0.2 Structure of Carboxylic Acid Derivatives Very effective resonance stabilization Amide resonance is a powerful stabilizing force and gives rise to a number of structural effects. Unlike the pyramidal arrangement of bonds in ammonia and amines, the bonds to nitrogen in amides lie in the same plane. The carbon-nitrogen bond has considerable double-bond character and, at 135 pm, is substantially shorter than the normal 147-pm carbon-nitrogen single-bond distance observed in amines The barrier to rotation about the carbon-nitrogen bond in amides is 75 to 85 kJ/mol (18-20 kcal/mol) R E=75-85 k/mol R er in ethane is only 12 I( kcal/mol) This is an unusually high rotational energy barrier for a single bond and indicates that the carbon-nitrogen bond has significant double-bond character, as the resonance picture PROBLEM 20.2 The 'H NMR spectrum of N, N-dimethylformamide shows a sep arate signal for each of the two methyl groups. Can you explain why? Electron release from nitrogen stabilizes the carbonyl group of amides and decreases the rate at which nucleophiles attack the carbonyl carbon. Nucleophilic reagents attack electrophilic sites in a molecule; if electrons are donated elec- trophilic site in a molecule by a substituent, then the tendency of that molecule to react with external nucleophiles is moderated An extreme example of carbonyl group stabilization is seen in carboxylate anions: O The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group. Resonance in carboxylate anions is more effective than resonance in carboxylic acids, acyl chlorides, anhydrides, esters, and amides Table 20. 1 summarizes the stabilizing effects of substituents on carbonyl groups to which they are attached. In addition to a qualitative ranking, quantitative estimates of the relative rates of hydrolysis of the various classes of acyl derivatives are given. A weakly stabilized carboxy lic acid derivative reacts with water faster than does a more stabilized one toa. Most methods for their preparation convert one class of carboxylic acid derivative nother, and the order of carbonyl group stabilization given in Table 20. 1 bears directly on the means by which these transformations may be achieved. A reaction that converts one carboxylic acid derivative to another that lies below it in the table is practical;a reaction that converts it to one that lies above it in the table is not. This is another way f saying that one carboxylic acid derivative can be converted to another if the reaction Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAmide resonance is a powerful stabilizing force and gives rise to a number of structural effects. Unlike the pyramidal arrangement of bonds in ammonia and amines, the bonds to nitrogen in amides lie in the same plane. The carbon–nitrogen bond has considerable double-bond character and, at 135 pm, is substantially shorter than the normal 147-pm carbon–nitrogen single-bond distance observed in amines. The barrier to rotation about the carbon–nitrogen bond in amides is 75 to 85 kJ/mol (18–20 kcal/mol). This is an unusually high rotational energy barrier for a single bond and indicates that the carbon–nitrogen bond has significant double-bond character, as the resonance picture suggests. PROBLEM 20.2 The 1 H NMR spectrum of N,N-dimethylformamide shows a sep￾arate signal for each of the two methyl groups. Can you explain why? Electron release from nitrogen stabilizes the carbonyl group of amides and decreases the rate at which nucleophiles attack the carbonyl carbon. Nucleophilic reagents attack electrophilic sites in a molecule; if electrons are donated to an elec￾trophilic site in a molecule by a substituent, then the tendency of that molecule to react with external nucleophiles is moderated. An extreme example of carbonyl group stabilization is seen in carboxylate anions: The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group. Resonance in carboxylate anions is more effective than resonance in carboxylic acids, acyl chlorides, anhydrides, esters, and amides. Table 20.1 summarizes the stabilizing effects of substituents on carbonyl groups to which they are attached. In addition to a qualitative ranking, quantitative estimates of the relative rates of hydrolysis of the various classes of acyl derivatives are given. A weakly stabilized carboxylic acid derivative reacts with water faster than does a more stabilized one. Most methods for their preparation convert one class of carboxylic acid derivative to another, and the order of carbonyl group stabilization given in Table 20.1 bears directly on the means by which these transformations may be achieved. A reaction that converts one carboxylic acid derivative to another that lies below it in the table is practical; a reaction that converts it to one that lies above it in the table is not. This is another way of saying that one carboxylic acid derivative can be converted to another if the reaction R C  O O R C O O Eact  75–85 kJ/mol (18–20 kcal/mol) C R R R O C N N R O R R R C  O NR 2 R NR 2 C O Very effective resonance stabilization 20.2 Structure of Carboxylic Acid Derivatives 779 Recall that the rotational barrier in ethane is only 12 kJ/mol (3 kcal/mol). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website