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12.5: Infrared Spectra of Some Common Functional Groups

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    183009
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    Common Group Frequencies Summary

    When analyzing an IR spectrum, it is helpful to overlay the diagram below onto the spectrum with our mind to help recognize functional groups.

    group frequency1 (1).gif

    Figure 1. Group frequency and fingerprint regions of the mid-infrared spectrum

    The region of the infrared spectrum from 1200 to 700 cm-1 is called the fingerprint region. This region is notable for the large number of infrared bands that are found there. Many different vibrations, including C-O, C-C and C-N single bond stretches, C-H bending vibrations, and some bands due to benzene rings are found in this region. The fingerprint region is often the most complex and confusing region to interpret, and is usually the last section of a spectrum to be interpreted. However, the utility of the fingerprint region is that the many bands there provide a fingerprint for a molecule.

    Group Frequencies - a closer look

    Detailed information about the infrared absorptions observed for various bonded atoms and groups is usually presented in tabular form. The following table provides a collection of such data for the most common functional groups. Following the color scheme of the chart, stretching absorptions are listed in the blue-shaded section and bending absorptions in the green shaded part. More detailed descriptions for certain groups (e.g. alkenes, arenes, alcohols, amines & carbonyl compounds) may be viewed by clicking on the functional class name. Since most organic compounds have C-H bonds, a useful rule is that absorption in the 2850 to 3000 cm-1 is due to sp3 C-H stretching; whereas, absorption above 3000 cm-1 is from sp2 C-H stretching or sp C-H stretching if it is near 3300 cm-1.

     
     

    Stretching Vibrations

    Bending Vibrations

    Functional Class

    Range (cm-1)

    Intensity

    Assignment

    Range (cm-1)

    Intensity

    Assignment

    Alkanes

    2850-3000 str CH3, CH2 & CH
    2 or 3 bands
    1350-1470
    1370-1390
    720-725
    med
    med
    wk
    CH2 & CH3 deformation
    CH3 deformation
    CH2 rocking

    Alkenes

    3020-3100
    1630-1680

    1900-2000
    med
    var

    str
    =C-H & =CH2 (usually sharp)
    C=C (symmetry reduces intensity)

    C=C asymmetric stretch
    880-995
    780-850
    675-730
    str
    med
    med
    =C-H & =CH2
    (out-of-plane bending)
    cis-RCH=CHR

    Alkynes

    3300
    2100-2250
    str
    var
    C-H (usually sharp)
    C≡C (symmetry reduces intensity)
    600-700 str C-H deformation

    Arenes

    3030
    1600 & 1500
    var
    med-wk
    C-H (may be several bands)
    C=C (in ring) (2 bands)
    (3 if conjugated)
    690-900 str-med C-H bending &
    ring puckering

    Alcohols & Phenols

    3580-3650
    3200-3550
    970-1250
    var
    str
    str
    O-H (free), usually sharp
    O-H (H-bonded), usually broad
    C-O
    1330-1430
    650-770
    med
    var-wk
    O-H bending (in-plane)
    O-H bend (out-of-plane)

    Amines

    3400-3500 (dil. soln.)
    3300-3400 (dil. soln.)
    1000-1250
    wk
    wk
    med
    N-H (1°-amines), 2 bands
    N-H (2°-amines)
    C-N
    1550-1650
    660-900
    med-str
    var
    NH2 scissoring (1°-amines)
    NH2 & N-H wagging
    (shifts on H-bonding)

    Aldehydes & Ketones

    2690-2840(2 bands)
    1720-1740
    1710-1720
    med
    str
    str

    str
    str
    str
    str
    C-H (aldehyde C-H)
    C=O (saturated aldehyde)
    C=O (saturated ketone)

    aryl ketone
    α, β-unsaturation
    cyclopentanone
    cyclobutanone

    1350-1360
    1400-1450
    1100

    str
    str
    med

    α-CH3 bending
    α-CH2 bending
    C-C-C bending

    Carboxylic Acids & Derivatives

    2500-3300 (acids) overlap C-H
    1705-1720 (acids)
    1210-1320 (acids)
    str
    str
    med-str

    str
    str
    str
    str
    str
    str
    O-H (very broad)
    C=O (H-bonded)
    O-C (sometimes 2-peaks)

    C=O
    C=O (2-bands)
    O-C
    C=O
    O-C (2-bands)
    C=O (amide I band)
    1395-1440







    1590-1650
    1500-1560
    med







    med
    med
    C-O-H bending







    N-H (1°-amide) II band
    N-H (2°-amide) II band

    Nitriles

    Isocyanates,Isothiocyanates,
    Diimides, Azides & Ketenes

    2240-2260

    2100-2270
    med

    med
    C≡N (sharp)

    -N=C=O, -N=C=S
    -N=C=N-, -N3, C=C=O
     

    Recognizing Group Frequencies in IR Spectra - a very close look

    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.

    octane (1).png

    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.

    1-octene.png

    Figure 4. Infrared Spectrum of 1-Octene

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

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

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

    1-hexyne.png

    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.

    toluene.png

     

    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.

    ethanol.png

    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

    - ?, ?-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.

    2-butanone.png

    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
      • alpha, beta-unsaturated aldehydes 1710-1685 cm-1

    Figure 9. shows the spectrum of butyraldehyde.

    butyraldehyde.png

    Figure 9. Infrared Spectrum of Butyraldehyde

    The carbonyl stretch C=O of esters appears:

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

    Figure 10. shows the spectrum of ethyl benzoate.

    ethyl benzoate.png

    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.

    hexanoic acid.png

    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

       

    nitromethane.png

    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.

    1-chloro-2-methylpropane.png

    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.

    Exercise

    1.  What functional groups give the following signals in an IR spectrum?

    A) 1700 cm-1

    B) 1550 cm-1

    C) 1700 cm-1 and 2510-3000 cm-1

    2.  How can you distinguish the following pairs of compounds through IR analysis?

    A) CH3OH (Methanol) and CH3CH2OCH2CH(Diethylether)

    B) Cyclopentane and 1-pentene.

    C)

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

    Source: SDBSWeb : http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, 2 December 2016)

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

    Answer

    1.

    2.

    A) A OH peak will be present around 3300 cm-1 for methanol and will be absent in the ether.

    B) 1-pentene will have a alkene peak around 1650 cm-1 for the C=C and there will be another peak around 3100 cm-1 for the sp2 C-H group on the alkene

    C) Cannot distinguish these two isomers. They both have the same functional groups and therefore would have the same peaks on an IR spectra.

    3.

    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.)

    4.

    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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