In the Laboratory
Roto-Vibrational Spectroscopy: A Hint for DCl Generation Eraldo L. Lehmann and Edvaldo Sabadini* Department of Physical-Chemistry, Institute of Chemistry, University of Campinas - UNICAMP, P.O. Box 6154, 13083-970, Campinas, SP, Brazil *
[email protected] The roto-vibrational spectral analysis of HCl is commonly present in the curriculum of undergraduate chemistry students. The roto-vibrational experiment involving gas-phase HCl is probably the first contact of students with the concept of quantization of the molecular rotation and vibration. The experiment becomes more attractive if the four isotopic compounds are investigated: 1H35Cl, 1H37Cl, 2H35Cl, and 2H37Cl. The two first isotopic compounds are easily generated from the reaction of NaCl and concentrated H2SO4 (1). The reaction is rapid and occurs in high yield, and for this reason, the cell used to collect the gas is easily filled. The reaction of benzoyl chloride and D2O generates the two deuterium isotopic compounds of hydrochloric acid (2). Additionally, the four isotopic compounds can be obtained if a small volume of H2O is added to the reaction (3). However, in contrast to the reaction of NaCl and H2SO4, the hydrolysis of benzoyl chloride is slower and occurs with a smaller yield, and for this reason, it is more difficult to know if the quantity of gas inside the cell is enough to obtain a good spectrum. Some authors have proposed different strategies to fill the cell. In one strategy, the DCl is first frozen at 77 K and then transferred to the infrared cell. This procedure is dangerous because the vapor pressure rapidly rises without control (3). Alternatively, DCl can be collected in a large flask (Florence flask) for subsequent expansion into the cell and in this case a vacuum line is used (3). The production of DCl at atmospheric pressure is, in many cases, interesting due to the simplicity of the experimental apparatus used. However, in any procedure, the determination of the ideal DCl concentration inside the gas cell is still a crucial step to obtaining a good spectrum. We describe a simple method to indicate if the quantity of DCl (and HCl) inside the infrared cell is adequate. A scheme of the experimental setup is shown in Figure 1, in which a thick and wet pH indicator paper is inserted at the exit of the infrared cell. When the cell is filled with DCl, the excess is eliminated and promotes the color change of the indicator. The stopcocks of the cell can be closed a few seconds after the visualization of color change. This simple solution was used for several semesters with success.
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Journal of Chemical Education
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Figure 1. The experimental setup use to generate DCl. (1) The mixture of benzoyl chloride and D2O is heated to 100 °C. (2) The beaker containing ice and ethanol is used to trap water vapors. (3) A wet pH indicator paper (indicated by the arrow) is inserted in to the exit tube of the infrared cell.
Hazards All the procedures must be performed in a fume hood, as dangerous vapors are produced. Hydrochloric acid, benzoyl chloride, and sulfuric acid are corrosive and cause burns to all body tissue. Literature Cited 1. Rieck, D. F.; Kundell, F. A.; Clements, P. J. J. Chem. Educ. 1989, 66, 682. 2. Shoemaker, D. P.; Garland., C. W.; Nibler, J. W. Experiments in Physical Chemistry, 6th ed.; McGraw-Hill: New York, 1996; pp 401-404. 3. Furlong, W. R.; Grubbs, W. T. J. Chem. Educ. 2005, 82, 124.
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Vol. 87 No. 12 December 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed900051k Published on Web 09/21/2010
