24.10 Spectroscopy of Amines

Infrared Spectroscopy

Primary and secondary amines can be identified by a characteristic N–H stretching absorption in the 3300 to 3500 cm^–1 range of the IR spectrum. Alcohols also absorb in this range (Section 17.11), but amine absorption bands are generally sharper and less intense than hydroxyl bands. Primary amines show a pair of bands at about 3350 and 3450 cm^–1 from the symmetric and asymmetric stretching modes, and secondary amines show a single band at 3350 cm^–1. Tertiary amines have no absorption in this region because they have no N–H bonds. Figure 24.9 recalls the IR spectrum of cyclohexylamine from Section 12.8.

Figure 24.9 IR spectrum of cyclohexylamine.

Nuclear Magnetic Resonance Spectroscopy

Amines are difficult to identify solely by ¹H NMR spectroscopy because N–H hydrogens tend to appear as broad signals without clear-cut coupling to neighboring C–H hydrogens. As with O–H absorptions (Section 17.11), amine N–H absorptions can appear over a wide range and are best identified by adding a small amount of D₂O to the sample. Exchange of N–D for N–H occurs, and the N–H signal disappears from the NMR spectrum.

Hydrogens on the carbon next to nitrogen are deshielded because of the electron-withdrawing effect of the nitrogen, and they therefore absorb further downfield than alkane hydrogens. N-Methyl groups are particularly distinctive because they absorb as a sharp three-proton singlet at 2.2 to 2.6 δ. This N-methyl resonance at 2.42 δ is easily seen in the ¹H NMR spectrum of N-methylcyclohexylamine (Figure 24.10).

Figure 24.10 Proton NMR spectrum of N-methylcyclohexylamine.

Carbons next to amine nitrogens are slightly deshielded in the ¹³C NMR spectrum and absorb about 20 ppm downfield from where they would absorb in an alkane of similar structure. In N-methylcyclohexylamine, for example, the ring carbon to which nitrogen is attached absorbs at a position 24 ppm downfield from any other ring carbon.

Mass Spectrometry

The nitrogen rule of mass spectrometry says that a compound with an odd number of nitrogen atoms has an odd-numbered molecular weight. Thus, the presence of nitrogen in a molecule is detected simply by observing its mass spectrum. An odd-numbered molecular ion usually means that the unknown compound has one or three nitrogen atoms, and an even-numbered molecular ion usually means that a compound has either zero or two nitrogen atoms. The logic behind the rule derives from the fact that nitrogen is trivalent, thus requiring an odd number of hydrogen atoms. For example, morphine has the formula C₁₇H₁₉NO₃ and a molecular weight of 285 amu.

Alkylamines undergo a characteristic α cleavage in the mass spectrometer, similar to the cleavage observed for alcohols (Section 17.11). C–C bond nearest the nitrogen atom is broken, yielding an alkyl radical and a resonance-stabilized, nitrogen-containing cation.

As an example, the mass spectrum of N-ethylpropylamine shown in Figure 24.11 has peaks at m/z = 58 and m/z = 72, corresponding to the two possible modes of α cleavage.

Figure 24.11 Mass spectrum of N-ethylpropylamine. The two possible modes of α cleavage lead to the observed fragment ions at m/z = 58 and m/z = 72.