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
- Example One: Isopropyl Benzene: Aromatic and Aliphatic Hydrocarbon:
- Example Two: 2-methyl propanol: Aliphatic alcohol
- Example Three: Butanoic Acid: Aliphatic Carboxylic Acid
- Example Four: Aniline: Aromatic and Primary Amine
- Example Five: Acetophenone: Aromatic and Ketone
- Example Six: Pentanal: Aliphatic aldehyde
- Example Seven: 1-Pentene: Alkene with aliphatic side-chain
- Example Eight: Propyl Acetate: Aliphatic ester
- Download the classroom (.pdf) Problem Set and its Key
Example Problem: Use the prompts below this spectrum to aid in your analysis its IR bands:
Any sp3 C-C-H?
Csp3–H stretch appears at 3000-2850cm-1
Their exact frequency varies according to functional group, but better diagnostics are found in 13C NMR
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
Since this is the fingerprint region, it is the absence of a signal rather than the presence of one that provides information.
Look them up if you encounter them (eg nitriles at ≈2400cm-1; alkynes at ≈2200cm-1)
- Acetophenone (Methylphenylketone)
- 4-Ethylbenzoic acid
|©Kathleen Armstrong||April, 2011|