Haloalkanes

• A haloalkane is an alkane with at least one halogen atom in place of a hydrogen atom.

  
• Polarity of haloalkanes

  • Halogens are much more electronegative than carbon, so the carbon-halogen bond is polar.
  • The &+ carbon is electron deficient (it doesn't have enough electrons).
  • This means it can be attacked by a nucleophile.
  • A nucleophile's an electron-pair donor. It donates an electron pair to somewhere without enough electrons.

  
• Examples of nucleophiles

  
• Nucleophilic substitution reactions

  • Haloalkanes react in nucleophilic substitution reactions. In a nucleophilic substitution reaction, a nucleophile attacks a polar molecule, kicks out a functional group and settles itself down in its place.
  • The carbon can only be bonded to four other atoms so the addition of the nucleophile breaks the bond between the carbon and the halogen - this is shown by another curly arrow. The pair of electrons from the carbon-halogen bond are taken by the halogen and become a lone pair.

  
• Hydrolysis of haloalkanes

  • In hydrolysis reactions, molecules are split up by water molecules (which also split apart).
  • Haloalkanes can be hydrolysed to make alcohols with either an aqueous alkali or water.
• Hydrolysis with aqueous alkali


  • Haloalkanes are hydrolysed by warm aqueous alkali (reflux), such as sodium hydroxide (NaOH) solution or potassium hydroxide (KOH) solution, in a nucleophilic substitution reaction.
  • The alcohol and the new nucleophile are produced.

  
• Hydrolysis with water


  • The water molecule is a weak nucleophile, but it will eventually substitute for the halogen
  • it's just a much slower reaction than the one using a warm aqueous alkali.
  • You get an alcohol produced again with the new nucleophile.

  
• Bond enthalpy and hydrolysis

  • How quickly different haloalkanes are hydrolysed depends on bond enthalpy.
  • Weaker carbon-halogen bonds break more easily so they react faster.
  • lodoalkanes have the weakest bonds, so they hydrolyse the fastest.
  • Fluoroalkanes have the strongest bonds, so they're the slowest at hydrolysing.
  • If you've got a molecule with more than one halogen in it, the halogen with the lowest bond enthalpy will get replaced first.
  • To hydrolyse haloalkanes with high bond enthalpies you need to use harsher reaction conditions - for example, a higher temperature.

  
• Comparing hydrolysis rates of haloalkanes

  • If you put silver nitrate solution in the mixture too, the silver ions will react with the halide ions as soon as they form, giving a silver halide precipitate:
  • Ag+(aq) + X- (aq) -> AgX (s)
  • You can use this reaction to compare the reactivities of different haloalkanes.
• Comparing hydrolysis rates of haloalkanes method:

  1. To do this, set up four flasks each containing the same amount of a different haloalkane, ethanol (as a solvent) and dilute silver nitrate solution.
  2. You can measure the rates of the reactions by timing how quickly each silver halide is precipitated, using the disappearing cross method.
  3. Put a piece of paper with a cross on it under each flask and measure how long it takes until you can't see the cross any more.
  4. The hydrolysis of haloalkanes by water can be very slow. So, you can warm all the reaction flasks to the same temperature.

  
• Identifying unknown haloalkanes

  • React a sample of the haloalkane in a test tube with water in the presence of silver nitrate solution and ethanol.
  • The colour of the precipitate that forms will tell you what halogen is present in the haloalkane — yellow means it's an iodoalkane, cream means it's a bromoalkane and white means it's a chloroalkane .