Infrared Spectroscopy

In order to assign functional groups to unknown molecules from their Infrared Spectra, it is important to familiarize yourself with not just the characteristic frequencies and their intensities, but also their shapes and characteristic patterns.

Consider the following examples, working through the spectral regions discussed in class and defined in your outline to Chapter 12:
You may use download a reference table here or use one in your textbook; Perkin Elmer FTIR at Foothill College

Download the classroom (.pdf) Problem Set, its Key, and Spectroscopy Tables (.pdf)

Example Problem: Use the prompts below this spectrum to aid in your analysis its IR bands:


Analysis of an IR spectrum:
Any OH or NH?

OH signals from alcohols are very strong, broad and appear at around 3400 cm-1

NH signals from amines are moderate in intensity. 1° amines appear as two signals and 2° amines appear as one signal around 3400-1

Any sp2 C=C-H?
Any sp3 C-C-H?
Csp2–H stretch appears at 3150-3000cm-1
Csp3–H stretch appears at 3000-2850cm-1
Any C=O?
Carbonyls are very strong, sharp signals at around 1700cm-1

Their exact frequency varies according to functional group, but better diagnostics are found in 13C NMR

Any Aryl rings?
The presence of an aryl (benzene) ring requires analysis of several regions of the IR spectrum. In addition to sp2 C-H stretches, Aryl rings will show sharp signals due to out-of-plane C=C-H bending modes at 900-690cm-1 that reveal the alkyl substitution pattern:
To distinguish a phenyl group from a vinyl (alkenyl) group, look for C=C stretch which for phenyl is typically at around 1600 cm-1 compared to the alkene C=C stretch which typically appear at around 1650 cm-1.
Although not always observable, look for diagnostic weak signals around 2000-1650cm-1
Any C-O?
The C-O single bond stretch is a fairly strong signal in fingerprint region at around 1100cm-1 (1000-1300cm-1)
Since this is the fingerprint region, it is the absence of a signal rather than the presence of one that provides information.

Any other outlying signals?
Although less common, any signals at frequency >1700cm-1 are diagnostic.
Look them up if you encounter them (eg nitriles at ≈2400cm-1; alkynes at ≈2200cm-1)
Which of the following structures is most consistent with this spectrum?
  1. Acetophenone (Methylphenylketone)
  2. 4-Ethylbenzoic acid
  3. Methylphenylether
  4. 4-Methoxyacetophenone
  5. Ethylbenzoate



©Kathleen Armstrong Nov, 2016