Wirt-Gast-Komplex β-Cyclodextrin und Phenolphthalein

1. Experiment is contained in the WAKER's Experimental Kit for Schools	yes
2. Experimental procedure has been modified	yes
3. Video clip available	yes als wmv oder mov
4. Flash animation available	yes Flash-Animation
5. Additional material: Worksheet 2

6. Host-Guest Complex: β-Cyclodextrin-Phenolphthalein

1 Materials, Chemicals, Time Needed

Scales
Spatula
Glass beaker (50 mL)
Glass beaker (20 mL)
Magnetic stirrer
Magnetic stir bar
Pipettes
Indicator paper
Watch glass

β-Cyclodextrin
Distilled water
1% solution of phenolphthalein in ethanol F
Sodium hydroxide solution conc = 0,1 mol•L^-1 C

If the solutions are already prepared, the experiment can be performed in 10 – 15 minutes. Follow-up experiments are also available (Substitution of the Guest Molecule and Dissociation of the Complex).

2 Procedure and Observations

The following experimental variants require 1% solution of phenolphthalein in ethanol. Add 5 drops of the phenolphthalein solution to 10 ml sodium hydroxide solution (concentration = 0.1 mol•L^-1) .

Variant I: Place a portion of solid β-cyclodextrin on a watch glass and add the prepared alkaline phenolphthalein solution drop by drop.
Variant II: Prepare a β-cyclodextrin solution from 0.2 g β-cyclodextrin and 20 ml water. To this solution add the alkaline phenolphthalein solution drop by drop until a faint, lasting pink color is obtained. Then measure the pH of the solution with indicator paper. For comparison, treat 20 ml distilled water with the same volume of alkaline phenolphthalein solution and then test the pH of the solution (Video of the experiment).

Variant I: The dark magenta phenolphthalein solution quickly fades when it is dropped on to the β-cyclodextrin.
Variant II: When the magenta phenolphthalein solution is added to the β-cyclodextrin solution, it immediately loses its color. Immediate loss of color can still be observed even after adding 5 ml phenolphthalein solution to the solution. The solution has a pH of 10 - 11. When the same volume of water is treated with 5 ml alkaline phenolphthalein solution, it clearly turns pink and has a pH of 10 – 11.

If a further 5 ml phenolphthalein solution is added to the β-cyclodextrin solution, a faint, lasting pink color can be observed.

Fig. 2.13: Results of experimental variant II

3 Discussion of Results

The decolorization of the phenolphthalein solution in the two variants cannot be attributed to the shift in pH towards the acid range, since the suspension or solution is demonstrably alkaline (pH = 10 – 11). When the magenta phenolphthalein solution is added to β-cyclodextrin or to the β-cyclodextrin solution, the cone-shaped β-cyclodextrin forms a host-guest complex with the guest molecule phenolphthalein, due to van der Waals interactions. Since the molecular structure is non-ionic and therefore less polar in acid and neutral conditions, it can be assumed that the phenolphthalein is complexated in this form, which would also explain the decolorization of the solution.

Fig. 2.14: Molecular structures of phenolphthalein at pH < 8.5 and pH > 9

UV-VIS spectroscopy of different substances and complexes has shown that the phenolphthalein is present in the cyclodextrin in the dianionic form. During the formation of the host-guest complex, the phenolphthalein dianion is complexated by the formation of three hydrogen bonds to the cyclodextrin molecule. Therefore the van der Waals forces between the guest molecule and the non-polar cavity of the cyclodextrin are not the significant forces in this complex.

Fig. 2.15: Schematic diagram of the hydrogen bonds between the β-cyclodextrin molecule and the phenolphthalein dianion

These intermolecular interactions cause the phenolphthalein molecule to twist more strongly around the central carbon atom. Delocalization of the conjugated π-electron is consequently impaired, so that the color disappears.

4 Tips and Comments

5 Supplementary Information

The observation that phenolphthalein decolorizes in the presence of cyclodextrin despite the alkaline solution forms the basis of one analytical method for determining the cyclodextrin content. In UV-VIS spectra, the cyclodextrin content can be determined by the absorption at 550 nm, where the phenolphthalein dianion has the most intense absorption band.

Further experiments also use the decolorization of the phenolphthalein solution to test for cyclodextrins. For instance, this simple experiment (Extraction from Textile Fresheners and Detection of Cyclodextrins) can be used to test textile fresheners for cyclodextrins. Cotton fabric finished with cyclodextrins is also tested in this way (Cyclodextrins in Textile Finishing). If the red-violet solution loses its color even when the fabric strip has been washed in water, then it is certain that the cyclodextrin molecules are bonded covalently to the cotton fiber via the anchor molecule.

6 References

Tausch, M.; von Wachtendonk, M.; Chemie 2000+ Band 3, C.C. Buchners Verlag Bamberg, 2005, S. 26
Buvári, A.; Barcza, L.; J. Chem. Soc. Perkin Trans. II, 1988, 1687-1690
Szejtli, J.; Osa, T.; Comprehensive Supramolecular Chemistry. Volume 3 Cyclodextrins, Elsevier Science Ltd., 1996, S. 254
Kuwabara, T.; Takamura, M.; Matsushita, A.; Ikeda, H.; Nakamura, A.; Ueno, A.; Toda, F.; J. Org. Chem., 1998, 63, 8729-8735