Infrared (IR) spectroscopy provides a rapid instrumental technique for identifying the major structural elements of organic (i.e., carbon-based) compounds. IR instruments measure the frequencies at which various bonds in a compound absorb radiation in the IR region of the electromagnetic spectrum. By convention, chemists state these frequencies in units of reciprocal centimeters (1/cm), or “wavenumbers.” The absorption frequencies of specific bonds tend to be distinctive. An O-H bond, for example, exhibits a broad absorption around 3400 1/cm. After obtaining the spectrum for a given compound, chemists use IR spectroscopy correlation tables to identify the types of bonds that occur in the compound. Methyl m-nitrobenzoate comprises a nitro group, or -NO2, and a methyl ester group, or C(=O)-O-CH3, attached to a benzene ring.
Identify the absorption peak of the C=O group of the ester by its strong absorption between 1735 and 1750 1/cm. This should be the strongest peak in the spectrum.
Locate the C-C(=O)-C stretch between 1160 and 1210 1/cm.
Sciencing Video Vault
Identify the two -NO2 stretches in the ranges of 1490-1550 and 1315-1355 1/cm.
Locate the two C=C aromatic stretches at about 1600 and 1475 1/cm.
Identify the C-H stretch of the -CH3 group located between 2800 and 2950 1/cm. A -CH3 bend absorbance should also occur near 1375 1/cm.
Identify the C-H bends associated with the benzene ring. Locate the ortho C-H at 735 to 770 1/cm. Locate the meta C-H at 880 1/cm, and between 690 and 780 1/cm. The para C-H should be between 800 and 850 1/cm.