
- refrigerants
- pesticides
Where is ozone layer found?
On the outer edge of the stratosphere
How far is ozone above Earth’s surface?
Between 10 to 40 km
What is the formula of ozone?
O3
How does ozone form?
By a reaction involves O2 and an oxygen radical, which requires UV light. (O2 + O ⇌ O3)
What does ozone do?
It absorbs most of the biologically damaging UV radiation from the sun called UV-B.
What is the disadvantage of ozone?
It is corrosive
What is CFC?
Chlorofluorocarbons
What bonds do CFCs don’t have?
C-H bonds
Why is CFC stable? What does it mean?
Because of the strength of its carbon-halogen bonds. This means that CFC has a long residence time (停留時間) in the troposphere.
Until when CFC is not stable anymore?
Until they reach the stratosphere
What does CFC do when it reaches the stratosphere? What does it mean to the ozone layer?
It starts to break down, forming a chlorine radical. It catalyses the breakdown of the Earth’s protective ozone layer.
How do CFCs break down the ozone layer?
Once CFC is in stratosphere, UV radiation causes homolytic fission and breaks the carbon-halogen bond.
What is homolytic fission?
The breaking of a covalent bond which each bonding atom receiving one electron from the bonded pair, forming two radicals.
Why does chlorine radical forms rather than other halogen radicals?
Because the C-Cl bond has the lowest bond enthalpy so it breaks the easiest. Once the C-Cl bond breaks, chlorine radical forms.
How is ozone (O3) broken down?
The chlorine radical is very reactive and reacts with an ozone molecule, breaking down the ozone into oxygen.
What is the initiation step of CF2Cl2?
CF2Cl2 > CF2Cl● + ●Cl
What are the propagation steps of CF2Cl2?
- Cl● + O3 > ●ClO + O2 (ozone is broken down by the chlorine radical)
- ●ClO + O > Cl● + O2 (forms chlorine radical again, which breaks down more ozone)
What is the overall equation of ozone being broken down?
O3 + O > 2O2
What type of radical also breaks down ozone?
Nitrogen oxide radical (NO●)
What is the propagation step of nitrogen oxide?
- NO● + O3 > NO2● + O2
- NO2● + O > NO● + O2
