CHAPTER NINETEEN Carboxylic Acids TABLE 19.3 Acidity of Some Substituted Benzoic Acids Ka (pka)* for different positions of substituent x Substituent in XC H4CO2H Ortho Para -5(4.2) .3×10-5(4.2) 63×10-5(42) 2.CH3 12×10-4(39) 53×10-5(4.3)42×10-5(44) 3.F 54×10-4(3.3) 14×10-4(39) 72×10-5(4.1) 4. CI (29) 15×10-4(3.8) 15×10-4(3.8) 14×10-3(2.9) 14×10-4(39) 92×10-5(40) 7. CH3o 81×10-5(4.1) 82×10-5(4.1) 34×10-5(4.5) 8.O2N 67×10-3(2.2)32×10-43.5) 38×10-4(34) *In water at25° group. In those cases the pk a values are clustered in the range 3.54 y of benzoic the carboxyl group. An o-nitro substituent, for example, increases the acidi acid 100-fold. Substituent effects are small at positions meta and para to the carboxyl 19.8 DICARBOXYLIC ACIDS Separate ionization constants, designated KI and K2, respectively, characterize the two successive ionization steps of a dicarboxylic acid HoC-CoH、H++HoC-Co-k1=65×10-2 Hydrogen oxalate pK1=1.2 occurs naturally in a number of plants including sorrel and eep houseplants out of the each of small children, who HOC-CO H++“OC—CO-K2=5.3×10-5 might be tempted to eat the aves or berries Hydrogen oxalate Oxalate The first ionization constant of dicarboxylic acids is larger than Ka for boxylic analogs. One reason is statistical. There are two potential sites for ionization rather than one, making the effective concentration of carboxyl groups twice as large Furthermore, one carboxyl group acts as an electron-withdrawing group to facilitate dis- sociation of the other. This is particularly noticeable when the two carboxyl groups are separated by only a few bonds. Oxalic and malonic acid, for example, are several orders of magnitude stronger than simple alkyl ervative es of acetic acid. Heptanedioic acid, in which the carboxyl groups are well separated from each other, is only slightly stronger han acetic acid HO,CCO,H HO, CCH, CO,H HO,C(CH,)5,H Oxalic acid Malonic acid Heptanedioic acid K16.5×10 K11.4×10-3 K13.1×10-5 (pk11.2) (pK12.8) (pK14.3) Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitethe carboxyl group. An o-nitro substituent, for example, increases the acidity of benzoic acid 100-fold. Substituent effects are small at positions meta and para to the carboxyl group. In those cases the pKa values are clustered in the range 3.5–4.5. 19.8 DICARBOXYLIC ACIDS Separate ionization constants, designated K1 and K2, respectively, characterize the two successive ionization steps of a dicarboxylic acid. The first ionization constant of dicarboxylic acids is larger than Ka for monocar￾boxylic analogs. One reason is statistical. There are two potential sites for ionization rather than one, making the effective concentration of carboxyl groups twice as large. Furthermore, one carboxyl group acts as an electron-withdrawing group to facilitate dis￾sociation of the other. This is particularly noticeable when the two carboxyl groups are separated by only a few bonds. Oxalic and malonic acid, for example, are several orders of magnitude stronger than simple alkyl derivatives of acetic acid. Heptanedioic acid, in which the carboxyl groups are well separated from each other, is only slightly stronger than acetic acid. HO2CCO2H Oxalic acid K1 6.5  102 (pK1 1.2) HO2CCH2CO2H Malonic acid K1 1.4  103 (pK1 2.8) HO2C(CH2)5CO2H Heptanedioic acid K1 3.1  105 (pK1 4.3) H K1  6.5  102 Oxalic acid pK1  1.2 HOC COH O O Hydrogen oxalate (monoanion) HOC CO O O K1 H K2  5.3  105 Oxalate pK2  4.3 (Dianion) OC CO O O Hydrogen oxalate (monoanion) HOC CO O O K2 748 CHAPTER NINETEEN Carboxylic Acids TABLE 19.3 Acidity of Some Substituted Benzoic Acids Substituent in XC6H4CO2H *In water at 25°C. 1. H 2. CH3 3. F 4. Cl 5. Br 6. I 7. CH3O 8. O2N Ortho 6.3  105 (4.2) 1.2  104 (3.9) 5.4  104 (3.3) 1.2  103 (2.9) 1.4  103 (2.8) 1.4  103 (2.9) 8.1  105 (4.1) 6.7  103 (2.2) Meta 6.3  105 (4.2) 5.3  105 (4.3) 1.4  104 (3.9) 1.5  104 (3.8) 1.5  104 (3.8) 1.4  104 (3.9) 8.2  105 (4.1) 3.2  104 (3.5) Para 6.3  105 (4.2) 4.2  105 (4.4) 7.2  105 (4.1) 1.0  104 (4.0) 1.1  104 (4.0) 9.2  105 (4.0) 3.4  105 (4.5) 3.8  104 (3.4) Ka (pKa)* for different positions of substituent X Oxalic acid is poisonous and occurs naturally in a number of plants including sorrel and begonia. It is a good idea to keep houseplants out of the reach of small children, who might be tempted to eat the leaves or berries. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website