Electronic structure of carbonyl compounds

The electronic structure of the carbonyl group is perturbed by the rest of the molecule. In classical terms, a carbonyl compound may be represented as a resonance hybrid of the canonical structure I to V, viz. 

Structures III and IV assist ionisation of the C-X bond, whereas structure II facilitates nucleophilic addition and consequently a bimolecular displacement of X. The various derivatives of carboxylic acids form a series with varying degrees of resonance stabilisation decreasing in the following order  

The amide group is resonance-stabilised to a greater extent than the ester group, as shown by the extensive evidence of X-ray analysis of the peptide bond10 and the carbon-nitrogen bond distance in amides8. 

---