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

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  • 19.6Substituents and Acid Strength

  • Substituent Effects on Aciditystandard of comparison is acetic acid (X = H)Ka = 1.8 x 10-5 pKa = 4.7

  • Substituent Effects on Acidityalkyl substituents have negligible effect

  • Substituent Effects on Acidityelectronegative substituents increase acidity

  • Substituent Effects on Acidityelectronegative substituents withdraw electrons from carboxyl group; increase K for loss of H+

  • Substituent Effects on Acidityeffect of electronegative substituent decreases as number of bonds between X and carboxyl group increases

    XKapKaH1.8 x 10-54.71.4 x 10-32.91.0 x 10-44.0ClCH2Cl3.0 x 10-54.5ClCH2CH2

  • 19.7Ionization ofSubstituted Benzoic Acids

  • Hybridization Effectsp2-hybridized carbon is more electron-withdrawing than sp3, and sp is more electron-withdrawing than sp2

  • 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

  • 19.8Dicarboxylic Acids

  • Dicarboxylic Acidsone carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separationOxalic acidMalonic acidHeptanedioic acid1.22.84.3pKa

  • 19.9Carbonic Acid

  • Carbonic AcidCO2+H2O99.7%0.3%

  • Carbonic AcidCO2+H2OH++

  • 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

  • Carbonic AcidH++Ka = 5.6 x 10-11Second ionization constant:

  • 19.10Sources of Carboxylic Acids

  • 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)

  • 19.11Synthesis of Carboxylic Acids by the Carboxylation of Grignard Reagents

  • Carboxylation of Grignard Reagentsconverts an alkyl (or aryl) halide to a carboxylic acid having one more carbon atom than the starting halideRXMgdiethyl etherRMgXCO2H3O+

  • Carboxylation of Grignard ReagentsCO dH3O+

  • Example: Alkyl Halide(76-86%)1. Mg, diethyl ether2. CO23. H3O+ClCO2H

  • Example: Aryl Halide(82%)1. Mg, diethyl ether2. CO23. H3O+

  • 19.12Synthesis of Carboxylic Acidsby thePreparation and Hydrolysis of Nitriles

  • 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+

  • ExampleNaCNDMSO(77%)H2OH2SO4heat(92%)

  • Example: Dicarboxylic AcidBrCH2CH2CH2BrNaCNH2OH2O, HClheat(77-86%)NCCH2CH2CH2CN(83-85%)

  • via Cyanohydrin1. NaCN2. H+(60% from 2-pentanone)H2OHCl, heat

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