19.6 Substituents and Acid Strength

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19.6 Substituents and Acid Strength. O. CH 2 COH. X. Substituent Effects on Acidity. standard of comparison is acetic acid ( X = H). K a = 1.8 x 10 -5 p K a = 4.7. O. CH 2 COH. X. X. K a. p K a. H. 1.8 x 10 -5. 4.7. 1.3 x 10 -5. 4.9. CH 3. CH 3 (CH 2 ) 5. 1.3 x 10 -5. 4.9. - PowerPoint PPT Presentation


<ul><li><p>19.6Substituents and Acid Strength</p></li><li><p>Substituent Effects on Aciditystandard of comparison is acetic acid (X = H)Ka = 1.8 x 10-5 pKa = 4.7</p></li><li><p>Substituent Effects on Acidityalkyl substituents have negligible effect</p></li><li><p>Substituent Effects on Acidityelectronegative substituents increase acidity</p></li><li><p>Substituent Effects on Acidityelectronegative substituents withdraw electrons from carboxyl group; increase K for loss of H+</p></li><li><p>Substituent Effects on Acidityeffect of electronegative substituent decreases as number of bonds between X and carboxyl group increases</p><p>XKapKaH1.8 x 10-54.71.4 x 10-32.91.0 x 10-44.0ClCH2Cl3.0 x 10-54.5ClCH2CH2</p></li><li><p>19.7Ionization ofSubstituted Benzoic Acids</p></li><li><p>Hybridization Effectsp2-hybridized carbon is more electron-withdrawing than sp3, and sp is more electron-withdrawing than sp2</p></li><li><p>Table 19.3 Ionization of Substituted Benzoic Acidseffect is small unless X is electronegative; effect is largest for ortho substituentpKaSubstituentorthometaparaH4.24.24.2CH33.94.34.4F3.33.94.1Cl2.93.84.0CH3O4.14.14.5NO22.23.53.4</p></li><li><p>19.8Dicarboxylic Acids</p></li><li><p>Dicarboxylic Acidsone carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separationOxalic acidMalonic acidHeptanedioic acid1.22.84.3pKa</p></li><li><p>19.9Carbonic Acid</p></li><li><p>Carbonic AcidCO2+H2O99.7%0.3%</p></li><li><p>Carbonic AcidCO2+H2OH++</p></li><li><p>Carbonic AcidCO2 is major species present in a solution of "carbonic acid" in acidic mediaCO2+H2OH++overall K for these two steps = 4.3 x 10-7</p></li><li><p>Carbonic AcidH++Ka = 5.6 x 10-11Second ionization constant:</p></li><li><p>19.10Sources of Carboxylic Acids</p></li><li><p>Synthesis of Carboxylic Acids: Reviewside-chain oxidation of alkylbenzenes (Section 11.13) oxidation of primary alcohols (Section 15.10)oxidation of aldehydes (Section 17.15)</p></li><li><p>19.11Synthesis of Carboxylic Acids by the Carboxylation of Grignard Reagents</p></li><li><p>Carboxylation of Grignard Reagentsconverts an alkyl (or aryl) halide to a carboxylic acid having one more carbon atom than the starting halideRXMgdiethyl etherRMgXCO2H3O+</p></li><li><p>Carboxylation of Grignard ReagentsCO dH3O+</p></li><li><p>Example: Alkyl Halide(76-86%)1. Mg, diethyl ether2. CO23. H3O+ClCO2H</p></li><li><p>Example: Aryl Halide(82%)1. Mg, diethyl ether2. CO23. H3O+</p></li><li><p>19.12Synthesis of Carboxylic Acidsby thePreparation and Hydrolysis of Nitriles</p></li><li><p>Preparation and Hydrolysis of Nitrilesconverts an alkyl halide to a carboxylic acid having one more carbon atom than the starting halidelimitation is that the halide must be reactive toward substitution by SN2 mechanismRXSN2H3O+heat+ NH4+</p></li><li><p>ExampleNaCNDMSO(77%)H2OH2SO4heat(92%)</p></li><li><p>Example: Dicarboxylic AcidBrCH2CH2CH2BrNaCNH2OH2O, HClheat(77-86%)NCCH2CH2CH2CN(83-85%)</p></li><li><p>via Cyanohydrin1. NaCN2. H+(60% from 2-pentanone)H2OHCl, heat</p><p>2122232323232623282930312323232312452388115131413</p></li></ul>


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