How to Identify Functional Groups in an IR Spectrum

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Infrared light (IR) is the portion of the electromagnetic spectrum with wavelengths longer than visible light (750 nanometers) but shorter than microwave radiation. IR can cause vibrational excitation of bonds in a molecule. You can imagine this effect as being a little like two balls of unequal size on a spring; certain frequencies of IR will cause the "spring" to stretch and compress. Atoms at either end of a bond can also rock, wag, scissor or twist with respect to each other. You don't need to remember the different types of motion, however, because what's important is the end effect: certain functional groups (groups of atoms) in molecules absorb IR at characteristic frequencies. Identifying some of the functional groups that are present can provide valuable clues to your molecule's structure.

Things You'll Need

  • Pencil
  • Paper
  • IR spectrum to analyze
  • Look at the printout of the IR spectrum for your molecule. Usually % transmittance (i.e. the % of IR emitted that actually made it through the sample) is on the y-axis and wavenumber is on the x-axis. The wavenumber is just 1 divided by the wavelength, so shorter wavelengths are at the left side of the printout and longer wavelengths at the right. A rough, ragged line runs across the graph near the top; here and there it will suddenly dip to a lower % transmittance, like a low valley or canyon in the graph. These dips or valleys are called peaks, because they denote the wavelengths/wavenumbers where the sample absorbed some or most of the IR light traveling through it. A really deep peak is called strong-intensity; a peak that only goes about halfway down is moderate intensity; and a peak that only goes maybe a quarter or a third of the way down is weak.

  • Look for peaks in characteristic regions of the graph. The stretching vibrations are the easiest to identify. Here are some of the bonds or functional groups that might be present in your molecule, together with some peaks typical of each:

    Alcohol: Strong, broad peak in the 3200 to 3650 range. (Note: all numbers here are wavenumbers.)

    Alkanes: Strong narrow peak with two or three bands in the 2850 to 3000 range.

    Alkenes: Medium narrow peak in the 3020 to 3100 range, together with a variable-strength peak in the 1630 to 1680 range.

    Amines: Weak peak (or two peaks if NH2) in the 3300 to 3500 range.

    Aldehydes and Ketones: Strong, sharp peak in the 1690 to 1750 range

    Esters: Strong peak in the 1735 to 1750 range; also strong peak with two bands in the 1000 to 1300 range.

    Alkynes: Strong sharp peak at about 3300, together with variable-strength peak in the 2100 to 2250 range.

    Carboxylic acids: Broad, strong peak in the 2500 to 3300 range, together with strong peak in the 1705 to 1720 range and medium peak in the 1210 to 1320 range.

  • Don't try to identify groups matching every single bond in the spectrum, and don't spend too much time on peaks below 1500. The region below 1500 is called the fingerprint region because absorptions in this region can be useful in determining whether two IR spectra "match" and belong to the same compound. This offers us a good way to identify compounds in some samples -- but it's not a very good way to identify functional groups, so in general, you're best off disregarding most of the peaks below 1500, with the exception of those mentioned above. Remember that the whole goal is just to find telltale peaks that tell you a specific functional group is present.

Tips & Warnings

  • Check out the link under the Resources section for pictures of some IR spectra and practice exercises. As with everything else in organic chemistry, the best way to get used to IR spectra is to practice.

References

  • Photo Credit Photos.com/Photos.com/Getty Images
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