The determination of the structure of a compound is a common task of organic chemists. In modern organic chemistry labs, researchers rely on spectroscopic data to determine the structure of a compound. Given the molecular formula (which is obtained from various procedures not covered here) and the NMR and IR spectra for a compound, it is possible to determine its structure. These powerful tools - IR and especially NMR - are used routinely by organic chemists.
It is the goal of the undergraduate lab courses to teach students enough about NMR and IR so that they, too, can use these spectrocopic methods to determine the structure of organic compounds (at least of simple molecules). The object of this tutorial is to introduce you to common functional groups one at a time and to walk you through sample problems in each category. The tutorial takes you through the practical aspect of solving problems; it assumes that you are familiar with IR "stretching and bending vibrations" and with NMR "shifts and splitting."
Suggested approach to solving spectroscopy problems:
Step 1: Calculate the degree of unsaturation to limit the number of possible structures. The degree of unsaturation indicates whether or not the compound has one or more double bonds.
Step 2: Look at the IR spectrum, especially the region of wavenumbers greater than 1500. Look for likely functional groups, working in conjunction with the degree of unsaturation result from Step 1 and with the molecular formula. For instance, if there is one double bond and an oxygen, it could be a carbonyl; if there is no double bond and an oxygen, it will be either an ether or an alcohol; if there are 4 double bonds, there might be an aromatic ring (and so on).
Step 3: Study the NMR to determine the connectivity of the compound. Draw the likely functional groups proposed in step 2 and see how they might fit together to be consistent with the given NMR spectrum.
Step 4: Draw a structure consistent with the unsaturation, IR, and NMR data. Check that you have the proper total number of carbons, oxygens, hydrogens (and etc.) as consistent with the molecular formula. Check that you have 4 bonds to each carbon. Check that the structure you have drawn "fits" with the NMR. If it doesn't, draw another possible structure and check again.
You can begin with the first item in the Table of Contents to the left: Degree of Unsaturation, and go on down the list in order. Of course, you can also jump around the functional groups in any order. Examples are solved problems and are given with each type of functional group; if you like, you can access the examples from the example directory. Reference charts are given for both IR and NMR spectroscopy.
If you want more practice, go to the Spectroscopy Problems located in another section of this web site.
The IR and NMR spectra used for the spectroscopy problems and for the spectroscopy tutorial were taken from the CD-ROM version of A Spectrum of Spectral Problems by D. Richard A. Tomasi. We have used these great problems for many years in the spectroscopy section of the organic chemistry lab and lecture courses. Here is their contact information:
Sunbelt R&T, Inc.
1946 S. 74 E. Ave.
Tulsa, OK 741112-7716
Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.