How do you differentiate between primary, secondary, and tertiary amines using spectroscopy?

Primary, secondary, and tertiary amines can be differentiated using spectroscopy by analysing their unique infrared (IR) and nuclear magnetic resonance (NMR) spectra.

Infrared (IR) spectroscopy is a powerful tool for identifying functional groups in organic compounds. Primary amines show two N-H absorption bands in the IR spectrum, typically between 3300 and 3500 cm-1. These bands are due to the N-H stretching vibrations. Secondary amines, on the other hand, only show one N-H absorption band in the same region. This is because secondary amines have only one N-H bond. Tertiary amines do not show any N-H absorption bands as they do not have any N-H bonds.

Nuclear Magnetic Resonance (NMR) spectroscopy provides further information about the structure of amines. In proton NMR, primary amines usually show a complex multiplet in the region of 0.5-3.0 ppm due to the protons on the nitrogen atom. Secondary amines typically show a singlet in the same region, while tertiary amines do not show any peaks in this region as they do not have any protons directly attached to the nitrogen atom.

In carbon-13 NMR, the carbon atom attached to the nitrogen atom in primary, secondary, and tertiary amines usually appears in the region of 30-60 ppm. However, the exact position can vary depending on the nature of the other groups attached to the nitrogen atom.

In summary, by analysing the IR and NMR spectra of an unknown amine, it is possible to determine whether it is a primary, secondary, or tertiary amine. The presence or absence of N-H absorption bands in the IR spectrum provides the first clue, while the pattern of peaks in the proton NMR spectrum provides further evidence. The carbon-13 NMR spectrum can also provide useful information, although it is usually less definitive than the other two techniques.