Investigation of the rates of hydrolysis (halogenoalkanes)

• Procedure Part 1

  1. Set up a water bath

  2. Take three test tubes and add 5 cm3 of ethanol to each one

  3. Add four drops of 1-iodobutane, 1-bromobutane and 1-chlorobutane to the respective test tubes

  4. Loosely place a bung in each test tube and place the test tubes in the water bath

  5. Take three clean test tubes and pour 5 cm3 of silver nitrate solution into each one, then place the test tubes in the water bath

  6. When the halogenoalkane–ethanol solutions have reached the temperature of the water bath, add one test tube of silver nitrate solution to one of the halogenoalkane–ethanol solutions and replace the bung, start the stop clock

  7. Measure the time taken for a precipitate to appear, stop the stop clock when the solution becomes cloudy

  8. Repeat for the other two halogenoalkanes
• Procedure Part 2


  Repeat Part 1 using 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane instead of the other halogenoalkanes
• NaOH can be used instead of water to hydrolyse the halogenoalkanes but then any excess NaOH must be neutralised by HNO3 before the AgNO3 is added. Otherwise a precipitate of Ag2O will form
• What is the objective ?

  To investigate the relative rates of hydrolysis of primary, secondary and tertiary

  halogenoalkanes and of chloro-, bromo- and iodoalkanes
• What are the safety procedures ?

  Wear eye protection

  avoid skin contact with the reactants

  must be no naked flames in the vicinity as ethanol/halogenoalkanes are highly flammable

  laboratory needs to be well ventilated to prevent inhalation fumes
• Hydrolysis of halogenoalkanes

  Nucleophilic substitution reaction

  A) S+

  B) S-

  C) H+

  D) X-

  E) water

  
• What are the important tips to consider?

  it is a nucleophilic substitution reaction

  the nucleophile is water

  NaOH can be used instead of water to hydrolyse the halogenoalkanes but then any excess

  NaOH must be neutralised by HNO3 before the AgNO3 is added. Otherwise a precipitate of

  Ag2O will form.
• Write an equation for the reaction of 1-bromobutane with water

  CH3CH2CH2CH2Br + H2O → CH3CH2CH2CH2OH + H+ + Br–
• Explain why ethanol is used in these reactions

  The halogenoalkanes are insoluble in water. Using ethanol ensures that the halogenoalkanes

  dissolve so they can react with the water molecules
• Explain why water is able to act as a nucleophile

  Water has lone pair(s) of electrons on the oxygen atom.
• Explain why water is used as the nucleophile rather than hydroxide ions

  If hydroxide ions were used, a precipitate of silver hydroxide would form instantly.
• Explain how it is not the electronegativity of the halogen that determines the relative rate of the nucleophilic substitution reaction

  The greater the electronegativity of the halogen the more ∂+ the carbon atom.

  A more ∂+ carbon will be attacked more readily by the nucleophile

  so it will result in a faster reaction

  but the reaction is slower/shows the opposite trend.