In the Laboratory
Zeolite 5A Catalyzed Etherification of Diphenylmethanol Jason Cooke,* Eric J. Henderson, and Owen C. Lightbody Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada; *
[email protected] Zeolites are a diverse class of materials with many useful properties that are of interest to chemical educators, such as their ability to act as drying agents (1), selective adsorbents (2–4), and as catalysts for organic transformations (5). As was described in the preceding article (4), zeolites can adsorb or
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exclude alcohols on the basis of compatibility with the size of the zeolite pores. One anomalous result was observed during this study, namely the disappearance of the OH stretching band in the IR spectrum when diphenylmethanol, (C6H5)2CHOH, was exposed to zeolite 5A pellets for one hour in a gently shaken dichloromethane solution. The disappearance of the OH band from diphenylmethanol occurs despite the fact that the smaller phenylmethanol (benzyl alcohol), C6H5CH2OH, is too large to enter the pores of zeolite 5A, as was demonstrated by the lack of change in the intensity of the phenylmethanol OH absorption under the same conditions (4). Accordingly, the loss of the OH band from diphenylmethanol cannot be attributed to adsorption in the zeolite pores. For the diphenylmethanol–zeolite 5A combination, separation of the supernatant dichloromethane solution from the zeolite pellets and removal of the solvent under vacuum yields an oily white solid. Additional product can be recovered by extracting the zeolite pellets with extra dichloromethane. The oily nature of the material is improved to a waxy white powder by triturating with hexane or by evacuation under high vacuum. A class of 50 students carried out the reaction as described above (0.2 g alcohol, 0.6–0.7 g zeolite 5A pellets, 2 mL CH2Cl2) and yields approaching or exceeding 75% for the waxy white powder were consistently achieved. The product has an IR spectrum that confirms the retention of an aromatic ring (or rings) along with an aliphatic C–H stretching band; given the presence of aromatic groups, the fingerprint region is understandably quite complex (Figure 1). The 1H and 13C{1H} APT NMR spectra1 (Figures 2 and 3) confirm the presence of a (C6H5)2CH moiety that is attached to a strongly electron-withdrawing atom. The data therefore indicate that the waxy white solid is the ether produced by net dehydration of two molecules of (C6H5)2CHOH:
Figure 2. 1H NMR spectrum of (C6H5)2CHOCH(C6H5)2 (in CDCl3).
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Chemical Shift (ppm) Figure 3. 13C{1H} APT NMR spectrum of (C6H5)2CHOCH(C6H5)2 (in CDCl3). The C and CH2 carbons are “up” in the spectrum and the CH and CH3 carbons are “down” in the spectrum.
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The final verification of the product’s identity can be achieved by mass spectrometry. Although the molecular ion is not observed, the fragmentation pattern closely matches that for (C6H5)2CHOCH(C6H5)2 as located in a computerized library search (6), with the largest identifiable fragment corresponding to [M-C6H6]+. The melting point (89–91 °C) and 1H NMR data also compare favorably with data previously reported in the literature (7). As not all undergraduate laboratories have orbital shakers, further investigation of the reaction determined that it can also
Journal of Chemical Education • Vol. 86 No. 5 May 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
be carried out using the same reagent and solvent quantities by either refluxing for one hour or by leaving the sealed vial overnight at room temperature to allow the reaction to proceed by diffusion. Further study of the conditions revealed that the practical minimum mass ratio of zeolite 5A:diphenylmethanol is 2.5:1 and that fully hydrated zeolite 5A did not cause etherification to occur. In essence, the dehydrated zeolite 5A catalyst is poisoned by the water that is produced during the reaction, and a certain minimum mass of dehydrated zeolite is therefore needed to take the reaction to completion. As none of the other studied zeolite–alcohol combinations produced similar transformations at room temperature (4), several attempts were made to see whether increased temperature or longer reaction times would promote the analogous dehydration of the other alcohols. For example, a solution of 2-propanol, (CH3)2CHOH, in CDCl3 was heated in a sealed tube over zeolite 5A pellets for 18 h at 70 °C, resulting only in anhydrous (CH3)2CHOH in the 1H NMR spectrum: δ 4.00 (septet of doublets, 3JHH = 6.5 Hz and 4.5 Hz, 1H, CH); δ 1.70 (d, 3JHH = 4.5 Hz, 1H, OH); and δ 1.18 (d, 3JHH = 6.3 Hz, 6H, CH3). Phenylmethanol did show some evidence of slow conversion under similar conditions, but several products were formed and the reaction was not pursued further. Zeolite 4A was also investigated more thoroughly, but did not show any catalytic activity in combination with diphenylmethanol, phenylmethanol, or 2-propanol. Thus, for the range of alcohols and zeolites that have been studied (4), the reaction appears to be specific to diphenylmethanol and zeolite 5A. Lewis (7, 8) and Brønsted (9) acids are known to promote dehydration of diphenylmethanol to form the