APPENDIX 3 - Table of infrared bands.

Valuable information about the functional groups in an organic compound may be obtained by an examination of the infrared spectrum. The absorption bands in the spectrum result from energy changes arising as a consequence of vibrations of the bonds (stretching and bending). The band positions for some common functional groups, expressed in wavenumbers (cm-1) are given below (see Vogel for detailed tables):

Class of Compound Functional Group Wavenumber (cm-1)
Alcohol O-H 3650 - 3200
Acid O-H 3300 - 2500*
Phenol O-H 3600 - 3000*
Acid C=O 1725 - 1700**
Ketone C=O 1725 - 1705**
Aldehyde C=O 1740 - 1720**
Ester C=O 1750 - 1730**
Amide C=O 1690 - 1680**
Primary amine N-H (stretch) (about 3500 and 3300)#
  N-H (deform) 1650 - 1580
Secondary amine N-H (stretch) 3450 - 3300
  N-H (deform) 1650 - 1550
Aromatic C=C (stretch) 1600
Alkene C=C (stretch) 1680 - 1620##

* Carboxylic acids and phenols have broad O-H stretches because of hydrogen bonding.

** Conjugation of the carbonyl group with a p electron system lowers the carbonyl absorption frequency so a,b-unsaturated and aromatic carbonyls absorb at lower frequencies.

# Two bands.

## Bands are often very weak.

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