3.3.3 Halogenoalkanes

• The trend in atomic radius going down group 7 is that the halogens become larger, which decreases the strength of the carbon-halogen bonds.
• Going down group 7, the carbon-halogen bond enthalpy decreases due to the increase in atomic radius of the halogens.
• Fluorine, being the smallest halogen, attracts the bonding pair of electrons most strongly towards its nucleus, giving the bond its strength.
• Iodine, being the largest halogen, forms the weakest carbon-halogen bond as the bonding pair of electrons is furthest from its nucleus.
• A mono-substituted haloalkane has the general formula CnH2n+1X, where X represents the halogen and is also represented as R-X, where R represents the alkyl chain.
• Going down group 7, the polarity of the carbon-halogen bond decreases because the electronegativity of the halogens decreases going down the group.
• Halogenoalkanes are not soluble in water due to their non-polarity and the lack of hydrogen bonds.
• The intermolecular forces between haloalkane molecules are dipole-dipole forces and van der Waals forces.
• Boiling points of halogenoalkanes increase with increasing carbon chain length and the further down the group the halogen is.
• In nucleophilic substitution, an atom or group of atoms is replaced by a nucleophile (an electron pair donor).
• The polarity of the carbon-halogen bond makes it susceptible to nucleophilic attack.
• Nucleophiles have a negative charge or dipole and are attracted to a positive charge or dipole on another molecule.
• The carbon-halogen bond breaks by heterolytic fission, meaning that the halogen takes both the electrons when the bond breaks.
• The halide ion produced in a nucleophilic substitution reaction is known as the leaving group as it is lost from the halogenoalkane.
• Nucleophilic substitution reactions are useful as they introduce new functional groups on to a molecule, allowing the synthesis of specific molecules e.g. pharmaceutical drugs.
• Reactivity of halogenoalkanes increases going down group 7 due to the decrease in bond enthalpy and bond polarity.
• Nucleophilic substitution of halogenoalkanes involves a species with a lone pair of electrons that will attack an electron deficient species.
• Bond enthalpy is the more important factor than bond polarity in the reactivity of halogenoalkanes. 
• Nucleophiles are a species with a lone pair of electrons that will attack an electron deficient species. 
• Common nucleophiles

  • hydroxide ions (OH-)
  • cyanide ions (CN-)
  • ammonia ions (NH3)
• When halogenoalkanes react with an aqueous solution hydroxide ions, a nucleophilic substitution reaction occurs, producing an alcohol and a halide ion. 

  e.g.

  1-iodopropane + potassium hydroxide → Propan-1-ol + potassium iodide
• Hydrolysis of haloalkanes involves warm aqueous sodium or potassium hydroxide.
• Mechanism for hydrolysis of haloalkanes

  1. The lone pair on the hydroxide ion are attracted and donated to the electron deficient carbon atom in the halogenoalkane (called nucleophilic attack).
  2. The donation of the electron pair leads to the formation of a covalent bond between the carbon and OH- ion, making an alcohol.
  3. The carbon-halogen bond breaks by heterolytic fission. Both electrons from the bond move to the halogen, forming a halide ion which leaves.
• When drawing a mechanism for nucleophilic substitution…

  1. Always include dipole on C-X bond
  2. Curly bond should come from the middle of the C-X bond and from the Nu to the carbon. 
  3. Show charge on nucleophile

  
• Nucleophilic substitution of haloalkane with cyanide ion extends the carbon chain by one carbon, which must be reflected in the name of the product. The product will be named with the suffix –nitrile.

  e.g.

  Chloroethane + potassium cyanide → propanenitrile + potassium chloride
• The reaction conditions for the nucleophilic substitution of haloalkanes with cyanide ions

  1. requires heating
  2. ethanolic potassium cyanide ions (dissolved in ethanol)
• Mechanism for nucleophilic substitution of haloalkanes with cyanide ions

  1. The lone pair on the cyanide ion are attracted and donated to the electron deficient carbon atom in the halogenoalkane (called nucleophilic attack).
  2. The donation of the electron pair leads to the formation of a covalent bond between the carbon and cyanide ion.
  3. The carbon-halogen bond breaks by heterolytic fission. Both electrons from the bond move to the halogen, forming a halide ion which leaves.
• Halogenoalkanes react with excess concentrated ethanolic ammonia and is carried out under pressure. The product is an alkylamine (R-NH2).

  e.g.

  Bromopropane + ammonia → propylamine + ammonium bromide.
• Reaction conditions for nucleophilic substitution of haloalkane with ammonia

  • requires heating
  • excess ethanolic ammonia (ethanol as a solvent to dissolve ammonia)
• Mechanism for nucleophilic substitution of haloalkane with ammonia

  1. The lone pair on the ammonia molecule are attracted and donated to the electron deficient carbon atom in the halogenoalkane (called nucleophilic attack).
  2. The donation of the electron pair leads to the formation of a covalent bond between the carbon and ammonia molecule.
  3. The carbon-halogen bond breaks by heterolytic fission. Both electrons from the bond move to the halogen, forming a halide ion. 
  4. An ammonia molecule removes a hydrogen from the NH3 on the carbon chain, forming an akylamine and an ammonium ion.
• Elimination reactions can be favoured by increasing the temperature, using a concentrated solution of sodium/potassium hydroxide and using ethanol as a solvent.
• Mechanism for the elimination of a halogenoalkane

  1. OH- ions act as a base and takes a proton (H+) from a carbon in the haloalkane, this forms water.
  2. The carbon now has a spare electron so forms a double bond with the carbon bonded to a halogen next to it
  3. This results in heterolytic fission of the carbon-halogen bond to form the double bond and the halogen is lost as an ion.
• Warming a haloalkane with hydroxide ions dissolved in ethanol, rather than water, causes an elimination reaction. An alkene is formed as a result.

  
• Reaction conditions for hydrolysis of haloalkanes

  • requires heating
  • aqueous solution of hydroxide ions (dissolved in water)
  • ethanol to dissolve haloalkane
• Explain how CFCs can contribute to global warming

  • it absorbs infrared radiation
  • molecule has polar bonds