Structure & Reactivity

Infrared Spectroscopy

IR5. Bonds to Common Heteroatoms: Oxygen

These bonds are pretty polar, so they show up strongly in IR spectroscopy. IR spectroscopy is therefore a good way to determine what heteroatom-containing functional groups are present in a molecule.

Compounds Containing C-O Single Bonds

Oxygen forms two bonds. An oxygen atom could be found in between two carbons, as in dibutyl ether, or between a carbon and a hydrogen as in 1-butanol. Dibutyl ether is an example of an ether and 1-butanol is an example of an alcohol.

       

If you look at an IR spectrum of dibutyl ether, you will see:

  • there are the usual sp3 C-H stretching and CH2 bending modes at 2900 and 1500 cm-1.
  • there is a strong peak near 1000 cm-1. This peak is due to the C-O stretching vibration.

Figure IR5.1. IR spectrum of dibutyl ether.1

 

If you look at an IR spectrum of 1-butanol, you will see:

  • there are sp3 C-H stretching and CH2 bending modes at 2900 and 1500 cm-1.
  • there is a strong C-O stretching mode near 1000 cm-1.
  • there is a very large peak around 3400 cm-1. O-H peaks are usually very broad like this one.

Figure IR5.2. IR spectrum of 1-butanol.1

 

Peak shapes are sometimes very useful in recognizing what kind of bond is present. The rounded shape of most O-H stretching modes occurs because of hydrogen bonding between different hydroxy groups. Because protons are shared to varying extent with neighboring oxygens, the covalent O-H bonds in a sample of alcohol all vibrate at slightly different frequencies and show up at slightly different positions in the IR spectrum. Instead of seeing one sharp peak, you see a whole lot of them all smeared out into one broad blob. Since C-H bonds don't hydrogen bond very well, you don't see that phenomenon in an ether, and an O-H peak is very easy to distinguish in the IR spectrum.

Problem IR5.1.

Even though there are only two C-O bonds in dibutyl ether, the C-O stretching mode is even stronger than the peak at 2900 cm-1 arising from 10 different C-H bonds. Explain why.

 

Problem IR5.2.

The IR spectrum of methyl phenyl ether (aka anisole) has strong peaks at 1050 and 1250 cm-1.1

a) Identify the type of bond corresponding to these two peaks.

b) Why are there two peaks for this type of bond in this molecule, and not just one?

c) Draw a second, zwitterionic resonance structure for methyl phenyl ether.

d) Use the zwitterionic resonance structure to explain why one of these bonds shows up at a higher frequency than the other one.

Figure IR5.3. IR spectrum of methyl phenyl ether.1

 

1. Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

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This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. 

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