Resumen de Spectra Interpretation of Organic Compounds (Pretsch, Erno; Clerc, Jean Thomas)
Erno Pretsch and Jean Thomas Clerc. VCH: Weinheim, 1997. 165 pp, CD-ROM included. ISBN 3-527-28826-0. $90.00.
This slim book purports to afford an interactive spectroscopy course via the iterative solution of 15 spectral problems. The text's working premise is that there is no single generalized approach to solving combined spectral problems. To the extent that Pretsch and Clerc have attempted to provide a mnemonic, they recommend (i) extracting the most accessible information from any of the spectra (MS, IR, 1H NMR, or 13C NMR) to obtain identifiable structural elements, (ii) generating all plausible structural combinations by combining the various constituent building blocks, and (iii) differentiating among the proposed structures by correlation with empirical spectral data. To assist in the third facet of this paradigm, an ancillary CD-ROM containing the SpecTeach version of the SpecTool software that runs on Windows 3.1, Windows 95, or Windows NT is provided with the text.
What is SpecTool? It is a Hypermedia application that consists of linked information units ("nodes") containing reference data, reference spectra, and computational tools. The links in SpecTool "map the thought patterns of a chemist interpreting the spectra." The logical basis of the links can best be described as a three-dimensional hyperspace in which compound type, spectroscopic method, and informational type represent the three axes. The TopPage consists of a two-dimensional matrix where the spectroscopic methods are the columns and the data types, the rows. Selection buttons provide access to the next hierarchical level of information.
Navigating the tightly linked database was not insurmountable even for someone accustomed to Mac-based software. But the lack of availability of IBM-based software resulted in my having access only to Version 3.1 of Windows, which led to some vexing problems with the spectral displays that were seemingly unavoidable. For example, a literature citation screen that almost completely covered the screen of interest, either spectral or explanatory, could not be moved or removed. Yet the authors claim that the accompanying version of SpecTool was finding that the interconnections between links were not always directly accessible. Return to the TopPage always circumvented any electronic problems but made use of the software more cumbersome.
Let's take a tour of how Pretsch and Clerc rationalize the structure of the unknown in problem #1. The 13C NMR proton-decouple spectrum is relatively simple with only 5 peaks; hence, this is an obvious starting point. From the combined DEPT135 and DEPT90 spectra, it is possible to deduce three structural subunits: a methyl, a methine, and a methylene group in the upfield region of the spectrum. Access to the SpecTool software allows the reader to ascertain the effect of common substituents on the chemical shifts of simple alkyl groups such as methyl, ethyl, propyl. The second conclusion drawn from the 13C NMR spectrum is that the peak at 177.0 ppm is consistent with a O=C-X group. After the numberof hydrogen atoms is derived from the 1H NMR integration, and the methyl doublet, the only first-order character in the spectrum, is assigned as a CH3-CH structural fragment, the presumed molecular mass of 100 is determined from the m/z ratio of the last significant signal in the mass spectrum (MS). An interesting feature of the software is the external MolForm program, which provides all possible molecular formulas that satisfy the molecular mass. By further restricting the field to those molecular formulas in which the number of hydrogen atoms is 8, in agreement with the 1H NMR integration, the number of possibilities is reduced from 17 to 3. It is only now that the authors move to the IR spectrum in an attempt to identify the functional group. The authors use the C=O stretching vibration at 1770 cm{1 to implicate an ester functionality. Access to the IR spectrum of an arbitrary ester allows the user to examine the effect of solvent absorption on the region around the C-O-C stretch. At this point the authors believe that the unknown is sufficiently well refined to propose two alternate structures: 2-methyl- or 3-methyl-g-butyrolactone. The final differentiation rests on such arguments as the estimated MS resolution to determine if the base peak at m/z 42 is due to ketene (CH2C=O) or propenyl (C3H6) ions and the effect of the vicinal methyl on the chemical shift of the diastereotopic CH2-O hydrogens. The high frequency of the carbonyl stretching vibration is examined to confirm the ring size of the lactone.
Supplemental information on MS, IR, and NMR spectroscopy is provided in later chapters. This includes degree of unsaturation (MS), the harmonic oscillator model (IR), first- and higher-order spectra (1H NMR), and the phenomenon of saturation, the nuclear Overhauser effect (NOE), and conformational equilibria (both 1H NMR and 13C NMR).
The final chapter is devoted to the structure drawing program ChemWindow®III used in conjunction with estimating NMR chemical shifts. My attempt to examine toluene as a simple model, however, resulted in an error message. Unfortunately, the capability of the shift estimation program is limited to the 15 solved examples in the text.
To obtain a student perspective on the book, I asked Jarred D. Bender, '99, who took organic chemistry with me last year, to examine the book. This student was especially adept at structure elucidation, so his opinion might not be representative of the average organic chemistry student. His overall opinion of the book was positive. He found the reliance on mass spectrometry somewhat overwhelming only because interpretation of mass spectroscopy fragmentation patterns is not a significant component of the sophomore-level organic course. He was especially impressed with the potential utility of the reference spectra available with SpecTool. His opinion of the authors' spectral interpretation approach was that it would be a valuable methodology for a skilled student who had already taken a course in instrumental analysis.
The absence of a systematic protocol for spectral identification and the authors' assumption of some prior experience in spectral interpretation restrict the utility of this text to advanced undergraduate courses. In fact, the authors state thatan introduction to instrumental analysis would provide sufficient background for the use of this volume. The lack of a reproducible method of spectroscopic analysis might prove very frustrating to students who will not understand why the authors have chosen to examine spectral data in the order in which they have for the solved problems. While the authors purposefully are not always explicit in what the students will find when they access SpecTool, thus introducing an element of discovery for the student, they are remiss in not providing a justification for their method. Despite these limitations, however, I believe the book would be suitable as a supplement in an advanced undergraduate spectroscopy course.
