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22.3: Spectroscopy of Amines

  • Page ID
    183152
  • 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

    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:

    eg. 2:

    eg. 3:

    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.

    ch 20 spectroscopy question solution.png

     

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