Question

What is the main PURPOSE or FUNCTION of IR spectrometry (to identify functional groups). What is the "fingerprint" region and what primary purpose does it serve? Describe how IR spectrometry could be used to monitor the reduction of nitrobenzene to aniline. What IR would decrease, and which would increase as the reaction was conducted?

Answer 1

IR spectroscopy is used to determine the functional group present in a sample.

Fingerprint region is in the range of 1500 to 500 cm^-1for the electromagnetic radiation. The importance of the fingerprint region is that each different compound produces a different pattern
of troughs in this part of the spectrum.

Nitro groups show two intense peaks in the IR spectrum: one between 1300-1400 cm^-1 for the symmetric stretching mode, the other one between 1500-1600 cm^-1 for the asymmetric stretching mode. . If an NH₂ group is present, two bands are commonly seen above 3000 cm-1 due to the symmetric and asymmetric stretching motions and two peaks at 1605 cm-1 and 1630 cm-1 correspond to the NH₂ bending absorption.

The peaks corresponding to nitro group were used to determine the decreasing concentration of nitrobenzene and the peaks indicated at 1605 and 1630 cm-1 were used to determine the increasing concentration of aniline during the hydrogenation reaction.