CHAPTER TWENTY Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution FIGURE 20.2 The three o bonds originating at X=Cl; acyl chloride X=OCR: acid anhydride planar. The p orbital of the arbonyl carbon, its oxyger nd the atom by which X= OR: ester group x is attached to the acyl group overlap to form an extended T syster through which the elec trons are delocalized donating properties of the substituent X. Generally, the less electronegative X is, the bet- ter it donates electrons to the carbonyl group and the greater its stabilizing effect Resonance stabilization in acyl chlorides is not nearly as pronounced as in other derivatives of carboxylic acids Weak resonance stabilization 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 T orbital of the carbonyl group is poor. Consequently, there is little delocalization of the electron pairs of chlo- rine into the T system. The carbonyl group of an acyl chloride feels the normal electron- withdrawing inductive effect of a chlorine substituent without a significant compensat ing electron-releasing effect due 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 Acid anhydrides are better stabilized by electron delocalization than are acyl chlo- rides. The lone-pair electrons of oxygen are delocalized more effectively into the car bonyl group Resonance involves both carbonyl groups of an acid anhydride The carbonyl group of an ester is stabilized more than is that of an anhydride Since both acyl groups of an anhydride compete for the oxygen lone pair, each carbonyl is stabilized less than the single carbonyl group of an ester. is more effective th Ester Acid anhydride Esters are stabilized by resonance to about the same extent as carboxylic acids bu h as amides. Nitrogen is less electronegative than oxygen and is a better electron-pair donor Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitedonating properties of the substituent X. Generally, the less electronegative X is, the bet￾ter it donates electrons to the carbonyl group and the greater its stabilizing effect. Resonance stabilization in acyl chlorides is not nearly as pronounced as in other derivatives of carboxylic acids: 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 orbital of the carbonyl group is poor. Consequently, there is little delocalization of the electron pairs of chlo￾rine into the system. The carbonyl group of an acyl chloride feels the normal electron￾withdrawing inductive effect of a chlorine substituent without a significant compensat￾ing electron-releasing effect due 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. Acid anhydrides are better stabilized by electron delocalization than are acyl chlo￾rides. The lone-pair electrons of oxygen are delocalized more effectively into the car￾bonyl group. Resonance involves both carbonyl groups of an acid anhydride. The carbonyl group of an ester is stabilized more than is that of an anhydride. Since both acyl groups of an anhydride compete for the oxygen lone pair, each carbonyl is stabilized less than the single carbonyl group of an ester. Esters are stabilized by resonance to about the same extent as carboxylic acids but not as much as amides. Nitrogen is less electronegative than oxygen and is a better electron-pair donor. is more effective than Ester R OR C O R C C R O O O Acid anhydride C C R R O O  O R C C R O O O R C O C R O O  R C  O Cl R Cl C O Weak resonance stabilization 778 CHAPTER TWENTY Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution O C X X  OH; carboxylic acid X  Cl; acyl chloride X  OCR; acid anhydride X O X  OR; ester X  NR2; amide FIGURE 20.2 The three bonds originating at the carbonyl carbon are coplanar. The p orbital of the carbonyl carbon, its oxygen, and the atom by which group X is attached to the acyl group overlap to form an extended system through which the elec￾trons are delocalized. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website