Chapter 15 - Haloalkanes
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What are haloalkanes
Compounds containing Carbon, Hydrogen and at least one halogen atom
What is different about a carbon – halogen bond compared to C–H
It is polar - Halogens are more electronegative
What are nucleophiles
Nucleophiles are negative ions, or molecules with a δ- atom, that can donate a lone pair of electrons to form a new covalent bond
What happens in the hydrolysis of a haloalkane under reflux
The halogen atom is replaced by an –OH group to form an alcohol
Describe the process of the nucleophillic substitution of a haloalkane
• OH- ion approaches the δ+ carbon on the opposite side of the molecule from the δ- halogen atom, to minimise repulsion • A lone pair of electorns on the OH- ion is donated to the δ+ carbon atom and a new bond is formed • The carbon-halogen bond breaks by heterolytic fission • The new organic product is an alcohol • A halide ion is also formed
What does the rate of the hydrolysis of haloalkanes depend on
The rate of hydrolysis of the haloalkanes depends upon the strength of the carbon-halogen bond
Which reacts fastest and why: * Fluoroalkanes * Bromoalkanes * Chloroalkanes * Iodoalkanes
• Iodoalkanes react the fastest (C-I bond weakest, so less energy required to break the C-I bond) • Fluoroalkanes react the slowest (C-F bond strongest, so more energy required to break the C-F bond). [Fluoroalkanes unreactive.]
How can you measure the rate of the hydrolysis of Primary Alkanes
• Comparing the rate of hydrolysis of 1-chlorobutane, 1-bromobutane, and 1-iodobutane • Ethanol and aqueous silver nitrate solution are added to the test tubes containing 3-4 drops of the haloalkanes. The test tubes are warmed in a water bath and the time taken for the precipitates of silver halide to appear are measured and recorded
Where is the ozone layer
The ozone layer is found at the outer edge of the stratosphere
Why is the ozone layer important
Ozone absorbs the biologically damaging high energy UV radiation (UV-B). - Sunscreen Effect Exposure to UVB causes sun burn and long term effects such as skin cancer and genetic damage
What is meant by the steady state of the ozone layer
ozone is continually being formed and broken down by the action of ultraviolet (UV) radiation
How is ozone formed and broken down, include equations
• Initially, very high energy UV radiation breaks oxygen molecules into oxygen radicals, O, via homolytic fission • O2 → 2O • A steady state is then set up involving O2 and the oxygen radicals in which ozone is constantly being formed and broken down • O2 + O ⇌ O3 • The reversible reaction occurs at the same rate
What are CFCs
chlorofluorocarbons (contain chlorine, fluorine, and carbon)
Why are CFCs stable
Due to the strength of the carbon-halogen bonds, CFCs are very stable compounds – chemically inert, non-toxic, low flammability
What are uses of CFCs
• Fridges • Aerosols
Write the equations for the catalysis of the breakdown of Ozone by CFCs using CFCl3 and state initiation step, propagation step, and overall equation
Initiation * CFCl3→ CFCl2* + Cl* Propagation * Cl* + O3 → ClO* + O2 * ClO* + O → Cl* + O2 Overall * O3 + O → 2O2
What common radical can also catalyse the breakdown of ozone
Nitrogen
Show the catalysis of the breakdown of ozone by radical X
• X* + O3 → XO* + O2 • XO* + O → X* + O2 Overall : O3 + O → 2O2