corresponding ether. To the best of our knowledge, the zeolite 5A promoted etherification of (C6H5)2CHOH has not been previously reported, although the same transformation has been reported to proceed in one minute when carried out over zeolite CaY (10). In the case of zeolite CaY, it was noted that both Lewis and Brønsted acidity of the zeolite could contribute to the observed etherification reactions (10), but in zeolite 5A, which is Ca-exchanged zeolite A (11), it is presumed that only Lewis acid sites would be present. Moreover, since diphenylmethanol should be too large to enter the pores of zeolite 5A, any catalytically active sites would be expected to be found on the surface of the pellet or near the entrance to a cavity. It is interesting to note from the larger screening study that most alcohols that were not admitted into the pores of a particular zeolite often decreased slightly in concentration owing to surface adsorption (4). In the case of diphenylmethanol, it is reasonable to conclude that this adsorption process is then followed by etherification. Although it is not precisely known why type 5A is the only zeolite we tested that caused the observed transformation, the reactivity of diphenylmethanol is perhaps not surprising given that aromatic secondary alcohols also gave the highest conversions in other Lewis acid-catalyzed etherification reactions (7, 8c). Thus, a study of the etherification of other secondary aromatic alcohols by zeolite 5A could be pursued as an open-ended research activity. Hazards Diphenylmethanol is an irritant. Dichloromethane is not flammable, but is a suspected carcinogen and should therefore
be handled with adequate ventilation, preferably in a fume hood. Hexane is a flammable liquid. Zeolite 5A is a desiccant and may cause skin irritation. Using powdered zeolite 5A is not recommended, but if it is chosen, appropriate precautions must be taken to avoid inhalation of the dust. Pellet zeolite 5A typically does not pose such a risk. Summary An experiment has been presented that demonstrates the ability of zeolites to act as heterogeneous catalysts for organic transformations. The zeolite 5A-promoted dehydration of (C6H5)2CHOH to produce (C6H5)2CHOCH(C6H5)2 is a straightforward exercise that could be easily incorporated into any synthetic undergraduate laboratory. Apart from illustrating the catalytic properties of zeolites, the reaction also produces a compound that provides an interesting spectroscopic problemsolving exercise. If included as part of a wider study on the selective adsorption of alcohols of varying sizes by zeolites (4), the zeolite 5A–diphenylmethanol combination will introduce an interesting twist that will require the students to think creatively. Therefore, the experiment can also be presented as an open-ended discovery-research exercise and could potentially be extended by investigating whether other activated secondary alcohols would also form ethers under similar conditions. Acknowledgments The Department of Chemistry at the University of Alberta is thanked for funding the project. The efforts of the undergraduate students who have successfully completed the experiment are gratefully acknowledged, as are the helpful suggestions of the graduate teaching assistants for the course. Note 1. APT stands for “attached proton test” and is a pulse sequence that causes the C and CH2 carbons to appear in a different phase than the CH and CH3 carbons.
Literature Cited 1. Smoot, A. L.; Lindquist, D. A. J. Chem. Educ. 1997, 74, 569– 570. 2. Balkus, K. J., Jr.; Ly, Kieu T. J. Chem. Educ. 1991, 68, 875–877. 3. Chao, P.-Y.; Chuang, Y.-Y.; Ho, G. H.; Chuang, S.-H.; Tsai, T.C.; Lee, C.-L.; Tsai, S.-T.; Huang, J.-F. J. Chem. Educ. 2008, 85, 1558–1561. 4. Cooke, J.; Henderson, E. J. J. Chem. Educ. 2009, 86, 606–609. 5. (a) Copperthwaite, R. G.; Hutchings, G. J.; van der Riet, M. J. Chem. Educ. 1986, 63, 632–634. (b) Bibby, D. M.; Copperthwaite, R. G.; Hutchings, G. J.; Johnston, P.; Orchard, S. W. J. Chem. Educ. 1986, 63, 634–637. 6. NIST/EPA/NIH Mass Spectral Library, Gaithersburg, MD, 2002. http://www.nist.gov/srd/nist1a.htm (accessed Feb 2009). 7. Kim, S.; Chung, K. N.; Yang, S. J. Org. Chem. 1987, 52, 3917– 3919 and references therein. 8. (a) Sket, B.; Zupan, M. J. Macromol. Sci.-Chem. 1983, A19, 643–652. (b) Ruicheng, R.; Shuojian, J.; Ji, S. J. Macromol. Sci.Chem. 1987, A24, 669–679. (c) Zhu, Z.; Espenson, J. H. J. Org.
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In the Laboratory Chem. 1996, 61, 324–328. (d) Miller, K. J.; Abu-Omar, M. M. Eur. J. Org. Chem. 2003, 1294–1299. (e) Firouzabadi, H.; Iranpoor, N.; Jafari, A. A. J. Mol. Catal. A: Chem. 2005, 227, 97–100. (f ) Yasuda, M.; Somyo, T.; Baba, A. Angew. Chem., Int. Ed. 2006, 45, 793–796. 9. (a) Pratt, E. F.; Jones, D. G. J. Org. Chem. 1965, 30, 4362–4363. (b) Toda, F.; Takumi, H.; Akehi, M. J. Chem. Soc., Chem. Commun. 1990, 1270–1271. (c) Kobayashi, S.; Iimura, S.; Manabe, K. Chem. Lett. 2002, 10–11. (d) Manabe, K.; Iimura, S.; Sun, X.-M.; Kobayashi, S. J. Am. Chem. Soc. 2002, 124, 11971–11978. 10. Yu, W.; Wen, M.; Zhao, G. Y.; Yang, L.; Liu, Z. L. Chin. Chem. Lett. 2006, 17, 15–18. 11. Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978; p 12.
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