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3.8: More Complicated Spectra

  • Page ID
    194821
  • Sometimes more complicated heteroatomic functional groups, containing bonds to more than one heteroatom, have slightly different spectra. Carboxylic acids feature a hydroxyl group bonded to a carbonyl. Hexanoic acid, a carboxylic acid in a six-atom chain, is one example.

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    If you look at the IR spectrum of hexanoic acid:

    • there are CH2 bending modes at 1500 cm-1.
    • there is a very strong C=O peak around 1700 cm-1.
    • there is a medium C-O peak around 1250 cm-1.
    • the sp3 C-H and O-H stretching modes are less clear.
    clipboard_efd71f64a4d5367e502ab188640f63ddf.png
    Figure \(\PageIndex{1}\): IR spectrum of hexanoic acid. Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

    At first, the O-H peak appears to be absent. The C-H stretch appears to be very broad. The wide peak between 3000 and 2600 cm-1 is really the usual C-H stretch with a broad O-H stretch superimposed on it. The low frequency vibration of this O-H bond is related to the partial dissociation of protons due to strong hydrogen bonding.

    Exercise \(\PageIndex{1}\)

    Isopentyl butanoate has a C-O stretch at 1200 cm-1. We saw earlier that an ether had a C-O stretch around 1000 cm-1. Explain the differences in these bond stretches.

    clipboard_ea4b09c625d215bc3a57a36e64440ef03.png

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    Figure \(\PageIndex{2}\): IR spectrum of isopentyl butanoate. Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

    Answer

    The Csp2-O bond is conjugated, so there is some double bond character, making the bond stronger and moving the IR peak to higher frequency.

    IResterconj.png

    Exercise \(\PageIndex{2}\)

    Locate an O-H, a C-O and a C=O bond stretch in an IR spectrum of 4-hydroxy-2-butanone.

    clipboard_ef48b63a72e6046c4253934cb63d3f366.png

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    Figure \(\PageIndex{3}\): IR spectrum of 4-hydroxy-2-butanone. Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

    Answer

    OH: 3400 cm-1 (strong, broad)

    C=O: 1700 cm-1 (strong)

    C-O: 1050 cm-1 (strong)