Spectroscopy of amines

Although an amine IR spectrum also contains absorptions resulting from vibrations of C—N bonds, these vibrations appear around 1000 to 1200 cm , in the same region as C—C and C—O vibrations. Therefore, they are not very useful fw identifying an amine. 

Infrared spectrum of 1-propanamine. Note the characteristic N—H stretching absorptions at 3300 and 3400 cm - 

The following partial IR spectra correspond to a primary amine, a secondary amine, and an alcohol. Give the functional group for each spectrum. 

Like the O— H protons of alcohols, the N — H protons of amines absorb at chemical shifts that depend on the extent of hydrogen bonding. The solvent and the sample concentration influence hydrogen bonding and therefore the chemical shift. Typical N—H chemical shifts appear in the range SI to 64. 

Another similarity between O—H and N — H protons is their failure, in many cases, to show spin spin splitting. In some samples, N—H protons exchange from one molecule to another at a rate that is faster than the time scale of the NMR experiment, and the N—H protons fail to show magnetic coupling. Sometimes the N — H protons of a very pure amine will show clean splitting, but these cases are rare. More commonly, the N—H protons appear as broad peaks. A broad peak should arouse suspicion of N — H protons. As with O—H protons, an absorption of N—H protons decreases or disappears after shaking the sample with D2O. 

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

Alkylamines undergo a characteristic a cleavage in the mass spectrometer, similar to the cleavage observed for alcohols (Section 13.12). A C-C bond nearest the nitrogen atom is broken, yielding an alkyl radical and a nitrogen-containing cation  

FIGURE 18.7 Mass spectmm of N-ethylpropylamine. The two possible modes ofa cleavage lead to the observed fragment ions at m/z = 58 and m/z = 72. 

We ve now seen all the common functional groups that occur in hiomolecules. Of those groups, amines are among the most abundant and have among the richest chemistry. In addition to proteins and nucleic acids, the majority of pharmaceutical agents contain amine functional groups and many of the common coenzymes necessary for biological catalysis are amines. 

Amines are organic derivatives of ammonia. They are named in the lUPAC system either by adding the suffix -amine to the name of the alkyl substituent or by considering the amino group as a substituent on a more complex parent molecule. 

Problem 24.25 Compound A, C6H12O, has an IR absorption at 1715 cm and gives compound B, C6H15N, when treated with ammonia and NaBH3CN. The IR and hT NMR spectra of B are shown. What are the structures of A and B  

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 

Liquids made of ions Usually wlien we think of ionic compounds, vve think of high-melting solids sodium chloride, magnesium sulfate, lithium carbonate, and so forth. But yes, there also ionic compounds that are liquid at room temperature, and they are gaining importance as reaction solvents, particularly for use in green chemistry i rocesses (see the Chapter 11 Focus On). 

Ionic liquids have been known for nearly a century the first to be discovered was ethylammonium nitrate, CH3CH2Nl l3 a melting point of 

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Spectroscopy of Amines A Deeper Look— Green Chemistry II Ionic Liquids... 

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