CHAPTER NINETEEN Carboxylic Acids TABLE 19.2 Effect of Substituents on Acidity of Carboxylic Acids Name of acid Structure constant k* Standard of comparison Acetic acid CH3CO2H 18×10-5 Alkyl substituents have a negligible effect on acidity Propanoic acid CH3 CH2 CO2H 1.3×10-5 4.9 2-Methylpropanoic acid (CH3)2CHCO2H 4.8 2, 2-Dimethylpropanoic acid (CH3)3CCO2H 0.9×10 5.1 Heptanoic acid CH3(CH2)5CO2H 1.3×10 4.9 c-Halogen substituents increase acidity Fluoroacetic acid 2.5×10-3 2.6 Chloroacetic acid CICH2 CO 14×10-3 2.9 2.9 Dichloroacetic acid Cl2CHCO 50×10-2 1.3 Trichlo 1.3×10-1 0.9 Electron-attracting groups increase acidity Aethoxyace CH3OCH2 CO2H 2.7×10 3.6 Cyanoacetic acid 34×10-3 Nitroacetic acid O2NCH2CO2H 2.1×10-2 1.7 wIn water at25°C Inductive effects fall off rapidly as the number of o bonds betw een the group and the substituent increases. Consequently, the acid-strengthening effect of gen decreases as it becomes more remote from the carboxyl group CICH,CO,H CICH,CHCO,H CICH,CHCHCO,H Chloroacetic acid 3-Chloropropanoid Ka=1.4×1 Ka=1.0×10 Ka=3.0×10 pKa=2.9 PROBLEM 19. 4 Which is the stronger acid in each of the following pairs? (a)(CH3)3CCH2 CO H or (CH3)3NCH2 CO2H (b)CH3 CH2CO2H or CH3 CHCO2H (c) CH3 CCO2H or CH2CHCO2H d)CH3 CH2CH2 CO2H or CH3 SCH2 CO2H Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteInductive effects fall off rapidly as the number of bonds between the carboxyl group and the substituent increases. Consequently, the acid-strengthening effect of a halo￾gen decreases as it becomes more remote from the carboxyl group: PROBLEM 19.4 Which is the stronger acid in each of the following pairs? (a) (CH3)3CCH2CO2H or (CH3)3N CH2CO2H (b) (c) (d) CH3CH2CH2CO2H CH3SCH2CO2H O X X O or CH3CCO2H O X or CH2œCHCO2H CH3CH2CO2H CH3CHCO2H OH W or ClCH2CO2H Chloroacetic acid Ka  1.4  103 pKa  2.9 ClCH2CH2CO2H 3-Chloropropanoic acid Ka  1.0  104 pKa  4.0 ClCH2CH2CH2CO2H 4-Chlorobutanoic acid Ka  3.0  105 pKa  4.5 746 CHAPTER NINETEEN Carboxylic Acids TABLE 19.2 Effect of Substituents on Acidity of Carboxylic Acids Name of acid *In water at 25°C. Acetic acid Standard of comparison. Alkyl substituents have a negligible effect on acidity. Propanoic acid 2-Methylpropanoic acid 2,2-Dimethylpropanoic acid Heptanoic acid -Halogen substituents increase acidity. Fluoroacetic acid Chloroacetic acid Bromoacetic acid Dichloroacetic acid Trichloroacetic acid Ionization constant Ka* 1.8  105 1.3  105 1.6  105 0.9  105 1.3  105 2.5  103 1.4  103 1.4  103 5.0  102 1.3  101 2.7  104 3.4  103 2.1  102 pKa 4.7 4.9 4.8 5.1 4.9 2.6 2.9 2.9 1.3 0.9 3.6 2.5 1.7 Structure CH3CO2H CH3CH2CO2H (CH3)2CHCO2H (CH3)3CCO2H CH3(CH2)5CO2H FCH2CO2H ClCH2CO2H BrCH2CO2H Cl2CHCO2H Cl3CCO2H CH3OCH2CO2H NPCCH2CO2H O2NCH2CO2H Electron-attracting groups increase acidity. Methoxyacetic acid Cyanoacetic acid Nitroacetic acid Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website