Haloalkanes

Naming haloalkanes

  • Compounds containing the elements carbon, hydrogen and at least one halogen.
  • A prefix is added to the name of the longest chain. 
  • When two or more halogens are present in the structure they are listed in alphabetical order. 
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Reactivity of haloalkanes

  • Halogen atoms are more electronegative than carbon atoms. The electron pair is therefore closer to the halogen atom than the carbon atom and the carbon - halogen bond is polar. 
  • The carbon has a slight positive charge and can attract species containing a lone pair of electrons. 
  • Species that donate a lone pair of electrons are known as nucleophiles. 
  • A nucleophile is 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. 
  • Nucleophiles include hydroxide ions, water molecules and ammonia molecules. 
  • The nucleophile replaces the halogen in a substitution reaction. The reaction mechanism is nucleophilic substitution. 
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Nucleophilic substitution

  • The nucelophile, OH- approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom. 
  • This direction of attack minimises repulsion between the nucleophile and the delta negative halogen atom. 
  • A lone pair of electrons on the hydroxide ion is attracted and donated to the delta positive carbon atom. 
  • A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.
  • The carbon - halogen bond breaks by heterolytic fission. 
  • The new organic product is an alcohol. A halide ion is also formed. 
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Carbon-halogen bond strength

  • In hydrolysis, the carbon - halogen bond is broken and the -OH group replaces the halogen. 
  • The rate of hydrolysis depends upon the strength of the carbon-halogen bond in the haloalkane. 
  • The C-F bond is the strongest and the C-I bond is the weakest. Less energy is required to break the C-I bond than any other carbon-halogen bond. 
  • Iodoalkanes react faster than bromoalkanes. 
  • Bromoalkanes react faster than chloroalkanes.
  • Fluroalkanes are unreactive as a large quantity is of energy is required to break the bond. 
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The ozone layer

  • The ozone layer is found at the outer edge of the stratosphere, at a height that varies from about 10 to 40km above the Earth's surface. 
  • Absorbs most of the biologocally damaging UV radiation from the sun's rays, allowing only a small amount to reach the Earth's surface. 
  • In the stratosphere, ozone is continually being formed and broken down by the action of ultraviolet radiation. Initially very high energy UV breaks oxygen molecules into oxygen molecules:

O2 ---> 2O

  • A steady state is then set up involving O2 and the oxygen radicals in which ozone forms and breaks down. The formation of ozone is the as the rate at which it is broken down:

O2 + O ---> O3 

  • Human activity, especially the use of CFCs has upset this delicate equilibrium. 
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CFCs and the ozone layer

  • CFCs and HCFCs were the most common compounds used as refrigerants, in air conditioning units, and as aerosol propellants. 
  • CFCs are very stable because of the strength of the carbon-halogen bonds. 
  • 1973 - Frank Sherwood Rowland and Mario Molina began to look at the impact of CFCs on the atmosphere. They concluded that CFCs remain stable until they reach the stratosphere.
  • The CFCs begin to break down forming chlorine radicals which catalyse the breakdown of the ozone layer. 
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How do CFCs deplete the ozone layer?

  • UV radiation breaks the carbon-halogen bond in CFCs by homolytic fission to form radicals. The C-Cl bond is the weakest and so is the bond that breaks. 
  • As radiation initiates the breakdown, this process is called photodissociation.

CF2Cl2 ---> CF2Cl. + Cl.

  • The chlorine radical formed is a very reactive intermediate. It can react with an ozone molecule, breaking down the ozone into oxygen. 

Cl. + O3 ---> ClO. + O2

ClO. + O ---> Cl. + O2

O3 + O ---> 2O2 

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