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Organic Chemistry

24.10 Spectroscopy of Amines

Organic Chemistry24.10 Spectroscopy of Amines

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.

The IR spectrum of cyclohexylamine, where peaks are observed at 900, 1500, 2900, and 3400 centimeter inverse. The peak around 3400 indicates the presence of an amine group.
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.

A reversible reaction in which N-methylmethanamine reacts with deuterium oxide to form a product in which hydrogen is replaced by deuterium. H D O is the second product.

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).

H N M R spectrum with peaks at shift 0 (T M S), several small multiplets between 1 and 2, and a large singlet at around 2.4.
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.

The structure of cyclohexyl methyl amine. The chemical shift values of the carbon atoms are mentioned.
Problem 24-25

Compound A, C6H12O, has an IR absorption at 1715 cm–1 and gives compound B, C6H15N, when treated with ammonia and NaBH4. The IR and 1H NMR spectra of B are shown. What are the structures of A and B?

An illustration shows two parts. The first part shows infrared spectrum of compound B C6H15N wherein the horizontal axis shows wavenumbers in cm to the power minus one starting from 4000 on the left side to 500 on the right side at the interval of 500 cm to the power minus one; the vertical axis shows transmittance in percentage starting from 0 to 100 at the interval of 20 percent. A stretch is shown spread at wavenumber 3500 (60%) and at 3000 (0%). Spikes of different sizes are shown throughout the graph. The second part shows 1H NMR spectrum of compound B. The horizontal axis shows chemical shift (delta) from 10 on the left to 0 ppm on the right at the intervals of 1 delta. The vertical axis shows increasing intensity. The left of spectrum shows chemical shifts 0.88 0.98 and 2.59; relative area are 9.00 5.00. and 1.00. A short peak is shown at 2.6 delta. A medium peak is shown at 1 delta followed by a tall peak at 0.9 delta. A short peak labeled pink “TMS” is shown at 0 delta.

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.

Alkyl amines undergo alpha cleavage to form a radical and carbocation with two reversible resonance forms enclosed inside parentheses.

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.

Mass spectrum of N-ethylpropylamine. M plus peak is at 87 and m by z peak is at 72 and 58. Alpha cleavage of the species leads to two fragment ions.
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.
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