1. Experiment is contained in the WACKER's Experimental Kit.

No

 2. Experimental procedure has been modified

/

 3. A separate experimental procedure has been devised

Yes

 4. Video clip available

Yes (wmv or mov).

 5. Flash animation available

No

 6. Other materials: Worksheet 7, Worksheet 4, Worksheet 1, Slide DH8

Rate of Hydrolysis of Chloromethylsilanes

TopDown 1 Materials, Chemicals, Time Needed
  • Retort
  • Retort clamps and sleeves
  • Magnetic hot plate
  • Magnetic stirrer bar
  • Thermometer (measuring up to 100 °C)
  • Conductivity meter
  • Voltmeter (3 V ~)
  • Ammeter (30 mA ~)
  • Variable ac source
  • Stop-watch
  • 3 glass beakers, 250 ml, tall
  • Glass beaker, 25 ml
  • 3 test tubes
  • 3 suitable rubber stoppers
  • Test-tube rack
  • Balance
  • Small conical flask
  • 3 pipets (15 ml, 20 ml, 25 ml)
  • Pipet bulb
  • Chlorotrimethylsilane (M), C, F
  • Dichlorodimethylsilane (D), Xi, F
  • Trichloromethylsilane (T), Xi, F
  • Universal indicator paper
  • Hydrochloric acid, c = 2 mol/l

The whole experiment takes about 35 minutes. Allow 20 minutes for preparation, including calibrating the measuring equipment, 11 minutes for the actual hydrolyses and the rest for rearranging and intermediate cleaning.

TopDown 2 Procedure and Observations
As chloromethylsilanes hydrolyze very easily to release hydrogen chloride, perform the experiment in a fume cupboard.
Wear safety glasses, rubber gloves and laboratory coat.

Introduce 50 ml water into each of three glass beakers. Measure each of the chloromethylsilanes into a test tube and keep it ready: add 10.86 g chlorotrimethylsilane to the first, 6.45 g dichlorodimethylsilane to the second and 4.98 g trichloromethylsilane to the third test tube. Seal the test tubes. Set up the measuring apparatus in a fume cupboard as shown in the following sketch.

TopDown Before conducting the individual measurements, dip the conductivity meter in hydrochloric acid, c = 2 mol/l, and adjust the voltage until a useful current reading (from 10 mA to 15 mA) is obtained without any visible gas formation on the electrodes. Clean the conductivity meter and suspend it in the water for the first hydrolysis measurement. Set the stirrer to a medium speed. Start the stop-watch at the same time as adding the chlorotrimethylsilane and observe the change in current strength. After the current has remained constant for 60 seconds, stop the experiment. Do not change the set voltage on the power supply.
Clean the conductivity meter and the thermometer and set up the experiment again for the next beaker of water. Perform the second measurement on dichlorodimethylsilane.
In both cases, the reaction mixture turns hazy when the chloromethylsilane is added, the current strength increases rapidly and reaches a constant value after about 25 seconds. When the reaction mixture is left to stand for a prolonged period, two liquid phases separate out.
Do not use the conductivity meter when hydrolyzing trichloromethylsilane as it might become damaged. Hydrolysis, and the subsequent condensation, occurs as soon as the trichloromethylsilane is added. A solid silicone is produced that would stick to the contacts of the conductivity meter.
The reaction conditions are summarized below:

  • Chlorotrimethylsilane 10.86 g + 50 g water
  • Dichlorodimethylsilane 6.45 g + 50 g water
  • Trichloromethylsilane 4.98g + 50 g water (current is not measured in this case)
  • Room temperature 295 K
  • Measuring voltage U = 1 V
  • Measured current in HCl (aq), c = 2 mol/l: I = 10 mA

TopDown The experimental results are summarized in the following table:

Time in s
Chlorotrimethylsilane
Current in mA
Dichlorodimethylsilane
Current in mA
5
6
10
10
9
10.5
15
10
10.5
20
10.5
10.5
25
10.5
10.5
30
10.5
10.5
60
10.5
10.5
120
10.5
10.5
180
10.5
10.5
240
10.5
10.5
300
10.5
10.5

TopDown 3 Discussion of Results

None of the three chloromethylsilanes is resistant to water and all three hydrolyze to methylsilanols and hydrogen chloride. The equation for the reaction of dichlorodimethylsilane is shown here as an example:
The chloride and hydronium ions formed cause the current strength to increase in proportion to their concentration. The change in current strength is therefore a direct measure of the state of the reaction and may be used to determine the rate of the reaction. The quantities of materials used in the experiment are calculated to produce hydrochloric acid in a concentration of c = 2 mol/l in each glass beaker if hydrolysis goes to completion.
A plot of the readings yields the following current-time curves:

It is clearly evident that chlorotrimethylsilane hydrolyzes somewhat more slowly than dichlorodimethylsilane, but the reaction is still fairly rapid (it is complete after 25 seconds). The amperometric measurement provides no information about the speed at which the silanols formed during the hydrolysis condense. A qualitative impression is given, however, by the phase separation that occurs after hydrolysis (see above).

Whereas the condensation of monofunctional trimethylsilanol finishes with the formation of low-viscosity hexamethyldisiloxane, the other two condensation reactions yield high-viscosity and solid products.

The equation for chain formation during the condensation of dimethylsilanediol is:

TopDown 4 Tips and Comments

  • On the basis of the results, only the hydrolysis of chlorotreimethylsilane is recommended for amperometric measurements – the other two chloromethylsilanes hydrolyze too quickly.
  • The structural similarity of chlorotrimethylsilane and 2-chloro-2-methylpropane (tert.-butyl chloride) is reason enough to perform the experiments described here on the two chlorine derivatives with a view to comparing nucleophilic substitution at the Si atom and at the C atom.
  • All hydrolysis experiments described here are exothermic. If the experiment is started at room temperature, the measurements will show a rise in temperature to about 40 °C.
    Some of the dichlorodimethylsilane hydrolysis product may be used after brief conditioning in the experiment "Burning of liquid silicones".
  • The dichlorodimethylsilane hydrolysis products are suitable for the experiment "Viscosity of silicone fluids", especially if samples from previous experiments that have been standing around for some time are available.

TopDown  5 Supplementary Information

Given the outstanding importance of hydrolysis and subsequent condensation of chloromethylsilanes in the synthesis of silicones, these simple, yet informative experiments should be taught in lectures and used for practical work at university, as well as for advanced chemistry students at school. In the form described here, they complement the information on nucleophilic substitution and industrial production of silicones, which is contained in the textbook indicated below.
TopBottom  6 References
M. Tausch, M. von Wachtendonk (editors), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993), (1998), S. 337f
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