19.7 lonization of substituted Benzoic acids SAMPLE SOLUTION (a)Think of the two compounds as substituted derivatives of acetic acid. a tert-butyl group is slightly electron-releasing and has only a mod- est effect on acidity. the compound(CH3)3 CO2H is expected to have an acid strength similar to that of acetic acid. a trimethylammonium substituent, on the other hand, is positively charged and is a powerful electron-withdrawing sub- stituent. The compound(CH3)3NCH2 CO, H is expected to be a much stronger acid han(CH3)3CCH2 CO, H. The measured ionization constants, shown as follows, con- firm this prediction (CH3)3CCH2 CO2 H ( CH3)3NCH2 CO2H Ka=5×10-6 (pKa=5.3) (pKa=1.8) Another proposal advanced to explain the acid-strengthening effect of polar sub- stituents holds that the electron-withdrawing effect is transmitted through the water mol ecules that surround the carboxylate ion rather than through successive polarization of g bonds. this is referred to as a field effect. both field and inductive contributions to the polar effect tend to operate in the same direction, and it is believed that both are Important It is a curious fact that substituents affect the entropy of ionization more than they do the enthalpy term in the expression AG°=△H°-T△S° The enthalpy term AHo is close to zero for the ionization of most carboxylic acids, regardless of their strength. The free energy of ionization AG is dominated by the C TASO term. Ionization is accompanied by an increase in solvation forces, leading to a ecrease in the entropy of the system; AS is negative, and -TASo is positive. Anions incorporate substituents capable of dispersing negative charge impose less order on 19.7 IONIZATION OF SUBSTITUTED BENZOIC ACIDS A considerable body of data is available on the acidity of substituted benzoic acids. Ben- zoic acid itself is a somewhat stronger acid than acetic acid. Its carboxyl group is attached to an sp-hybridized carbon and ionizes to a greater extent than one that is attached to an sp-hybridized carbon. Remember, carbon becomes more electron-withdrawing as its CH:CO,H CHa=CHCOH CO,H crylic acid K=1.8×10-5K K=63×10-5 (pKa4.8) (pKa4.3) PROBLEM 19.5 What is the most acidic neutral molecule characterized by the formula C3H, O2? Table 19. 3 lists the ionization constants of some substituted benzoic acids. The largest effects are observed when strongly electron-withdrawing substituents are ortho to Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteSAMPLE SOLUTION (a) Think of the two compounds as substituted derivatives of acetic acid. A tert-butyl group is slightly electron-releasing and has only a mod￾est effect on acidity. The compound (CH3)3CCH2CO2H is expected to have an acid strength similar to that of acetic acid. A trimethylammonium substituent, on the other hand, is positively charged and is a powerful electron-withdrawing sub￾stituent. The compound (CH3)3N CH2CO2H is expected to be a much stronger acid than (CH3)3CCH2CO2H. The measured ionization constants, shown as follows, con- firm this prediction. Another proposal advanced to explain the acid-strengthening effect of polar sub￾stituents holds that the electron-withdrawing effect is transmitted through the water mol￾ecules that surround the carboxylate ion rather than through successive polarization of bonds. This is referred to as a field effect. Both field and inductive contributions to the polar effect tend to operate in the same direction, and it is believed that both are important. It is a curious fact that substituents affect the entropy of ionization more than they do the enthalpy term in the expression G°  H° TS° The enthalpy term H° is close to zero for the ionization of most carboxylic acids, regardless of their strength. The free energy of ionization G° is dominated by the TS° term. Ionization is accompanied by an increase in solvation forces, leading to a decrease in the entropy of the system; S° is negative, and TS° is positive. Anions that incorporate substituents capable of dispersing negative charge impose less order on the solvent (water), and less entropy is lost in their production. 19.7 IONIZATION OF SUBSTITUTED BENZOIC ACIDS A considerable body of data is available on the acidity of substituted benzoic acids. Ben￾zoic acid itself is a somewhat stronger acid than acetic acid. Its carboxyl group is attached to an sp2 -hybridized carbon and ionizes to a greater extent than one that is attached to an sp3 -hybridized carbon. Remember, carbon becomes more electron-withdrawing as its s character increases. PROBLEM 19.5 What is the most acidic neutral molecule characterized by the formula C3HxO2? Table 19.3 lists the ionization constants of some substituted benzoic acids. The largest effects are observed when strongly electron-withdrawing substituents are ortho to CH3CO2H Acetic acid Ka  1.8  105 (pKa 4.8) Acrylic acid Ka  5.5  105 (pKa 4.3) CH2 CHCO2H Benzoic acid Ka  6.3  105 (pKa 4.2) CO2H (CH3)3CCH2CO2H Weaker acid Ka  5  106 (pKa  5.3) (CH3)3NCH2CO2H Stronger acid Ka  1.5  102 (pKa  1.8) 19.7 Ionization of Substituted Benzoic Acids 747 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website