The Synthesis And Characterization Of Ferrocene Essay — страница 4

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peaks that could be attributed to the 1,2-diacetylferrocene complex at 917 cm-1 or a doublet due to the 1,3-diacetylferrocene complex at 922 and 905 cm-1 (8). The experimental UV-Vis spectra of ferrocene and acetylferrocene were obtained in acetonitrile and Beers law was used to calculate the molar absorptivity. The UV spectrum for ferrocene shows maxima at 330 nm (2 = 52) and 440 nm (2 = 90), and a rising short-wavelength absorption at 225 nm (2 = 5051). This is comparable with the reported spectrum in ethanol (3). The UV spectrum for acetylferrocene shows maxima at 219 nm (2 = 2.2 x 104), 266 nm (2 = 5268) and 320 nm (2 = 1124). Except for the calculated molar absorptivity of the peak at 219 nm, this is comparable with the reported spectrum in 95% ethanol (8). The students also

observed peaks assigned to ferrocene in their acetylferrocene samples. The electrochemistry component of this laboratory was the first time that most students were exposed to cyclic voltammetry and the bulk electrolysis technique. An Amel System 5000 Potentiostat was used for all measurements. For cyclic voltammetry, the electrochemical cell was a 100 mL beaker equipped with a Ag/AgCl reference electrode (student prepared), a BAS (West Lafayette, IN) platinum-disk working electrode (2 mm diameter) and a large (1 cm2) platinum flag counter electrode. After having verified a flat background of tetrabutylammonium hexafluorophosphate (0.01 M) supporting electrolyte in acetonitrile in the range 0.0 to 1.0 V vs. Ag/AgCl, cyclic voltammograms of ferrocene and acetylferrocene

(approximately 3.2 x 10-3 M) were obtained at scan rates of 100 500 mV/sec. A typical cyclic voltammogram of ferrocene showed a reversible oxidation at E1/2 = +0.35 V vs. Ag/AgCl with Ep/2 = 0.057V. A typical cyclic voltammogram of acetylferrocene also showed a reversible oxidation at E1/2 = +0.58 V vs. Ag/AgCl with Ep/2 = 0.044V. Small peaks for ferrocene were also visible in the acetylferrocene cyclic voltammogram. These results are comparable to the reported E of acetylferrocene at +0.27 V vs. the ferrocene/ferrocenium couple (6). A second new electrochemical component that was recently introduced into this laboratory is the bulk electrolysis of ferrocene to ferrocenium. The electrochemical cell was a 100 mL beaker equipped with an Ag/AgCl reference electrode (student

prepared), a BAS (West Lafayette, IN) reticulated vitreous carbon (RVC) working electrode and an extremely large platinum flag counter electrode. After having verified a flat background of tetrabutylammonium hexafluorophosphate (0.01 M) supporting electrolyte in acetonitrile in the range 0.0 to 1.0 V vs. Ag/AgCl, the bulk electrolysis of ferrocene (approximately 7.5 x 10-4 M) was achieved on several occasions. As expected, a new peak in the UV-Vis was observed at 620 nm and the solution changed color from orange to blue. Unfortunately to date, these experimental conditions are not reproducible. As a supplement to their standard chemical characterization, students used the CAChe molecular modeling program to build a ferrocene molecule in both the eclipsed and staggered

conformations and to remove an electron to obtain information about the ferrocenium cation. The results of this modeling were then discussed in relation to their experimental observations. When the students have synthesized and derivatized ferrocene, they have an experimental background for comparison of the unsubstituted ferrocene versus the acetylated ferrocene. They also have a clear understanding of the potential R groups that are chemically practical. This is especially meaningful if the student has completed organic chemistry and is able to relate the familiar benzene substituents with the ferrocene molecule. We have found that if a student proceeds through the iterative question before understanding the acetylation experiment, they design strange, wondrous and impractical

molecules with the aid of the CAChe system. It must be stressed that molecular modeling is only a tool. The input is influenced to a large degree by the understanding of the operator which may be enhanced with guidance from the instructor. A natural progression at the completion of the two syntheses is the introduction of the iterative question. Students are asked to design a ferrocene with specific properties such as a different colored ferrocene. This question is answered with the aid of CAChe modeling where electronic spectra of the gas phase ferrocene and the substituted ferrocene may be generated by ZINDO (Zerners Intermediate Neglect of Differential Overlap). A more comprehensive iterative project involves both library work and molecular modeling. The students are asked to