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Chemistry LibreTexts

24.10 Spectroscopy of Amines

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  • Page ID
    91038
  • Objectives

    After completing this section, you should be able to

    1. identify the region of the infrared spectrum that shows absorptions resulting from the N$\ce{-}$H bonds of primary and secondary amines.
    2. describe a characteristic change that occurs in the infrared spectrum of an amine when a small amount of mineral acid is added to the sample.
    3. use 1H NMR spectra in determining the structure of an unknown amine.
    4. use the “nitrogen rule” of mass spectrometry to determine whether a compound has an odd or even number of nitrogen atoms in its structure.
    5. predict the prominent peaks in the mass spectrum of a given amine.
    6. use the mass spectrum of an unknown amine in determining its structure.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • nitrogen rule

    Study Notes

    You should note the spectroscopic similarities between amines and alcohols: both have infrared absorptions in the 3300–3360 cm−1 region, and in both cases, the proton that is attached to the heteroatom gives rise to an often indistinct signal in the 1H NMR spectrum.

    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.

    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.

     

    2amine1.gif1amine2.gif3amine1.gif

     

     

    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:

    eg. 2:

    eg. 3:

     

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