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20.3: Spectroscopy of Amines

Last updated

May 30, 2020
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- 20.2: Basicity of Amines and Ammonium Salt Formation
- 20.4: Synthesis of Amines

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NMR

The hydrogens attached to an amine show up ~ 0.5-5.0 ppm. The location is dependent on the amount of hydrogen bonding and the sample's concentration.

The hydrogens on carbons directly bonded to an amine typically appear ~2.3-3.0 ppm.

Addition of D₂O will normally cause all hydrogens on non-carbon atoms to exchange with deuteriums, thus making these resonances "disappear." Addition of a few drops of D₂O causing a signal to vanish can help confirm the presence of -NH.

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IR

The infrared spectrum of aniline is shown beneath the following table. Some of the characteristic absorptions for C-H stretching and aromatic ring substitution are also marked, but not colored.

Amine Class	Stretching Vibrations	Bending Vibrations
Primary (1°)	The N-H stretching absorption is less sensitive to hydrogen bonding than are O-H absorptions. In the gas phase and in dilute CCl₄ solution free N-H absorption is observed in the 3400 to 3500 cm^-1 region. Primary aliphatic amines display two well-defined peaks due to asymmetric (higher frequency) and symmetric N-H stretching, separated by 80 to 100 cm^-1. In aromatic amines these absorptions are usually 40 to 70 cm^-1 higher in frequency. A smaller absorption near 3200 cm^-1 (shaded orange in the spectra) is considered to be the result of interaction between an overtone of the 1600 cm^-1 band with the symmetric N-H stretching band. C-N stretching absorptions are found at 1200 to 1350 cm^-1 for aromatic amines, and at 1000 to 1250 cm^-1 for aliphatic amines.	Strong in-plane NH₂ scissoring absorptions at 1550 to 1650 cm^-1, and out-of-plane wagging at 650 to 900 cm^-1 (usually broad) are characteristic of 1°-amines.
Secondary (2°)	Secondary amines exhibit only one absorption near 3420 cm^-1. Hydrogen bonding in concentrated liquids shifts these absorptions to lower frequencies by about 100 cm^-1. Again, this absorption appears at slightly higher frequency when the nitrogen atom is bonded to an aromatic ring. The C-N absorptions are found in the same range, 1200 to 1350 cm^-1(aromatic) and 1000 to 1250 cm^-1 (aliphatic) as for 1°-amines.	A weak N-H bending absorption is sometimes visible at 1500 to 1600 cm^-1. A broad wagging absorption at 650 to 900 cm^-1 may be discerned in liquid film samples.
Tertiary (3°)	No N-H absorptions. The C-N absorptions are found in the same range, 1200 to 1350 cm^-1 (aromatic) and 1000 to 1250 cm^-1 (aliphatic) as for 1°-amines.	Aside from the C-N stretch noted on the left, these compounds have spectra characteristic of their alkyl and aryl substituents.

Mass Spectrometry and the Nitrogen Rule

The nitrogen rule states that a molecule that has no or even number of nitrogen atoms has an even nominal mass, whereas a molecule that has an odd number of nitrogen atoms has an odd nominal mass.

eg. 1:

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eg. 2:

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eg. 3:

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Exercise

7. Oh no! The labels have fallen off two samples: Q and R. The elemental analysis for the samples indicated the following composition: compound Q is 81.15% C, 8.34% H, and 10.52% O and compound R is 71.08% C, 6.72% H, 10.36% N, and 11.84% O. Fortunately, we can analyze the samples using IR and ¹H NMR spectroscopy. Name and draw the bond-line structures for compounds Q and R using the information provided. Support your answer by correlating the spectral data to the compound structures.

ch 20 spectroscopy question.png

Answer

7. Vial 1 contains compound R which is acetanilide. Vial 2 contains compound Q which is N-ethyl-3-methylaniline.

Contributors and Attributions

Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University)
Prof. Steven Farmer (Sonoma State University)
William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry

Gamini Gunawardena from the OChemPal site (Utah Valley University)

NMR

IR

Mass Spectrometry and the Nitrogen Rule

Contributors and Attributions

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