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3.11: Infrared Spectra of Some Common Functional Groups

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    167035
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    Objective

    After completing this section, you should be able to use an infrared spectrum to determine the presence of functional groups, such as alcohols, amines and carbonyl groups, in an unknown compound, given a list of infrared absorption frequencies.

    Study Notes

    In Chapter 12.7, you should have learned, in broad terms, where a few key absorptions occur. Otherwise, to find the characteristic infrared absorptions of the various functional groups, refer to this IR table.

    Spectral Interpretation by Application of Group Frequencies

    One of the most common application of infrared spectroscopy is to the identification of organic compounds. The major classes of organic molecules are shown in this category and also linked on the bottom page for the number of collections of spectral information regarding organic molecules.

    Hydrocarbons

    Hydrocarbons compounds contain only C-H and C-C bonds, but there is plenty of information to be obtained from the infrared spectra arising from C-H stretching and C-H bending.

    In alkanes, which have very few bands, each band in the spectrum can be assigned:

    • C–H stretch from 3000–2850 cm-1
    • C–H bend or scissoring from 1470-1450 cm-1
    • C–H rock, methyl from 1370-1350 cm-1
    • C–H rock, methyl, seen only in long chain alkanes, from 725-720 cm-1

    Figure 3. shows the IR spectrum of octane. Since most organic compounds have these features, these C-H vibrations are usually not noted when interpreting a routine IR spectrum. Note that the change in dipole moment with respect to distance for the C-H stretching is greater than that for others shown, which is why the C-H stretch band is the more intense.

    Spectrum showing the carbon-hydrogen stretch with wavenumbers 2971 and 2863; carbon hydrogen scissoring with wavenumber 1470; carbon-hydrogen methyl rock at 1383; and long-chain methyl rock with wavenumber 726.

    Figure 3. Infrared Spectrum of Octane

    In alkenes compounds, each band in the spectrum can be assigned:

    • C=C stretch from 1680-1640 cm-1
    • =C–H stretch from 3100-3000 cm-1
    • =C–H bend from 1000-650 cm-1

    Figure 4. shows the IR spectrum of 1-octene. As alkanes compounds, these bands are not specific and are generally not noted because they are present in almost all organic molecules.

    Spectrum with double bonded carbon-hydrogen stretch with wavenumber 3083, single bonded carbon-hydrogen stretch with wavenumbers 2966 and 2863, carbon-hydrogen scissoring with wavenumber 1465, carbon hydrogen methyl rock with wavenumber 1378, and double bonded carbon-hydrogen bend with wavenumber 917.

    Figure 4. Infrared Spectrum of 1-Octene

    In alkynes, each band in the spectrum can be assigned:

    • –CC– stretch from 2260-2100 cm-1
    • –CC–H: C–H stretch from 3330-3270 cm-1
    • –CC–H: C–H bend from 700-610 cm-1

    The spectrum of 1-hexyne, a terminal alkyne, is shown below.

    Spectrum with triple bonded carbon-hydrogen stretch with wavenumber 3324, carbon-hydrogen stretch with wavenumbers 2971 and 2879, carbon-carbon triple bond stretch with wavenumber 2126, carbon-hydrogen scissoring with wavenumber 1470, carbon-hydrogen methyl rock with wavenumber 1383, and triple bonded carbon-carbon-hydrogen bend with wavenumber 636.

    Figure 5. Infrared Spectrum of 1-Hexyne

    In aromatic compounds, each band in the spectrum can be assigned:

    • C–H stretch from 3100-3000 cm-1
    • overtones, weak, from 2000-1665 cm-1
    • C–C stretch (in-ring) from 1600-1585 cm-1
    • C–C stretch (in-ring) from 1500-1400 cm-1
    • C–H "oop" from 900-675 cm-1

    Note that this is at slightly higher frequency than is the –C–H stretch in alkanes. This is a very useful tool for interpreting IR spectra. Only alkenes and aromatics show a C–H stretch slightly higher than 3000 cm-1.

    Figure 6. shows the spectrum of toluene.

    Spectrum with aromatic carbon-hydrogen stretch with wavenumbers 3099, 3086, and 3032, a;lyl carbon-hydrogen stretch with wavenumber 2925, carbon-carbon aromatic ring stretching with wavenumbers 1614, 1506, and 1465, in-plane carbon-hydrogen bending with wavenumbers 1086 and 1035, and out-of-plane carbon-hydrogen bending with wavenumber 735.

    Figure 6. Infrared Spectrum of Toluene

    Functional Groups Containing the C-O Bond

    Alcohols have IR absorptions associated with both the O-H and the C-O stretching vibrations.

    • O–H stretch, hydrogen bonded 3500-3200 cm-1
    • C–O stretch 1260-1050 cm-1 (s)

    Figure 7. shows the spectrum of ethanol. Note the very broad, strong band of the O–H stretch.

    Spectrum with oxygen-hydrogen stretch with wavenumber 3391, carbon-hydrogen stretch with wavenumber 2961, and carbon-oxygen stretches with wavenumbers 1102 and 1055.

    Figure 7. Infrared Spectrum of Ethanol

    The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears:

    • C=O stretch - aliphatic ketones 1715 cm-1
    • α, β -unsaturated ketones 1685-1666 cm-1

    Figure 8. shows the spectrum of 2-butanone. This is a saturated ketone, and the C=O band appears at 1715.

