Experiments - Hydrolysis of chloromethylsilanes

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

Hydrolysis of Chloromethylsilanes

1 Materials, Chemicals, Time Needed

Test-tube rack
6 test tubes
3 pipets (2 ml)
Pipet bulb
Graduated cylinder (20 ml)
Dropping pipet
Glass rod
Suction bottle
Büchner funnel
Filter adapter
Water-jet pump

Chlorotrimethylsilane (M), C, F
Dichlorodimethylsilane (D), Xi, F
Trichloromethylsilane (T), Xi, F
Universal indicator paper
Filter paper

The whole experiment takes about 15 minutes, with 5 minutes for preparation, 3 minutes for hydrolysis and 3 minutes for the pH test. The remaining time is needed for preparing the silicone resin powder.

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.

In the fume cupboard, fill three test tubes with 6 ml, 12 ml and 18 ml water. To three other test tubes, add 2 ml each of the different chloromethylsilanes. Proceed as follows:
Add 6 ml water to the test tube containing chlorotrimethylsilane and observe the reaction. The mixture initially turns hazy and then two liquid phases form.
Add 12 ml water to the test tube containing dichlorodimethylsilane and observe the reaction. A vigorous reaction occurs, and a gas is evolved. The lower, initially hazy phase gradually turns clear, just like the upper phase.
Add 18 ml water to the test tube containing trichloromethylsilane and observe the reaction. Strong evanescence occurs, a gas is evolved and a solid forms that sticks firmly to the walls of the test-tube and is difficult to remove.

When the reaction is complete in each case, measure the pH of the aqueous phase. In all three cases, a strongly acidic pH of 1 is measured.
Stir the trichloromethylsilane hydrolysis product with the glass rod, filter through the Büchner filter, wash and dry under vacuum.
The photo shows the hydrolyses shortly after the reactions are complete. The hydrolysis product of dichlorodimethylsilane (D, see photo) is a slightly viscous liquid whereas the corresponding product of chlorotrimethylsilane (M, see photo) has a low viscosity similar to that of the three chloromethylsilanes. The trichloromethylsilane hydrolysis product (T, see photo), when dried, is a coarse white powder that crumbles finely in the hand.

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 methylsilanols condense immediately to larger molecules. In the case of the monofunction trimethylsilanol, condensation ceases with the formation of hexamethyldisiloxane, a silicon compound whose molecules have a structure similar to that of an ether. Hexamethyldisiloxane is a low-viscosity liquid with a fairly high vapor pressure.
Dimethylsilanediol condenses to larger molecules that may be either chain-like or cyclic. These are polydimethylsiloxanes, better known as silicones. The longer the chains, the more viscous is the product. The formation of chain-like silicone macromolecules is shown below:

Methylsilanetriol, the trimethylsilane hydrolysis product, condenses very rapidly to extensively crosslinked silicone macromolecules.

4 Tips and Comments

The hydrolysis of trichloromethylsilane is one of the few reactions in which a macromolecular material can be produced so easily, quickly and convincingly. Experienced experimenters can use this example as a demonstration.
The hydrolysis of dichlorodimethylsilane and the spontaneous condensation of the product are also suitable for demonstration purposes, both in experimental lectures and in school lessons.

5 Supplementary Information

These experiments complement the information on nucleophilic substitution and industrial production of silicones which is contained in the textbook indicated below.

6 References

M. Tausch, M. von Wachtendonk (editors), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993), (1998), S. 337f., S. 345f., S. 340