4. Halogenoalkanes

• A free radical is an atom or group with an unpaired electron
• Halogenoalkanes are used as refrigerants, propellants, solvents, flame retardants, anaesthetics and pharmaceuticals
• The C–X bond (X being the halogen) in halogenoalkanes is polar because the halogen atom is more electronegative than the C atom
• The halogenoalkanes are insoluble or only very slightly soluble in water
• Halogenoalkanes are soluble in organic solvents and due to their ability to mix with other hydrocarbons are used extensively as dry cleaning fluids and degreasing agents
• Steps in free radical substitution:

  • Initiation
  • Propagation
  • Termination
• Step 1. Initiation

  The molecules of chlorine absorb energy from UV light and the Cl-Cl covalent bond breaks symmetrically. Each atom from the bond leaves with one electron from the shared pair of electrons. Two chlorine atoms are formed. Each atom has an unpaired electron in the outer shell.

  • They are called free radicals and are extremely reactive
• Step 2. Propagation

  The highly reactive chlorine free radicals, Cl●, react with the methane molecules forming hydrogen chloride gas and leaving a methyl free radical, ●CH3. In turn the methyl free radical reacts with a second chlorine molecule forming chloromethane, CH3Cl, and another chlorine free radical. This continuing process of producing free radicals is known as a chain reaction.
• The continuing process of producing free radicals is known as a chain reaction
• In order for the reaction to stop two free radicals must collide and react to form a molecule
• Halogenoalkanes are the organic product of the photochemical reaction of the halogens with alkanes in UV light.

  • They are produced via a free radical substitution mechanism in a chain reaction
• A nucleophile is an electron pair donor
• The nucleophiles we learn about are:

  • hydroxide ions
  • cyanide ions
  • ammonia
• Conditions for the reaction of halogenoalkanes with an aqueous solution of hydroxide ions

  • Dissolve in a small volume of ethanol
  • Add an aqueous solution of sodium hydroxide
  • Reflux gently
• The rate of the nucleophilic substitution of the halogenoalkanes depends on the ease of breaking the carbon–halogen bond
• Conditions for the reaction of halogenoalkane with an aqueous solution of cyanide ions:

  • Dissolve in a small volume of ethanol
  • Add an aqueous solution of potassium cyanide
  • Reflux gently
• Conditions for the reaction of halogenoalkane with a concentrated solution of ammonia:

  • Dissolve in a small volume of ethanol
  • Add a concentrated solution of ammonia in excess
  • In a sealed container under pressure
• Properties of halogenoalkanes

  • Halogenoalkanes are polar molecules and contain polar bonds. The polar bonds are important in their reactions.
  • Because halogens are more electronegative than carbon, the carbon atoms carry a partial positive charge and the halogen atoms carry a partial negative charge.
  • Halogenoalkanes undergo substitution reactions with ammonia, hydroxide ions, and cyanide ions.
• • A nucleophile is an electron pair donor that is attracted to a nucleus.
  • Nuclei have positive charges, so nucleophiles tend to be negatively charged.
  • Molecules with lone pairs are also nucleophilic.
• • An electrophile is a molecule that is attracted to a pair of electrons.
  • Electrons have negative charges, so electrophiles tend to be positively charged.
• Hydroxide reacting with a halogenoalkane can either eliminate to produce an alkene or substitute to produce an alcohol.

  • To promote the elimination reaction, we can:
  • Use ethanol as the solvent rather than water
  • Heat the reaction under reflux conditions
• Reflux involves using a reflux condenser to condense gases back into the reaction flask.
• In the elimination mechanism, the hydroxide is behaving more like a base than a nucleophile
• Ozone has the formula O3.

  • O3 is much less stable than O2.
  • O3 is formed from O2 and UV light.
  • O3 is present in low concentrations.
• Reducing ozone:

  Chlorofluorocarbons undergo photolysis in the atmosphere.

  • This process creates chlorine radicals.
  • Chlorine radicals react with ozone (O3).
• A reduction in the ozone layer means that more of the Sun’s harmful UV rays reach the earth’s surface, which can cause a variety of environmental and health problems.
• The use of some halogenoalkanes has been restricted due to the effect of chlorofluorocarbons (CFCs) on the atmosphere.
• Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer.
• Ozone absorbs ultraviolet radiation.
• Chemists have now developed alternative chlorine free compound as cfcs have been banned