How is water hydrolyzed

Rate of hydrolysis of chloromethylsilanes

1 materials, chemicals, time required
  • tripod
  • Stand clamps and sleeves
  • Magnetic stir plate
  • Magnetic stir bars
  • Thermometer (up to 100 ° C)
  • Conductivity tester
  • Voltmeter (3 V ~)
  • Ammeter (30 mA ~)
  • adjustable AC voltage source
  • stopwatch
  • 3 beakers, 250 ml, tall shape
  • Glass beaker, 25 ml
  • 3 test tubes
  • 3 matching rubber stoppers
  • Test tube rack
  • Libra
  • small Erlenmeyer flask
  • 3 pipettes (15 ml, 20 ml, 25 ml)
  • Peleus ball
  • Chlorotrimethylsilane (M), C, F
  • Dichlorodimethylsilane (D), Xi, F
  • Trichloromethylsilane (T), Xi, F
  • Hydrochloric acid, c = 2 mol / l

The entire experiment takes about 35 minutes. 20 minutes are set aside for the preparation with setting up the measuring apparatus, 11 minutes for the actual hydrolysis and the remaining minutes for conversion and intermediate cleaning.

2 Carrying out and observing the experiment
Since the chloromethylsilanes hydrolyze very easily and produce hydrogen chloride, the experiment must be carried out under the fume hood.
Safety glasses, rubber gloves and lab coats go without saying.

Three beakers are filled with 50 ml of water each. The chloromethylsilanes are provided in three test tubes: 10.86 g of chlorotrimethylsilane are placed in the first test tube, 6.45 g of dichlorodimethylsilane in the second and 4.98 g of trichloromethylsilane in the third. The test tubes are closed. The measuring device is set up under the fume cupboard according to the following experimental sketch.

Before the individual measurements are carried out, the conductivity tester is immersed in hydrochloric acid, c = 2 mol / l, and the voltage is set in such a way that a usable current measured value (from 10 mA to 15 mA) can be read without gas formation on the electrodes is. The conductivity tester is cleaned and hung in the water for the first hydrolysis. Then the stirrer is set to medium speed. At the same time as the chlorotrimethylsilane is added, the stopwatch is started and the change in current intensity is monitored. If the current strength is constant for approx. 60 s, the experiment is ended. The voltage set on the power supply unit is not changed.
The conductivity tester and the thermometer are cleaned and the experimental setup is restored with the next beaker of water. The second measurement is carried out with dichlorodimethylsilane.

In both cases, the reaction mixture becomes cloudy when the chloromethylsilane is added, the current strength increases rapidly and reaches a constant value after approx. 25 s. If left to stand for a long time, two liquid phases separate from each other in both cases.

Trichloromethylsilane is hydrolyzed without a conductivity tester, as this could be damaged. Like the subsequent condensation, the hydrolysis takes place immediately when the trichloromethylsilane is poured in. A silicone precipitates as a solid, which would stick the contacts of the conductivity tester.

The summary of the reaction conditions is as follows:

  • Chlorotrimethylsilane: 10.86 g + 50 g water
  • Dichlorodimethylsilane: 6.45 g + 50 g water
  • Trichloromethylsilane: 4.98 g + 50 g water (no current measurement here)
  • Room temperature: 295 K
  • Measurement voltage: U = 1 V
  • Measurement current in HCl (aq), c = 2 mol / l: I = 10 mA

The measured values ​​obtained are shown in the following table:

Current in mA
Current in mA

3 Evaluation of the experiment

All three chloromethylsilanes are not resistant to water and hydrolyze to methylsilanols and hydrogen chloride. The reaction equation for dichlorodimethylsilane is given as an example:
The resulting chloride and hydronium ions cause the current to increase proportionally to their concentration. The change in the current strength provides direct information about the progress of the reaction and can be used to determine the reaction speed. The amounts of substance used in the experiment are calculated in such a way that hydrochloric acid with a concentration of c = 2 mol / l is present in the respective beaker when hydrolysis is complete.
The graphical representation of the measured values ​​results in the following current-time curves:
It can be clearly seen that the hydrolysis of chlorotrimethylsilane takes place somewhat more slowly than that of dichlorodimethylsilane, but still relatively quickly (it is complete after 25 s). The amperometric measurements do not provide information about the speed with which the condensation of the silanols that arise during hydrolysis takes place. This gives a qualitative impression based on the phase separation after hydrolysis (see above).
While in the case of the monofunctional trimethylsilanol the condensation ends with the low-viscosity hexamethyldisiloxane, macromolecular, highly viscous or solid products are formed in the other two cases.
The reaction equation for the chain formation during the condensation of dimethylsilanediol is:

4 tips and notes

  • Based on the results obtained, only the hydrolysis of chlorotrimethylsilane can be recommended for amperometric measurements because the other two chloromethylsilanes hydrolyze too quickly.
  • The structural analogy between chlorotrimethylsilane and 2-chloro-2-methylpropane (tert-butyl chloride) gives rise to the experiment described here with the two chlorine derivatives in order to compare the nucleophilic substitution on the Si atom with that on the C atom.
  • All hydrolyses described here are exothermic. The temperature measurement shows an increase to approx. 40 ° C if you start at room temperature.
  • Some of the hydrolysis product of dichlorodimethylsilane can, after a short period of aging, be used in the experiment "burning off liquid silicones".
  • The products of dichlorodimethylsilane hydrolysis are well suited for the "viscosity of silicone oils" experiment, especially when product samples are available from different batches that have been standing for a long time.

5 Additional factual information

In view of the outstanding importance of the hydrolysis and subsequent condensation of chloromethylsilanes in the synthesis of silicones, these simple and meaningful experiments should also find their way into teaching, experimental lectures and internships at the university, but also in upper secondary level. In the form described here, they supplement the content on nucleophilic substitution and the technical production of silicones presented in the textbook given below.
6 literature
M. Tausch, M. von Wachtendonk (Ed.), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993), (1998), pp. 337f
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