How to Read Infrared Spectroscopy

Organic chemists often use infrared (IR) spectroscopy in combination with other techniques to assign a chemical structure to an unknown compound. When you place a sample of your compound in the spectroscope, it gets scanned by a light beam containing a range of wavelengths from throughout the infrared spectrum. Different bond types absorb different wavelengths of light, so you'll be able to tell if your compound contains, for instance, an oxygen-hydrogen bond, or a carbon-oxygen double bond. This information is illustrated in the IR spectrum. Note that you cannot fully identify the structure of your compound by looking at the spectrum; you will only denote the presence or absence of particular bond types.

Things You'll Need

  • Literature listing of infrared absorbances of functional groups
  • Experimental infrared spectrum
Show More

Instructions

  1. Spectral Peak Correlation to Bond and Molecule Types

    • 1

      An alcohol is defined by the presence of an O-H bond. On your spectrum, a peak in the 3600-3100/cm range typically signals the presence of this bond.

      Additional spectral peaks around 3100-3000, 1600-1580 and 1500-1450/cm correspond to the bonds in an aromatic ring. If you find these in addition to the O-H bond peak, your compound is likely a phenol.

    • 2

      Next, see if there is a sharp, thin peak around the 1750-1625/cm range. This corresponds to the carbonyl group, or a carbon-oxygen double bond. This group is present in ketones and aldehydes.

      Carbonyl groups are also found in carboxylic acids, which also contain the alcohol function. However, the proximity of the O-H bond shifts the carbonyl peak into a more narrow range (1730-1660/cm).

      Esters also contain carbonyl groups, and the peak is also affected by the neighboring C-O bond. If you see correspondence in the 1300-1000/cm range plus the characteristic carbonyl peak, your compound is likely an ester.

    • 3

      Amines, like alcohols, also absorb in the low 3000/cm range. An amine contains nitrogen bonded to one, two or three carbons, with the other sites occupied by hydrogen. The peak for an alcohol is broad, whereas that for an amine is more narrow. You can also distinguish between a primary amine (nitrogen bonded to one carbon) and a secondary amine (nitrogen bonded to two carbons) on the spectrum. A primary amine produces two peaks on the spectrum in the 3500-3200/cm range, whereas a secondary amine only produces one.

Learnify Hub © www.0685.com All Rights Reserved