Keeping the "Un" in Unknowns in Qualitative Identification Exercises Using FTIR When FTIR instrumentation and its accompanying library searching abilities come into the door of undergraduate instrumental analysis or qualitative organic laboratories, the meaningfulness of unknown identification need not take the proverbial flight out the window. In the spring semester of 1991, our instrumental analysis students were given unknowns that had been diluted with hexachloro-1,3-butadiene.This compound, known to the students only as the "diluent", was chosen because of its relatively simple IR spectrum and its lack of a protan NMR spectrum. Although carbon tetrachloride has the same characteristics, it was too commonly available to use as the diluent since the students had bemme very proficient a t using the FTIR s o h a r e and a t subtracting spectra from each other. The FTIR spectnvn of the diluent was obtained and was not stored in the computer. Also, the hardware key to the Sadtler library was removed. A hard copy of the diluent's spectrum was given to the students along with the normal instructions for the safe handling of the unknowns, namely working in the hood and using gloves. The sole battle of hexachlora-1,3-butsdiene was hidden until the experiment was completed. The students were instructed to shade in the bands belonging to the diluent, and 75% were able to identify the unknown successfully by comparison of the FTIR and proton NMR This success rate falls into the normal range for this experiment. spectra with those found in spectral In the spring of 1992 we were not able to use this method because several instrumental analysis students were also enrolled in our inorganic prep course in which they used hexachloro-1,3-butadieneas a mulling agent. As a result, the instrumental analysis students were more successful at identifying their unknowns, mainly because of the computer's library search, but were less successful a t in-dass identification exercises. 'Pouchert, C.J. me Aldtiich Libraryof infrarsdSpctra,2nd ed.: Aldrich Chemical: Milwaukee, 1975. 2~ouchert.C.J.:Campbell, J.R. The AMrich Librery of NMRSpecfra; Ald"ch Chemical: Milwaukee. 1974. Judy V. Chauvin and Jack E. Field Nicholls State University Thibodaux, LA 7031 0
Volume 69
Number 8 August 1992
661