    Spectrum with carbon-hydrogen stretch with wavenumber 2991 and carbon-double-bond-oxygen stretch with wavenumber 1715.

    Figure 8. Infrared Spectrum of 2-Butanone

    If a compound is suspected to be an aldehyde, a peak always appears around 2720 cm-1 which often appears as a shoulder-type peak just to the right of the alkyl C–H stretches.

    • H–C=O stretch 2830-2695 cm-1
    • C=O stretch:
      • aliphatic aldehydes 1740-1720 cm-1
      • α, β-unsaturated aldehydes 1710-1685 cm-1

    Figure 9. shows the spectrum of butyraldehyde.

    Spectrum with alkyl carbon-hydrogen stretch with wavenumber 2976, aldehyde carbon hydrogen stretches with wavenumbers 2827 and 2725, and carbon-double-bond-oxygen stretch with wavenumber 1731.

    Figure 9. Infrared Spectrum of Butyraldehyde

    The carbonyl stretch C=O of esters appears:

    • C=O stretch
      • aliphatic from 1750-1735 cm-1
      • α, β -unsaturated from 1730-1715 cm-1
    • C–O stretch from 1300-1000 cm-1

    Figure 10. shows the spectrum of ethyl benzoate.

    Spectrum showing aromatic carbon-hydrogen stretch with wavenumber 3078, alkyl carbon-hydrogen stretch with wavenumber 2986, carbon-double-bond-oxygen stretch with wave number 1726, and carbon oxygen stretches with wavenumbers 1117.

    Figure 10. Infrared Spectrum of Ethyl benzoate

    The carbonyl stretch C=O of a carboxylic acid appears as an intense band from 1760-1690 cm-1. The exact position of this broad band depends on whether the carboxylic acid is saturated or unsaturated, dimerized, or has internal hydrogen bonding.

    • O–H stretch from 3300-2500 cm-1
    • C=O stretch from 1760-1690 cm-1
    • C–O stretch from 1320-1210 cm-1
    • O–H bend from 1440-1395 and 950-910 cm-1

    Figure 11. shows the spectrum of hexanoic acid.

    Spectrum with oxygen hydrogen stretch and carbon-hydrogen stretch with wave number 2971, carbon-double-bond-oxygen stretch with wavenumber 1721, oxygen-hydrogen bend with wavenumber 1419, carbon-oxygen stretch with wavenumber 1296, and oxygen-hydrogen bend 948.

    Figure 11. Infrared Spectrum of Hexanoic acid

    Organic Nitrogen Compounds

    • N–O asymmetric stretch from 1550-1475 cm-1
    • N–O symmetric stretch from 1360-1290 cm-1

    Spectrum showing nitrogen oxygen stretch with wavenumbers 1573 and 1383.

    Figure 12. Infrared Spectrum of Nitomethane

    Organic Compounds Containing Halogens

    Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorene, or iodine.

    • C–H wag (-CH2X) from 1300-1150 cm-1
    • C–X stretches (general) from 850-515 cm-1
      • C–Cl stretch 850-550 cm-1
      • C–Br stretch 690-515 cm-1

    The spectrum of 1-chloro-2-methylpropane are shown below.

    Spectrum showing carbon-hydrogen stretch with wavenumbers 2976 and 2873, terminal alkyl halide carbon-hydrogen wag with wavenumber 1271, and carbon-chlorine stretch with wavenumber 743.

    Figure 13. Infrared Spectrum of 1-chloro-2-methylpropane

    For more Infrared spectra Spectral database of organic molecules is introduced to use free database. Also, the infrared spectroscopy correlation table is linked on bottom of page to find other assigned IR peaks.

    Exercises

    Exercise \(\PageIndex{1}\)

    The following spectrum is for the accompanying compound. What are the peaks that you can identify?

     

    12.8.png

     

    12.10.gif

    Answer

    Frequency (cm-1)     Functional Group

     

     

    3200                            C≡C-H

     

     

    2900-3000                   C-C-H, C=C-H

     

     

    2100                            C≡C

     

     

    1610                            C=C

     

     

    (There is also an aromatic undertone region between 2000-1600 which describes the substitution on the phenyl ring.)

     

     

    Exercise \(\PageIndex{2}\)

    What absorptions would the following compounds have in an IR spectra?

     

    12.8(2).png

     

    Answer

    A

     

     

    Frequency (cm-1)     Functional Group

     

     

    2900-3000                   C-C-H, C=C-H

     

     

    1710                            C=O

     

     

    1610                            C=C

     

     

    1100                            C-O

     

     

     

     

     

    B

     

     

    Frequency (cm-1)     Functional Group

     

     

    3200                            C≡C-H

     

     

    2900-3000                   C-C-H, C=C-H

     

     

    2100                            C≡C

     

     

    1710                            C=O

     

     

     

     

     

    C

     

     

    Frequency (cm-1)     Functional Group

     

     

    3300 (broad)               O-H

     

     

    2900-3000                   C-C-H, C=C-H

     

     

    2000-1800                   Aromatic Overtones

     

     

    1710                            C=O

     

     

    1610                            C=C

     

     

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