C13 halogenoalkanes

• halogenoalkanes contain polar bonds because the halogen atoms are much more electronegative than the carbon atoms
• in a carbon-halogen bond, the carbon is delta-positive and the halogen is delta-negative
• the main intermolecular forces in halogenoalkanes are dipole-dipole attractions, and van der Waals forces
• the carbon-halogen bond is not polar enough to make halogenoalkanes soluble in water, but they do mix with hydrocarbons
• the boiling point of halogenoalkanes increases with increased chain length because the larger the molecule, the stronger the van der Waals forces
• the boiling point of halogenoalkanes increases going down group 7 because the halogen atom has more electrons so the van der Waals forces are stronger
• when halogenoalkanes react, typically the carbon-halogen bond breaks
• the factors which determine how readily the carbon-halogen bond breaks are the bond polarity and the bond enthalpy
• a nucleophile is a species that has a lone pair of electrons with which it can form a covalent bond by donating the electrons to an electron-deficient carbon atom
• common nucleophiles are:

  • the hydroxide ion
  • ammonia
  • the cyanide ion
• nucleophilic substitution is when a nucleophile attacks a positively charged atom, replacing the group that was attached to it
• nucleophilic substitution can be used to turn halogenoalkanes into alcohols, amines or nitriles
• nucleophilic substitution mechanism:

  • lone pair of nucleophile attacks delta-positive carbon
  • carbon has too many bonds so other group on carbon leaves, taking the electrons from the bond with it
  • if the nucleophile is ammonia, the nitrogen atom also has too many bonds, so a hydrogen is attacked by another ammonia nucleophile so is lost, giving the electrons to the nitrogen atom
• halogenoalkanes typically react by nucleophilic substitution, but under different conditions they react by elimination
• in an elimination reaction, a hydrogen halide is eliminated from the halogenoalkane, leaving a double bond in its place, so an alkene is formed
• the conditions for elimination are high temperature and alcoholic hydroxide ions, usually KOH or NaOH
• the conditions for nucleophilic substitution are low temperature and aqueous hydroxide ions, usually KOH or NaOH
• elimination mechanism:

  • lone pair of nucleophile attacks a hydrogen on the carbon next to the carbon-halogen bond as the hydrogen is slightly delta-positive
  • the electrons from the C-H bond are given to the C-C bond, forming a C=C bond
  • the carbon now has too many bonds so the halogen leaves, taking the electrons from the carbon-halogen bond with it
• chlorofluorocarbons (CFCs) are halogenoalkanes containing chlorine and fluorine instead of hydrogen atoms
• CFCs are very unreactive under normal conditions, and were used as aerosols, solvents and refrigerants, until they had to be banned for environmental reasons
• CFCs end up in the atmosphere where they decompose to form chlorine atoms, which decompose ozone O3, forming a hole in the ozone layer