512 CARBOXYLIC ACIDS attached to the carboxyl group in benzoic acid stabilizes benzoate anion better than the cor responding sp-hybridized carbon stabilizes cyclohexanecarboxylate. Benzoic acid is stronger acid CO,H CO2H Benzoic acid K.1.2×10-5 (d) Its five fluorine substituents make the pentafluorophenyl group more electron-withdrawing than an unsubstituted phenyl group. Thus, pentafluorobenzoic acid is a stronger acid than benzoic acid COH CO,H Pentafluorobenzoic acid Benzoic acid 1×10-4 Ka6.7×10 (pK3.4) (e) The pentafluorophenyl substituent is electron-withdrawing and increases the acidity of a car- boxyl group to which it is attached. Its electron-withdrawing effect decreases with distance Pentafluorobenzoic acid is a stronger acid than p-(pentafluorophenyl)benzoic acid F COH COH Pentafluorobenzoic acid p-(Pentafluorophenyl)benzoic acid K241×10-4 (K, not measured in water; comparable (pk3.4) th benzoic acid in acidity) (f) The oxygen of the ring exercises an acidifying effect on the carboxyl group This effect is largest when the oxygen is attached directly to the carbon that bears the carboxyl group Furan-2-carboxylic acid is thus a stronger acid than furan-3-carboxylic acid COH Furan-2-carboxylic acid Furan-3-carboxylic acid K (g) Furan-2-carboxylic acid has an oxygen attached to the carbon that bears the carboxyl group, whereas pyrrole-2-carboxylic acid has a nitrogen in that position. Oxygen is more Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Websiteattached to the carboxyl group in benzoic acid stabilizes benzoate anion better than the cor￾responding sp3 -hybridized carbon stabilizes cyclohexanecarboxylate. Benzoic acid is a stronger acid. (d) Its five fluorine substituents make the pentafluorophenyl group more electron-withdrawing than an unsubstituted phenyl group. Thus, pentafluorobenzoic acid is a stronger acid than benzoic acid. (e) The pentafluorophenyl substituent is electron-withdrawing and increases the acidity of a car￾boxyl group to which it is attached. Its electron-withdrawing effect decreases with distance. Pentafluorobenzoic acid is a stronger acid than p-(pentafluorophenyl)benzoic acid. ( f) The oxygen of the ring exercises an acidifying effect on the carboxyl group. This effect is largest when the oxygen is attached directly to the carbon that bears the carboxyl group. Furan-2-carboxylic acid is thus a stronger acid than furan-3-carboxylic acid. (g) Furan-2-carboxylic acid has an oxygen attached to the carbon that bears the carboxyl group, whereas pyrrole-2-carboxylic acid has a nitrogen in that position. Oxygen is more Furan-2-carboxylic acid (pKa 3.2) Ka 6.9  104 CO2H O Furan-3-carboxylic acid (pKa 3.9) Ka 1.1  104 CO2H O Pentafluorobenzoic acid Ka 4.1  104 (pKa 3.4) p-(Pentafluorophenyl)benzoic acid (Ka not measured in water; comparable with benzoic acid in acidity) CO2H F F F F F CO2H F F F F F Pentafluorobenzoic acid Ka 4.1  104 (pKa 3.4) Benzoic acid Ka 6.7  105 (pKa 4.2) CO2H CO2H F F F F F Benzoic acid Ka 6.7  105 (pKa 4.2) CO2H Cyclohexanecarboxylic acid Ka 1.2  105 (pKa 4.9) CO2H 512 CARBOXYLIC ACIDS __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website