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Infrared spectra of alcohols and phenols

O-H stretch: The free O-H stretch is a small sharp peak at 3650-3600.

The hydrogen-bonded O-H stretch is a broad strong peak at 3400-3300.

For the neat liquid, all the alcohol is considered hydrogen bonding, so the broad peak is the only one of the two visible. As the alcohol is dissolved in a non-hydrogen bonding solvent, the free O-H peak becomes more visible:


The right-side picture is of an extremely diluted alcohol, or a gas.

C-O-H bend: Broad weak peak at 1440-1220, often obscured by CH3 bends.

C-O: Stretch occurs at 1260-1000

1-Hexanol:


2-Butanol:


Para-cresol:


The free O-H stretch occurs near 3640, 3630, 3620, 3610 for primary, secondary, tertiary, and phenolic alcohols respectively. But resolving frequency differences this small requires careful calibration, so these distinctions are of no use.

Intramolecular hydrogen bonding usually shifts the O-H stretch to a lower frequency. For example, for methyl silicate it is situated about 3200, around 150 less then the average phenol.

The strong C-O stretch at 1260-1000 can be used to determine the structure of the alcohol, notably more easy than with the O-H stretch:

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12 comments:

  1. Anonymous24 February 2015 at 13:37

    Can you please help outline the differences between free and bonded OH ? Im struggling to understand this bit please?

    ReplyDelete
    Replies
    1. My ZaHrA29 November 2020 at 15:17

      bonded oh means interaction of oh with the environment which makes the peak broad and intense but round at the bottom.

      Delete
    2. Anonymous31 August 2022 at 03:05

      What is the difference between free O-H bond and bonded O-H

      Delete
  2. Tamil Parent15 September 2015 at 17:29

    Hydrogen bonded alcohols (H...OH- or H on O, N or F) show lower frequency than free or non bonded alcohols OH groups.
    Tamil PArent

    ReplyDelete
  3. Unknown23 May 2016 at 03:42

    nice

    ReplyDelete
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  5. Unknown13 February 2019 at 06:36

    THANKS

    ReplyDelete
  6. Unknown13 February 2019 at 06:38

    Can u describe the weak peak unless conjugated in IR spectrogram

    ReplyDelete
  7. Anonymous1 March 2019 at 03:01

    *breaks down door* hey it's me goku

    ReplyDelete
  8. Anonymous1 March 2019 at 03:02

    so no cock and ball torture?

    ReplyDelete
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  10. Unknown1 April 2019 at 22:45

    Why are phenols c-o of a higher wavenumber than alcohols?

    ReplyDelete

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