Contributing structures carboxylic acid derivatives

Because carboxylic acid derivatives (RCOZ) all contain an atom Z with a nonbonded electron pair, three resonance structures can be drawn for RCOZ, compared to just two for aldehydes and ketones (Section 20.1). These three resonance structures stabilize RCOZ by delocalizing electron density. In fact, the more resonance structures 2 and 3 contribute to the resonance hybrid, the more stable RCOZ is. 

The second effect derives from the relative resonance stabilization of the carboxylic acid derivatives. As follows, each derivative can be written with contributing structures that will be stabilizing to some extent. The second contributing structure that we show for each carboxylic acid derivative has a positive charge on the carbonyl carbon. This structure reflects the electrophilicity of these carbons. However, for ea derivative, it is the other contributing structures that reflect the relative resonance stabilization of the derivatives. 

The main evidence for the assumption that the dipolar form contributes greatly to the amide structure in many amino acid derivatives is provided by X-ray data. Thus in diketopiperazine the distance between the carboxyl carbon and nitrogen atoms was found to be 1.33 A., as... 

---