In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Enchanted Glass submitted by:
Sándor Szabó L. Chemical Research Center, Hungarian Academy of Sciences, POB 17, H-1525 Budapest, Hungary Károly Mazák Institute of Pharmaceutical Chemistry, Semmelweis University, H-1092 Budapest, Hungary ˝ Knausz Dezs o Department of General and Inorganic Chemistry, L. Eötvös University, H-1117 Budapest, Hungary Márta Rózsahegyi* Department of Inorganic and Analytical Chemistry, L. Eötvös University, Pázmány s. 1/A, H-1117 Budapest, Hungary;
[email protected] checked by:
Gordon A. Parker Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI 48128-1491
Silicon is the most abundant element of the earth’s crust, after oxygen. Silicon and its compounds have several interesting and important applications that can be listed when teaching about this element. They are used in semiconductors, glass, building materials, and the organic siloxane polymers called silicones. However, few memorable demonstrations have been published. This is especially true for silicones, which have a wide range of application owing to their heat resistance and hydrophobicity (1–7). The following experiments present the hydrophobizing and organophilic properties of silicones, enriching the teaching about silicon compounds and demonstrating structure– property relationships. It is well known that glass is hydrophilic and becomes wet with water. The reason for this is that glass possesses on its surface polar OH groups that can take part in hydrogen bonds. With appropriate procedures, the surface of glass can be rendered hydrophobic. One way is to substitute the hydrogens of the OH groups with trimethylsilyl groups, thus silylating the surface. Several silylating agents can be used for this purpose. In our experiments we used trimethylsilyl N,Ndimethylcarbamate, which is commercially available1 or can be prepared according to a published procedure (8): 2(CH3)2NH + CO2 → (CH3)2NC(O)O᎑(NH2(CH3)2)+ (CH3)2NC(O)O᎑(NH2(CH3)2)+ + ClSi(CH3)3 → (CH3)2NC(O)OSi(CH3)3 + (NH2(CH3)2)+Cl ᎑ Preparatory Procedures
Preparation of Trimethylsilyl N,N-Dimethylcarbamate Dimethylamine (54.2 g, 1.2 mol) was condensed into a 500-mL three-necked round-bottomed flask equipped with a stirrer, a drying tube, a reflux condenser, and a gas inlet and anhydrous dichloromethane (100 mL) was added. The dimethylammonium salt of N,N-dimethylcarbamate was obtained by saturating the amine with dry carbon dioxide gas with cooling
and stirring. The gas inlet tube was removed and a dropping funnel was inserted in its place, and a solution of trimethylchlorosilane (65.3 g, 0.6 mol) in dichloromethane (50 mL) was added with cooling and stirring. The mixture was allowed to warm to room temperature, and then stirred for a further 0.5 h. The aminehydrochloride was removed by filtration, and the precipitate was washed with anhydrous dichloromethane. The solvent was distilled from the filtrate and 100 mL of n-pentane was added to the residue to precipitate any aminehydrochloride still present; the precipitate was removed by filtration. The pentane was distilled from the solution and the product was purified by vacuum distillation. The agent must be kept from moisture. Yield: 91.8–92.7 g (95–96%), bp 28–29 °C/1.5 mbar.
