Your may have noticed on the data sheet that the wavenumber for the absorption of C=O bonds very much depends the functional group, whether it be a ketone for an acyl chloride or a carboxylic acid. Clearly the frequency of radiation that must be absorbed for the C=O bond to vibrate more strongly is affected by the other groups bonded to the carbon atom – in essence, these other groups have an effect on the strength of the C=O bond.
| Bond | Functional group | Wavenumber / cm-1 |
| C=O | Aldehyde | 1720 – 1740 |
| Ketone | 1705 – 1725 | |
| Carboxylic acid | 1700 – 1725 | |
| Ester | 1735 – 1750 | |
| Amide | 1630 – 1700 | |
| Acyl chloride, acid anhydride | 1750 – 1820 | |
| Carboxylic acid | 2500 – 3300 |
The first thing to say is that it is not about electronegativity. Chlorine and nitrogen have similar electronegativities and yet the C=O stretch in an acyl chloride is at a significantly higher wavenumber than the C=O stretch in an amide.
It all depends on whether the groups bonded to the carbon atom of the carbonyl group are electron donating or electron withdrawing, and this is a balance between the inductive and conjugative effects of these groups.
Electron donating groups …
- Alkyl groups are electron donating (the electrons are attracted to the 𝛅+ carbon of the C=O bond). This lowers the polarity of the C=O bond with the result that the C=O bond is a little longer / weaker and vibrates at a lower wavenumber – this is a positive inductive effect.
- Amides can exist in two resonance forms and the resulting electron donating conjugative effect is far stronger than the negative inductive effect of the electronegative nitrogen atom attracting electron density from the carbon.
- Unsaturated molecules may also exist in resonance forms where an electron donating conjugative effect results the bond vibrating at a lower wavenumber.
Electron withdrawing groups …
- The chlorine atom in an acyl chloride is electronegative and attracts electron density away from the carbon in the C=O bond. This increases the polarity of the C-O bond with the result that the C=O bond is shorter / stronger and vibrates at a higher wavenumber – this is a negative inductive effect.
- The lone pair of electrons on a chlorine atom are in a 3p orbital (in nitrogen the lone pair is in a 2p orbital). Since there is a poor overlap between the 3p orbital on chlorine and the 2p orbital on carbon, the formation of a C-Cl ∏ bond is not favoured so there is no possibility of an electron donating conjugative effect as we see in amides.