The chemistry of haloalkanes

• Aliphatic haloalkanes = the halogen is joined to a straight or branched carbon chain
• Haloalkanes can be classified as primary, secondary, and tertiary
• In haloalkanes the halogen in the carbon-halogen bond is more electronegative than the carbon. This makes the carbon-halogen bond polar.
• In haloalkanes, the carbon atom attached to the halogen is slightly positive, attracting nucleophiles e.g:

  hydroxide ions

  water molecules

  ammonia molecules
• Nucleophile = atom or group of atoms attracted to an electron deficient molecule/atom where it donates a pair of electrons to form a new covalent bond
• When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction.

  A new compound is produced containing a different functional group. The reaction mechanism is nucleophilic substitution.
• Hydrolysis = a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule. This results in the molecule being split into two products
• In the hydrolysis of a haloalkane the halogen atom is being replaced by an -OH :

  1. the nucleophile, OH-, approaches the carbon attached to the halogen from the opposite side of the molecule away from the halogen
  2. this direction of attack by the OH- minimises the repulsion between the nucleophile and the slightly negative halogen atom
  3. a lone pair of electrons on the OH- is attracted and domnated to the slightly positive carbon atom
  4. a new bond is formed between the oxygen atom from the OH- and the carbon atom
  5. the carbon-halogen bond breaks
  6. the new organic product is an alcohol and halide ion
• Haloalkanes can be converted to alcohols using aqueous sodium hydroxide. The reaction is very slow at room temp so the mixture is heated under reflux to obtain good yield of product
• The rate of hydrolysis depends on the strength of the carbon-halogen bond in the haloalkane. C-F is the strongest and unreactive because of fluorine's electronegativity. C-I is the weakest because I is the least elecronegative
• Measuring the rate of hydrolysis of primary haloalkanes

  Comparing 1-chlorobutane, 1-bromobutane, and 1-iodobutane:

  The general equation for the hydrolysis of these haloalkanes is:

  $CH_3CH_2CH_2CH_2X +$$H_2O -> CH_3CH_2CH_2CH_2OH +$$H^+$$+$$X^-$ X represents the halogen.

  The rate of reaction can be followed by carrying out the reaction in the presence of silver nitrate.

  As the reaction takes place, halide ions are produced which react with Ag+ ions to form a precipitate of the silver halide.

  The nucleophile in the reaction is water, which is present in the aqueous silver nitrate
• When reacting haloalkanes with aqueous silver nitrate to compare rates of hydrolysis, the reaction is carried out in the presence of an ethanol solvent because haloalkanes are insoluble in water. Ethanol allows the water and the haloalkane to mix and produce a single solution rather than two layers.
• Hydrolysis of haloalkanes:

  1. set up three test tubes as follows: Tube 1 = add 1cm3 of ethanol and 2 drops of 1-chlorobutane. Tube 2 = 1cm3 of ethanol and 2 drops of 1-bromobutane. Tube 3 = 1cm3 of ethanol and 2 drops of 1-iodobutane
  2. stand test tubes in water bath at 60 degrees c
  3. place a test tube containing 0.1 moldm-3 silver nitrate in the water bath and allow all tubes to reach a constant temp
  4. add 1 cm3 of the silver nirtate quickly to each of the test tubes. Immediately start a stop clock
  5. observe the test tubes for 5 mins and record the time taken for precipitate to form
• Haloalkanes reacting with aqueous silver nitrate:

  • 1-chlorobutane = white precipitate forms slowly
  • 1-bromobutane = cream precipitated forms slower than iododbutane but faster than chlorobutane
  • 1-iodidbutane = yellow precipitate forms rapidly