C15 - Haloalkanes Flashcards
What’s a nucleophile?
An atom or group of atoms with a negative charge which is attracted to positive charges / nuclei. It reacts by donating an electron pair.
What’s hydrolysis?
A chemical reaction involving water or an aqueous solution of hydroxide that causes the breaking of a bond in a molecule.
What type of reaction is hydrolysis?
Nucleophilic substitution
What occurs during hydrolysis?
1) The nucleophilic (OH-) approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom.
2) The direction of the attack by the OH- ion minimises repulsion between the nucleophile and the slight negative hydrogen atom.
3) A lone pair of electrons on the hydroxide ion is attracted and donated to the slight positive carbon atom.
4) A new bond is formed between the oxygen atom and hydroxide ion and carbon atom.
5) The carbon-halogen bond breaks by heterolytic fission.
6) The new organic product is an alcohol. A halide ion is also formed.
How are alcohols formed from haloalkanes?
By nucleophilic substitution using aqueous sodium hydroxide under reflux.
How does the rate of hydrolysis vary among various haloalkanes?
The rate of hydrolysis depends on the strength of the carbon-halogen bond in the haloalkane.
C-F bonds are the strongest and are unreactive as they require a large amount of energy for the bond to break.
C-I bonds are the weakest so rate of hydrolysis is much greater.
How can the rate of hydrolysis of primary haloalkanes be identified?
What is required for this reaction?
By reacting haloalkanes (1-chlorobutane, 1-bromobutane and 1-iodobutane) with aqueous silver nitrate.
Halide ions are produced due to the nucleophile (water) from the aqueous silver nitrate, producing a silver halide precipitate.
An ethanol solvent is also required as it allows water and the haloalkane to mix and form a single layer.
What is the rate of hydrolysis of a halo alkane dependent on?
Bond enthalpies.
Chlorobutane reacts slower as C-Cl bonds are very strong.
Iodobutane re acts faster as C-I bonds are the weakest.
What are organohalogens?
Molecules that contain at least one carbon-halogen bond, used for refrigerants, solvents, pesticides etc.
What’s the ozone layer?
A layer at the outer edge of the stratosphere (10-40km above surface of Earth).
A small fraction of gases make up the ozone however the absorb UV-B radiation from the sun, allowing only a small amount to reach the Earth’s surface.
What is ozone?
How is ozone produced?
O3
Within the stratosphere, it’s continually being broken down and formed by UV radiation.
Initially, high energy breaks oxygen into radicals
(O2 -> 2O)
O2 and oxygen radicals then form ozone and break apart again, a reversible reaction.
What are CFCs and what is their effect?
Chlorofluorocarbons
It was found that they remain stable until they reach the stratosphere then form chlorine radicals which catalyse the breakdown of the ozone.
Due to the C-halogen, they remain stable in the troposphere. Once in the stratosphere, UV radiation provides energy for homolytic fission to form radicals.
What’s photodissociation?
The break down (of chlorofluorocarbons) due to radiation.
What happens during the photo dissociation of chlorofluorocarbons?
1) Cl° + O3 -> ClO° + O2
2) ClO° + O -> Cl° + O2
Overall: O3 + O -> 2O2
Nitrogen oxide is also responsible for the depletion of the ozone layer.
