Reactions of Haloalkanes (OCR AS Chemistry)

Reactivity of Haloalkanes 

• Haloalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
• The carbon-halogen bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge

Due to the large difference in electronegativity between the carbon and halogen atom, the C-X bond is polar

• Because of this, haloalkanes will undergo two key types of reaction
• Nucleophilic substitution reactions
  • A halogen is substituted for another atom or group of atoms
  • The products formed when haloalkanes undergo this type of reaction are alcohols, amines and nitriles

• Elimination reactions
  • A hydrogen halide is eliminated during the reaction
  • The key product formed from this type of reaction is an alkene

Formation of alcohols

• The nucleophile in this reaction is the hydroxide, OH^- ion
• An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
• Halogenoalkanes are generally insoluble in water
  • This means that there would be an organic layer of the halogenoalkane and an aqueous layer of sodium hydroxide
  • Any reaction would only occur where the two layers meet
  • The use of ethanol allows the aqueous sodium hydroxide layer and the organic halogenoalkane layer to mix, allowing the reaction to proceed
• This reaction is very slow at room temperature, so the reaction mixture is warmed
• This is an example of a hydrolysis reaction and the product is an alcohol
  • The rate of this reaction depends on the type of halogen in the haloalkanes
  • The stronger the C-X bond, the slower the rate of the reaction
  • In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
  • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

The halogen is replaced by the nucleophile, OH^- 

• This reaction could also be done with water as the nucleophile, but it is very slow
  • The hydroxide ion is a better nucleophile than water as it carries a full negative charge
  • In water, the oxygen atom only carries a partial charge

A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than the aqueous alkali

Measuring the rate of hydrolysis

• Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
• Set up three test tubes in a 50 ^oC water bath, with a mixture of ethanol and acidified silver nitrate
• Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stop watch
• Time how long it takes for the precipitates to form
• The precipitate will form as the reaction progresses and the halide ions are formed
• A white precipitate will form from the chloroalkane, a cream precipitate will form from the bromoalkane and a yellow precipitate will form from the iodoalkane
  • The yellow precipitate will form the fastest
  • This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I^- ions to form the fastest
  • The white precipitate will form the slowest
  • This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl^- ions to form the slowest