Reading and Interpreting IR Spectra Solved Problems

Many students are unsure how much information they should be able to obtain from an infrared spectrum. In Chapter 13, we will use IR together with NMR and other information to determine the entire structure. For the present, concentrate on getting as much information as you can from the IR spectrum by itself. Several solved spectra are included in this section to show what information can be inferred. An experienced spectroscopist could obtain more information from these spectra, but we will concentrate on the major, most reliable, features. 

Study this section by looking at each spectrum and writing down the important frequencies and your proposed functional groups. Then look at the solution and 

Compound 1 This spectrum is most useful for what it does not show. There is a carbonyl absorption at 1714 cm and little else. There is no aldehyde C—H, no hydroxyl O—H, and no N—H. The carbonyl absoiption could indicate an aldehyde, ketone, or acid, except that the lack of aldehyde C—H stretch eliminates an aldehyde, and the lack of 0 — H stretch eliminates an acid. There is no visible C=C stretch and no unsaturated C — H absorption above 3000 cm , so the compound appears to be otherwise saturated. The compound is probably a simple ketone. 

Compound 2 The absorption at 1650 cm is so intense that it probably indicates a carbonyl group. A carbonyl group at this low frequency suggests an amide. The doublet (a pair of peaks) of N — H absorption around 3300 cm also suggests a primary amide, R—CONH2. Since there is no C—H absorption above 3000 cm , this is probably a saturated amide. 

Compound 3 The sharp peak at 2246 cm results from a nitrile C=N stretch. (An alkyne C=C absorption would be weaker and below 2200 cm .) The absence of C=C stretch or C—H stretch above 3(X)0 cm suggests that the nitrile is otherwise saturated. 

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The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

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