How is infrared spectroscopy helpful in identifying functional groups?

Infrared spectroscopy helps identify functional groups by analysing the specific absorption patterns they produce in an infrared spectrum.

Infrared (IR) spectroscopy is a powerful analytical tool used in chemistry to identify different types of molecules, including functional groups. It works on the principle that molecules absorb specific frequencies of infrared light which correspond to the vibrations of different chemical bonds. Each functional group has a unique set of vibrational frequencies, which are like its 'fingerprint'. By comparing the absorption pattern in the infrared spectrum of a sample to known patterns of different functional groups, we can identify which functional groups are present in the molecule.

The process begins by passing infrared radiation through a sample. The molecules in the sample absorb some of this radiation, causing them to vibrate at characteristic frequencies. These vibrations can be stretching (where the bond length changes) or bending (where the bond angle changes). The amount of radiation absorbed at each frequency is measured and plotted on a graph to produce an infrared spectrum.

The x-axis of the spectrum represents the frequency of the infrared radiation, usually expressed in terms of wavenumber (the number of wavelengths per centimetre). The y-axis represents the amount of radiation absorbed. Peaks on the graph indicate frequencies at which a lot of radiation is absorbed, corresponding to the vibrational frequencies of different bonds in the molecule.

Functional groups are specific groupings of atoms within molecules that have their own distinct behaviours in chemical reactions. They also have their own characteristic absorption patterns in an infrared spectrum. For example, the C=O bond in a carbonyl group absorbs infrared radiation at a wavenumber of around 1700 cm-1, producing a strong, sharp peak in that region of the spectrum.

By looking at the positions, shapes and intensities of the peaks in the spectrum, we can identify the functional groups present in the molecule. This is often done by comparing the spectrum to a reference table or database of known spectra. In this way, infrared spectroscopy provides a valuable method for identifying functional groups and understanding the structure of organic molecules.