Silylating Reaction The silylating reaction is as follows: –OH + (CH3)2NC(O)OSi(CH3)3 → (CH3)2NH + CO2 + –OSi(CH3)3
Sample Preparation The characteristics of hydrophobic glass are presented with glass beads of different size (1–5 mm diam), capillaries (1–1.5 mm i.d.), beakers, and glass sheets. All objects to be used should be activated to make their surfaces comparable and the ensuing reactions more effective. First the glass objects should be degreased by soaking in hexane. Then they should be soaked in 2 M HCl for at least 30 min, which increases the number of Si–OH groups on the glass surface. After the glass is treated with hydrochloric acid, residual acid should be removed from the surface with distilled water; the remaining water is removed with anhydrous ethanol. After these washing procedures, the objects should be dried in air. After drying, the glass beads should be placed in a dry vessel fitted with a ground-glass stopper. We prepare a 5–10% solution of the pure silylating agent (8) in hexane and add enough of the solution to cover the glass beads, then stopper
JChemEd.chem.wisc.edu • Vol. 78 No. 3 March 2001 • Journal of Chemical Education
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In the Classroom
the vessel. After 10 min the beads should be taken out of the solution and dried on a watch glass. An additional drying process should be carried out for 15 min at 120 °C. The glass capillary should be placed in a long test tube having a ground glass stopper and should be covered with the solution of the silylating agent. Then the test tube should be stoppered. After 10 minutes, the capillary should be taken out of the solution and dried in air. Further procedures are identical with those for the glass beads. The beaker should be filled to the brim with the reagent solution and placed in a vessel that is stoppered with a ground glass stopper. After 10 min the solution should be poured out of the beaker, and the beaker should be placed in the solution of the silylating agent with the orifice down, so that the brim of the beaker is covered by the solution. Further procedures are identical with those described above. The glass sheets (microscopic slides) should be placed into the solution of the silylating agent. Further procedures are identical with those described for the glass capillary. The pure silylating agent can be stored for extended periods if protected from moisture. The hexane solution and the unused silylating agent can be disposed of by adding water, which reacts to give dimethylamine, carbon dioxide, and hexamethyldisiloxane, which can then be disposed of as organic waste. Hazards All procedures should be carried out under the hood. Contact of skin with the silylating agent should be avoided. Trimethylsilyl-N,N-dimethylcarbamate is flammable (USA); highly flammable (EU). Do not breathe vapor; avoid contact with skin and eyes; keep away from source of ignition— no smoking; keep container tightly closed in a cool, wellventilated place; take precautionary measures against static discharges; wear suitable protective clothing, gloves, and eye/ face protection. The compound is moisture sensitive.
considerable difference in water-levels is observed. In the silylated capillary the water surface is flat and is lower than in the other capillary, where it is concave and higher. If this capillary system is dried and filled with hexane, a similar phenomenon can be observed, but the effects are reversed. It is in the silylated capillary where the liquid level is higher. If the system is dried once again and filled with ethanol, the liquid levels will be almost identical. Experiment 4. A drop of water should be placed on a silylated and an unsilylated microscope slide. On the latter slide, especially when it is activated, the water drop spreads out as a shapeless blotch, whereas on the silylated slide it assumes the shape of a slightly flattened sphere. Experiment 5. A silylated and an unsilylated beaker should be washed with a 0.1 M AgNO3 solution. Then a 0.1 M KI solution should be poured into the beakers. In the unsilylated beaker the solution becomes quite turbid, whereas in the other the solution remains completely transparent, with no sign of precipitate formation.
Explanation of Experiments Experiment 1. The silylated beads do not become wet with water. Buoyancy and the surface tension originating at the boundary surface of glass, water, and air compensate for the weight of the beads; thus they float on the surface of the water, slightly deforming it. This deformation generates a small slope, and successively added beads move toward the already floating beads and stick together because of their organophilicity. The unsilylated beads become wet with water, the surface tension cannot compensate for their weight, and they sink to the bottom of the dish. Experiment 2. In the case of oil contamination on the surface of water, the adhesion of oil to the silylated, apolar beads is considerable; therefore it adheres strongly to the
Experiments with Glasses of Modified Surface Experiment 1. A crystallizing dish should be filled with water up to three-quarters of its volume. If we carefully place unsilylated glass beads (1–5 mm diam) on the surface of the water, using tweezers, they will sink to the bottom of the dish. In contrast, silylated beads float on the surface (Fig. 1). By close examination we can see that water draws away from the beads and its surface curves downward in the vicinity of the beads. When beads are placed one by one on the surface of water, not too far from one another, they move closer and closer until they stick together (Fig. 2), displaying various figures. Experiment 2. If the silylated beads (
