Investigate the hydrolysis of halogenoalkanes.

The following experiments examine the effect on the rate of hydrolysis when (a) the halogen is changed and (b) primary, secondary and tertiary halogenoalkanes and a halogenoarene are used. Ethanol is used as a common solvent for the halogenoalkane and for the silver nitrate solution. Water (from the silver nitrate solution) is used as hydrolysis agent in place of hydroxide ions, which tend to react too quickly for comparisons to be made.

Environmental care:

The aqueous residues will contain the heavy metal ions of Ag and the organic residues will contain halogenated hydrocarbons so it is important not to dispose of any waste down the sink. Place all residues in the marked beaker in the fume cupboard. At the end of the practical, the two immiscible layers in the combined waste can be separated and the aqueous layer transferred to the 'heavy metals waste' container and the remainder to the 'organic waste' container.

Safety:

· As only small quantities are being used there are no particular hazards associated with this practical. However, it is worth noting that some chlorinated organic compounds e.g. dioxin, 2,4,5-trichlorophenoxyethanoic acid (Agent Orange) and polychlorinated biphenyls (PCB's) can be extremely poisonous.

· Ethanol is highly flammable, so I will aboid exposing it to direct flame throughout the experiment.

· Wear eye protection at all times.

Prediction for Experiment

Due too the mechanism of being nucleophilic attack, the bond enthalpy is the important factor of the reaction because it is the Carbon-Halogen bond that is the major determinant in terms of how willing a halogenoalkane will be to undergo nucleophilic substitution; much more so than the size of the +(ve) charge By looking at the bond enthalpies of the three chemicals, it is possible to see that the bond enthalpies decrease from C-Cl to C-I.

Bond Bond Enthalpy /kJ mol-1

C-Cl 346

C-Cl 290

C-I 228

As a consequence of this difference in bond enthalpy, I predict that the 1-iodobutane will be quickest to form a precipitate by being displaced fastest. Next I predict will be the 1-bromobutane and the 1-chlobutane will be the last to form a precipitate because it has the highestt bond enthalpy.

The reaction will be easy to follow due to the use of aqueous silver nitrate as an indicator. This is because silver nitrate is a salt which when dissolved in water will form Ag+ and NO3- ions, neither of which react with the halogenoalkanes. The Ag+ however, will react with halide ions to form insoluble products which appear as precipitates. This turns an otherwise undistinguishable reaction into one which is observable by measuring the time it takes for a precipitate to form.

Procedure/Method:

. In each of three test-tubes place 1 cm3 of ethanol.

2. Using separate teat pipettes place 2 drops of 1-chlorobutane in the first test-tube, 2 drops of 1-bromobutane in the second and 2 drops of l-iodobutane in the third.

3. Stand all three test-tubes in a beaker of water at about 60-65 oC and place another test-tube containing about 5 cm3 of 0.05 mol.dm-3 silver nitrate solution in the warm water.

4. Wait until the contents of all the test-tubes have reached approximately 60 oC and then place 1 cm3 of the silver nitrate solution into each of the other test-tubes, and quickly shake each tube to mix the contents.

5. Add a bung to the top of test tubes 1to3 to avoid loss of solution due to precipitation.

6. Record the time with three identical timers (same margin of error) and stop the timer when it's corresponding test tube displays a change. i.e. formation of a precipitate.

7. Put your results into a table

Evaluation

In this experiment, I felt I was able to obtain results which confirm my prediction and show 1-chlorobutane as the slowest to form a precipitate and 1-iodobutane as the fastest to form a precipitate.

Errors

Maintaining the temperature in the beakers at 600c was difficult but I don't think this was a major error since the deviations from the optimum temperature were slight and all the beakers would have undergone any temperature changes at the same time so I don't think this had a big effect on my results.

Improvements

If I did this experiment again, I would definately do repeats of the experiment in order to be able to find an average result and look for a definite correlation between time taken for precipitate to form and bond enthalpy of the halogenolakane in question.