740 CHAPTER NINETEEN Carboxylic Acids 19. 4 ACIDITY OF CARBOXYLIC ACIDS Carboxylic acids are the most acidic class of compounds that contain only carbon, hydro- gen, and oxygen. With ionization constants Ka on the order of 10(pKa s 5), they ar much stronger acids than water and alcohols. The case should not be overstated, how- ever. Carboxylic acids are weak acids; a 0. 1 M solution of acetic acid in water, for exam To understand the greater acidity of carboxylic acids compared with water and alcohols, compare the structural changes that accompany the ionization of a representa- tive alcohol (ethanol) and a representative carboxylic acid(acetic acid). The equilibria that define K ar CH3CH2OH H+ CHCHO IHCH3CH,0 ICH3CH,OH Ethoxide ion lonization of acetic acid CH3COH H+ CHz CO k、田CH3CO2 [CH3CO2H) 18×10-5 From these Ka values, the calculated free energies of ionization (AGo) are 91 kJ/mol are calculated from equilib-(21.7 kcal/mol) for ethanol versus 27 kJ/mol (6.5 kcal/mol) for acetic acid. An energy rium constants according to diagram portraying these relationships is presented in Figure 19.3. Since it is equilibria, the relationship not rates, of ionization that are being compared, the diagram shows only the initial and △G°=-RTn final states. It is not necessary to be concerned about the energy of activation, since that affects only the rate of ionization, not the extent of ionization. The large difference in the free energies of ionization of ethanol and acetic acid reflects a greater stabilization of acetate ion relative to ethoxide ion lonization of ethanol yields an alkoxide ion in which the negative charge is localized on oxygen Solvation forces are the chief means by which ethoxide ion is stabilized. Acetate ion is also sta- bilized by solvation, but has two additional mechanisms for dispersing its negative charge that are not available to ethoxide ion: 1. The inductive effect of the carbonyl group. The carbonyl group of acetate ion is electron-withdrawing, and by attracting electrons away from the negatively charged oxygen, acetate anion is stabilized. This is an inductive effect, arising in the polar- ization of the electron distribution in the o bond between the carbonyl carbon and the negatively charged oxygen. Positively polarized CH2 group has carbon attracts elec- ons from negatively H on electron density CH3-CH, at negatively 2. The resonance effect of the carbonyl group. Electron delocalization, expressed by resonance between the following Lewis structures, causes the negative charge in acetate to be shared equally by both oxygens. Electron delocalization of this type is not available to ethoxide ion Back Forward Main MenuToc Study Guide ToC Student o MHHE Website19.4 ACIDITY OF CARBOXYLIC ACIDS Carboxylic acids are the most acidic class of compounds that contain only carbon, hydro￾gen, and oxygen. With ionization constants Ka on the order of 105 (pKa 5), they are much stronger acids than water and alcohols. The case should not be overstated, how￾ever. Carboxylic acids are weak acids; a 0.1 M solution of acetic acid in water, for exam￾ple, is only 1.3% ionized. To understand the greater acidity of carboxylic acids compared with water and alcohols, compare the structural changes that accompany the ionization of a representa￾tive alcohol (ethanol) and a representative carboxylic acid (acetic acid). The equilibria that define Ka are Ionization of ethanol Ionization of acetic acid From these Ka values, the calculated free energies of ionization (G°) are 91 kJ/mol (21.7 kcal/mol) for ethanol versus 27 kJ/mol (6.5 kcal/mol) for acetic acid. An energy diagram portraying these relationships is presented in Figure 19.3. Since it is equilibria, not rates, of ionization that are being compared, the diagram shows only the initial and final states. It is not necessary to be concerned about the energy of activation, since that affects only the rate of ionization, not the extent of ionization. The large difference in the free energies of ionization of ethanol and acetic acid reflects a greater stabilization of acetate ion relative to ethoxide ion. Ionization of ethanol yields an alkoxide ion in which the negative charge is localized on oxygen. Solvation forces are the chief means by which ethoxide ion is stabilized. Acetate ion is also sta￾bilized by solvation, but has two additional mechanisms for dispersing its negative charge that are not available to ethoxide ion: 1. The inductive effect of the carbonyl group. The carbonyl group of acetate ion is electron-withdrawing, and by attracting electrons away from the negatively charged oxygen, acetate anion is stabilized. This is an inductive effect, arising in the polar￾ization of the electron distribution in the bond between the carbonyl carbon and the negatively charged oxygen. 2. The resonance effect of the carbonyl group. Electron delocalization, expressed by resonance between the following Lewis structures, causes the negative charge in acetate to be shared equally by both oxygens. Electron delocalization of this type is not available to ethoxide ion. CH3 C   O Positively polarized O carbon attracts elec￾trons from negatively charged oxygen. CH2 group has negligible effect on electron density at negatively charged oxygen. CH3 CH2 O Acetic acid CH3COH O Acetate ion CH3CO O H Ka  [H][CH3CO2 ] [CH3CO2H]  1.8  105 Ethanol CH3CH2OH H Ethoxide ion CH3CH2O Ka  [H][CH3CH2O] [CH3CH2OH]  1016 740 CHAPTER NINETEEN Carboxylic Acids Free energies of ionization are calculated from equilib￾rium constants according to the relationship G°  RT In Ka Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website