15 Haloalkanes Flashcards
What are haloalkanes?
Compounds containing the elements carbon, hydrogen and at least one halogen.
What are nucleophiles?
- An atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond.
E.g hydroxide ions, water molecules, ammonia molecules
What is nucleophilic substitution?
A reaction in which a nucleophile is attracted to an electron deficient carbon atom, and replaces an atom or group of atoms on the carbon atom.
What is hydrolysis?
- A reaction that breaks a chemical compound into two compounds, the H and OH in a water molecule becomes incorporated into the two compounds.
What happens during the hydrolysis of a haloalkane?
1) The nucleophile, OH-, approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom.
2) This direction of attack by the OH- ion minimises repulsion between the nucleophile and the sigma negative halogen atom.
3) A lone pair of electrons on the hydroxide ion is attracted and donated to the sigma positive carbon atom.
4) A new bond is formed between the oxygen atom of the hydroxide ion and the 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 can haloalkanes be converted to alcohols?
Using aqueous sodium hydroxide heated under reflux.
Which carbon-halogen bond is the strongest?
C-F bond
Which carbon-halogen bond is the weakest?
C-I bond
When does rate of hydrolysis increase?
Rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.
How do you measure the rate of hydrolysis?
1) Set up 3 test tubes with 1cm^3 of ethanol and two drops of 1-chlorobutane, 1-bromobutane, 1-iodobutane.
2) Stand the test tubes in a water bath at 60 degrees.
3) Place a test tube containing 0.1moldm-3 silver nitrate in the water bath and allow all tubes to reach a constant temperature.
4) Add 1cm^3 of the silver nitrate quickly to each of the test tubes. Immediately start a stop-clock.
5) Observe the test tubes for five minutes and record the time taken for the precipitate to form.
What colour precipitate would 1-chlorobutane form?
A white precipitate forms very slowly.
What colour precipitate would 1-bromobutane form?
A cream precipitate forms slower.
What colour precipitate would 1-iodobutane form?
A yellow precipitate forms rapidly.
What are organohalogen compounds?
Organohalogen compounds are molecules that contain at least one halogen atom joined to a carbon chain.
What happens in the stratosphere?
- High energy UV breaks oxygen molecules into oxygen radicals.
O2-> 2O - A steady state is then set up involving O2 and the oxygen radicals in which ozone forms and then breaks down.
O2 + O -> O3
Why are CFCs very stable?
They are very stable because of the strength of the carbon-halogen bonds within their molecules.
What is photodissociation?
- When UV radiation initiates the breakdown of CF2Cl2.
- UV radiation provides sufficient energy to break a carbon-halogen bond in CFCs by homolytic fission to form radicals.
- CF2Cl2-> CF2Cl. + Cl.
What happens in propagation step 1?
Cl. +O3 -> ClO. + O2
What happens in propagation step 2?
ClO. + O -> Cl. + O2
What is the overall equation?
O3 + O -> 2O2
What other radical catalyses the breakdown of ozone?
Nitrogen oxide radicals
What happens in propagation of nitrogen oxide?
Step 1: NO. + O3 -> NO2. + O2
Step 2: NO2. + O -> NO. + O2
What are CFC used as?
- Refrigerants
- Aerosol propellants
How are nitrogen oxide radicals formed?
- They are formed naturally during lightning strikes and as a result of aircraft travel in the stratosphere
