Question

What is a “functional group” ? Explain why infrared spectroscopy can often be used to detect the presence of a functional group in a molecule

Answer 1

FUNCTIONAL GROUP

===

a group of atoms responsible for the characteristic reactions of a particular compound.

Certain atomic movements give rise to bands that occur in approximately the same position in a large variety of compounds and are only slightly affected by the rest of the molecule. These vibrations are assigned to groups of atoms termed functional groups. Although absorption bands are characteristic of the molecule as a whole, it is a useful approximation to consider that molecular vibrations are localized in particular functional groups. This allows one to relate absorption band position with a particular functional group and to tabulate these relationships. The tables showing the positions where the functional group absorptions occur are called correlation tables. The intensity of the absorption bands is also shown on good correlation tables.

Most of the spectral features that allow us to readily identify functional groups are found in the left part of the spectrum. The right hand portion of the spectrum is more complex, and each peak is not readily identified with a particular part of the molecule. The entire spectral pattern is unique for a given compound.

The steps used by a chemist to find information about molecular structure from the IR spectrum are as follows:

1. Obtain a spectrum of the material on an IR spectrophotometer.

2. Using information from correlation tables and absorbances from the functional group region of the spectrum, identify the functional groups that are present or sometimes more importantly absent.

3. Compare this spectrum with those of known compounds or obtain a known sample of a suspected material and run its spectrum for comparison.

Many of the group frequencies vary over a wide range because the bands arise from complex interacting vibrations within the molecule. Absorption bands may, however, represent predominantly a single vibrational mode. Certain absorption bands, for example, those arising from C-H, O-H, and C=O stretching modes, remain within fairly narrow regions of the spectrum.

The two important areas for a preliminary examination of a spectrum are the region 4000-1300 cm-1 and the 909-650 cm-1 region. The high frequency portion of the spectrum is called the functional group region. The characteristic stretching frequencies for important functional groups such as OH, NH, and C=O occur in this portion of the spectrum. The absence of absorption in the assigned ranges for the various functional groups can usually be used as evidence for the absence of such groups from the molecule. The absence of absorption in the 1850-1540 cm-1 region excludes a structure containing a carbonyl group. Strong skeletal bands for aromatics and heteroaromatics fall in the 1600-1300 cm-1 region of the spectrum. These skeletal bands arise from the stretching of the carbon-carbon bonds in the ring structure.

The lack of strong absorption bands in the 909-650 cm-1 region generally indicates a nonaromatic structure. Aromatic and heteroaromatic compounds display strong out-of-plane C-H bending and ring bending absorption bands in this region.

The intermediate portion of the spectrum, 1300-909 cm-1 is usually referred to as the fingerprint region. The absorption pattern in this region is complex, with bands originating in interacting vibrational modes. Absorption in this intermediate region is probably unique for every molecular species.

Conclusions reached after examination of a particular band should be confirmed by examination of other portions of the spectrum if possible. For example the assignment of a carbonyl band to the presence of an ester should be confirmed by observation of a strong band in the C-O stretching region, 1300-1100 cm-1.