This QuickTime movie, featuring HMC students, was produced to illustrate what goes on in a typical General Chemistry (Chemistry 25) laboratory experiment. It was first screened for the Board of Trustees at their annual Saddlerock retreat in November of 2000.
The spectra and synthetic scheme presented in the movie are hard to see in this format, so they are reproduced below:
Gas chromatography is capable of separating complex mixtures based upon the amount of time required to elute from a column. The resulting TIC shows only one peak, eluting at 6.14 min. This means that our product is pure--there are no significant byproducts.
Mass spectroscopy shows the mass-to-charge ratio (m/z) of the ionized molecule and fragments of that molecule. The largest peak at highest m/z usually corresponds to the molecular ion, M+. The m/z of the molecular ion is the gram molecular weight of the molecule. Vanillin, shown on the left, has a gram molecular weight of 152. The product, shown on the right, has a gram molecular weight of 194.
Infrared spectroscopy allows us to determine the functional groups, or elements of molecular structure, present in a molecule, based upon the absorbance of specific frequencies of infrared light. Comparing the infrared spectrum of the product (bottom pannel) to that of vanillin (top pannel), we see that the broad band present in vanillin at 3050 cm-1 is not present in the product. The presence of a peak in this region of the spectrum correlates with the presence of an O-H in the molecule. In addition, a new peak is present in the product at 1750 cm-1; peaks in this region of the spectrum correspond to a C=O stretch.
The net bookkeeping is that an O-H originally present in vanillin is no longer present in the product. In addition, there is a new C=O in the product.
The peaks below 1500 cm-1 are more difficult to directly correlate with molecular structure and are beyond our abilities to interpret at the level of sophistication appropriate to general chemistry. Come back and take organic chemistry to learn more!
After analyzing the spectra and the materials used in our reaction, we can construct the synthetic scheme shown above, wherein the acetate group present in acetic anhydride (shown in blue) substitutes for the the H in the OH of vanillin (shown in red). The resulting product, vanillyl acetate, is consistent with the spectral data shown above. Furthermore, the reaction with acetic anhydride is reminiscent of the reactions of all acid anhydrides with water to give the corresponding acid.
Revised: March 20, 2002
URL
http://www.hmc.edu/www_common/chemistry/curriculum/sna60.html