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CH4/535

Structure Determination by Spectroscopic Methods

(CRN 38538, 38539)

Carbonyl Stretching Vibrations

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The C=O group is one of the most easily recognized peaks in an IR spectrum. The change in dipole moment is significant, making this an intense band, and there are few other groups that give rise to absorbances in the 1600-1850 cm-1 range. Furthermore, the precise position can be readily correlated with resonance effects: the higher the C=O bond order, the higher the frequency. Resonance donation of electron density, conversely, lowers bond order and decreases the absorbance frequency.

Ketones: normally 1720 cm-1

Cyclohexanone
: 1710 cm-1
Aldehydes: Somewhat higher in frequency, 1735 cm-1
Cyclohexanecarbaldehyde: 1740 cm-1 Note also the doublet at 2706 and 2807 cm-1, which is a characteristic aldehyde C-H feature.
Conjugated aldehydes and ketones: Presence of a conjugated Π bond or an aromatic ring will lower the frequency by 20-30 cm-1. There will often be combinations with the C=C stretch.
Acrolein: 1734, 1710 cm-1


Benzaldehyde: 1711 cm-1


Methyl vinyl ketone: 1707 cm-1


Acetophenone: 1690 cm-1
Ring size can play an important role for cyclic ketones; small rings impose strain and increase the net C=O bond order.
Cyclobutanone: 1810 cm-1


Cyclopentanone: 1774 cm-1


Cyclohexanone: 1710 cm-1


Cycloheptanone: 1722 cm-1
Carboxylic acids & derivatives: highly dependent on resonance donation versus electronegativity.
Benzoyl chloride: 1790 cm-1. Note that this is also lowered by conjugation; alkyl substituted acyl chlorides will be higher.


Propionic anhydride: In-phase and out-of-phase combinations at 1830 and 1770 cm-1. This is typical behavior for an anhydride.


Ethyl acetate: 1734 cm-1

2-Ethylhexanoic acid: 1699 cm-1. Note that the broad O-H band is also a dead giveaway.


Acetamide: 1684 cm-1

Last updated: 9/22/2018