Electron delocalization carboxylic acid derivatives

Amides are carboxylic acid derivatives. The amide group is recognized by the nitrogen connected to the carbonyl group. Amides are neutral compounds the electrons are delocalized into the carbonyl (resonance) and thus, are not available to bond to a hydrogen ion. 

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. 

Because the carbon-chlorine bond is so long—typically on the order of 180 pm for acyl chlorides—overlap between the 3p orbitals of chlorine and the tt orbital of the carbonyl gronp is poor. Conseqnently, there is little delocalization of the electron pairs of chlorine into the tt system. The carbonyl group of an acyl chloride feels the normal electron-withdrawing inductive effect of a chlorine substituent without a significant compensating electron-releasing effect dne to lone-pair donation by chlorine. This makes the carbonyl carbon of an acyl chloride more susceptible to attack by nucleophiles than that of other carboxylic acid derivatives. 

There are large differences in the reactivity of the various carboxylic acid derivatives, such as amides, esters, and acyl chlorides. One important factor is the resonance stabilization provided by the heteroatom substituent, which is in the order N > O > Cl. Electron delocalization reduces the electrophilicity of the carbonyl group and the corresponding stabilization is lost in the tetrahedral intermediate. 

Second, the weaker the basicity of Y, the smaller is the contribution from the resonance contributor with a positive charge on Y (Section 17.2) the less the carboxylic acid derivative is stabilized by electron delocalization, the more reactive it will be. 

Steric effects on the reactivity of benzoic acid derivatives are a little more complicated. The first significant point is that the aromatic carboxylic acids and esters are often some two orders of magnitude less reactive than the corresponding aliphatic compounds. Chapman et al. 7, have explained this difference in terms of three co-operative factors (i) the stabilization of the initial state by delocalization in the case of the aromatic compounds, (ii) inductive electron-withdrawal by the ring, which is significant in esterification... 

---