Structures from Spectra

Purpose: We will practice on combining data from infrared and nuclear magnetic resonance spectroscopies to identify unknown compounds. This is practice so that you can run and analyze your own samples in this course. To narrow the field of possibilities for the following problems we will assume that the unknown has a boiling point of 150±30°C. The table below will give an idea of the number of carbon atoms one can expect that fit within this boiling point range for a given type of molecule. This information is combined with the functional group information from the IR and information about the environments of the hydrogen and carbon atoms within molecules from the CMR and PMR spectra. The goal, then, is to come as close to a complete identification of the unknown as possible based solely on the boiling point and the spectra.

Preparation: M&B sections 17.4 through 17.7, 17.10 through 17.13 and 17.17 -17.18. Print out the unknowns data sets indicated at the end of this document for the first class meeting.  Be prepared to staple one set (3 spectra) to a page in your lab notebook (which you will be given out on the first lab day). Leave a blank page between each set to record notes and comments.  Review your nomenclature and draw out molecules as necessary.  Also, bring McMurry, the "Microscale" lab text by Mayo, et al. and a lock to the first lab.

Be prepared to annotate each spectrum with the inferences drawn - both the presence of peaks and the absence of peaks are important data. We will discuss possible structures of some of these molecules in class.

The compounds below all boil between 120-180^oC. The boiling point range for molecules varies fairly predictably depending on the type of compound (functional groups it contains) and the number of carbon atoms in the molecule.  These are summarized below.  There is no point in memorizing the boiling points. One needs only to get an appreciation of how many carbon atoms, more or less, to expect to work with when analyzing the spectra.

Even within this relatively small boiling point range, the number of possible isomers from branching appears to make the identification task formidable. However, help from spectral information, especially NMR, is on the way.

Quick Procedures for IR and NMR Analysis

Infrared spectra

Things to look for in decreasing order of importance (M&B p.592):

1. the C-H absorption(s) between 3100 and 2850 cm-1. An absorption above 3000 cm-1 indicates C=C, either alkene or aromatic. Confirm the aromatic ring by finding peaks at 1600 and 1500 cm-1 confirm alkenes with an absorption at 1640-1680 cm-1.

2. the carbonyl (C=O) absorption between 1690-1760; this strong band indicates either an aldehyde, ketone, carboxylic acid, ester, amide, anhydride or acyl halide.

3. the O-H or N-H absorption between 3200 and 3600 cm-1. This indicates either an alcohol, N-H containing amine or amide, or carboxylic acid.

4. the C-O absorption between 1080 and 1300 cm-1. These peaks are rounded like the O-H and N-H peak in 3. and are prominent. Carboxylic acids, esters, ethers, alcohols and anhydrides all contain this peak.

5. the -CC- and -CN absorptions at 2100-2260 cm-1 are small but exposed.

• Again, don't memorize everything, but do know where to look.  For more detailed information see McMurry, Ch. 12.5 (especially Table 12.1 on p. 440 and associated spectra provided therein).
• Don't try to interpret every peak in the IR.  The absence of peaks is important in eliminating possible functional groups.
• There are other helpful IR hints but they are best given with examples and with NMR data.

NMR Spectra

The four factors (McMurry Ch. 13) for NMR signals were given in Chem 3010.  You should review these ideas:

a) the 'kinds' of hydrogens and carbons determines the number of signals in the spectrum;

b) the electron density surrounding the hydrogens and carbons determine the chemical shifts (positions) of each signal;

c) the number of hydrogens (but not of carbons) in each group determine the area (intensity) of each signal; and

d) the number of nearby hydrogens determine the splitting of the signal (in CMR "nearby" means "bonded to the carbon being observed" and in PMR "nearby" means "protons of other groups closer than 4 bonds away from the hydrogen being observed").

A quick guide of chemical shifts from McMurry pp. 465, 466 and 480:

Laboratory Procedure

We will analyze the spectral  problems assigned.  The analysis of each should be recorded in your notebook.  You should find these analyses very useful throughout the semester so it is definitely worth your efforts to learn this material early.

Reacquaint yourself with the usual presentation of IR spectra in wavenumbers (cm^-1) or microns. A useful relationship between the two is that 1 cm = 10,000 microns . Notice that parts of the scale often change depending on the region of the spectrum displayed (x axis).

Likewise, reacquaint yourself with the typical presentation for proton and carbon NMR spectra on a ppm scale.  Most of the samples presented here were recorded in "deuteriated" chlorofom (CDCl₃) solvent.  Deuterium is a heavier isotope of hydrogen with an extra neutron.   Deuterium nuclei are different from normal hydrogen (i.e. protons) in NMR too, so they are not observed in a PMR experiment.  However, you will see residual hydrogen signals from the chloroform since a small percentage of molecules will still contain hydrogen. Where would you expect to see signals due to residual protons on chloroform (CHCl₃)?  This residual solvent signal is used as an internal standard to calibrate the chemical shift scale.  Remember not to confuse this signal with those from the compound you are analyzing.

We will be analyzing sample data sets during the first laboratory meeting.  You will need to print copies of MOLECULES 1-6 which contain three spectra when each link is individually opened.  You will need the last six molecules at a later date.  These spectra will be used to identify the unknown molecular structure and we will be discussing Molecules 1-6  during the first lab period.  BE READY!!!

***Please note that printing from web browsers is problematic and depends on the settings for your individual computer.  Copies of these spectra are available at the library reserve and can be copied using a photocopier.

References:

"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991.

"McMurry": McMurry, John, Organic Chemistry, 4th ed., Brooks/Cole Publishing Co.: Pacific Grove, CA, 1995.

The "Handbook": any of the recent editions of Weast, R.D. Handbook of Chemistry and Physics; The Chemical Rubber Co.: Cleveland, 1960-present.

Revised on 8/12/99.