Amines are broadly classified as aliphatic and aromatic amines. Aliphatic amines are further classified as primary (1º), secondary (2º) and tertiary (3º), based on the number of organic groups attached to the Nitrogen atom. The N-atom is sp3 hybridized which gives the amines a trigonal pyramidal shape. The residual negative charge on the non-bonding electron pair is the cause of basicity of amines, which can be exploited while detecting them.
Spectroscopic methods for structural analysis of amines
- Infrared spectroscopy (IR): Peak occurs in the range of 3500-3300 cm-1 due to vibrations of N-H bond. Primary amines give 2 bands, secondary gives one, a vanishingly weak one for aliphatic and a stronger one for aromatic secondary amines. Tertiary amines do not show a band as they do not have an N-H bond.
- NMR spectroscopy: Primary and secondary amines show N-H proton signals in the region δ 0.5-5. However, it has a broad spectrum and is difficult to identify. Hence they are replaced by Deuterium atoms and number of N-H bonds can be determined. The α-Carbon protons absorb in δ 2.2-2.9. Since β-Carbon protons are not as de-shielded as α-Carbon protons, they absorb in the range of δ 1.0 – 1.7.
- Mass spectroscopy: The most intense peak in the mass spectrum of aliphatic amines which arises from α-cleavage occurs at m/z = 30. In case of secondary and tertiary amines, fragmentation by β-cleavage leads to peaks at m/z = 58 and 86 respectively. Cyclic aliphatic amines usually produce intense peaks at m/z= 92.
Apart from the above, there are also chemical methods for elucidating the structure of amines. Eg: Hinsberg test. If the alkyl groups attached to a tertiary amine are all different, the compound will be chiral, and can be resolved using HPLC.
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- Ullman’s Encyclopedia, 2003. 9th edition. Volume 2. Wiley-VCH.
- Donald Pavia, 2007. Spectroscopy. Cengage Learning.
- C. S. Creuser, 1990. Analytical applications of spectroscopy. Royal Society of Chemistry.