Haloalkanes

Question 1

What is a nucleophile?

Answer 1

• An atom/group of atoms that is attracted to an electron deficient.
• An electron pair donor.

Question 2

What nucleophiles are attracted to haloalkanes normally?

Answer 2

• hydroxide ions, -OH
• water molecules, H2O
• ammonia molecules, NH3

Question 3

What is an example of nucleophilic substitution with haloalkanes?

Answer 3

HYDROLYSIS

• A chemical reaction involving water/solution containing hydroxide that causes breaking of bond in a molecule.
• This always results in molecule being split into two products.

Method:

1. Nucleophile approaches other side of carbon attached to halogen.
2. This approach actually minimises repulsion between nucleophile and slightly negative halogen atom.
3. A lone pair of electrons on hydroxide ion is attracted and donated to carbon atom so new bond is formed between oxygen of -OH and carbon atom.
4. Carbon-halogen bond is broken by heterolytic fission.
5. New organic compound is an alcohol and halide ion is also formed.

Question 4

What trend is there between carbon-halogen bonds and their rate of hydrolysis?

Answer 4

• The rate of hydrolysis increases as strength of carbon-halogen bond decreases.
• Fluoroalkanes are unreactive as large quantity of energy is required to break the bond.

Question 5

How has the use and production of some organohalogen compunds harmed nature?

Answer 5

• Chlorofluorocarbons (CFCs) are an example of how it has harmed the ozone layer.
• Ozone layer protects against UV-B radiation from the sun which has been proven to cause sunburn and is being researched, it is in the stratosphere.
• If there is an increased exposure to UV-B then there will be a rise in skin cancer and genetic damage to all organisms.

Question 6

How do CFCs deplete the ozone layer?

Answer 6

• Have strong carbon-halogen bonds so remain in troposphere before travelling up to stratosphere and undergoing photo-disassociation to trigger a two step propagation.
• Repeats in a charge reaction, about 100,000 molecules of ozone can be broken down by single molecule CFC.

Question 7

What other radicals catalyse breakdown of ozone?

Answer 7

• Nitrogen oxide radicals are formed naturally during lightning strikes.