Chap19 - CHAPTER 19 Carboxylic Acids 19-2 Structural and...

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1/27/12 1 CHAPTER 19 Carboxylic Acids Structural and Physical Properties of Carboxylic Acids 19-2 Formic acid is planar. The molecular structure of formic acid is roughly planar which is characteristic of carboxylic acids in general.
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1/27/12 2 The carboxy group is polar and forms hydrogen- bonds to other polarized molecules. The carboxy function is strongly polar and forms hydrogen bonds to other polarized molecules such as water , alcohols and other carboxylic acids . An Example NMR and IR Spectroscopy of Carboxylic Acids 19-3 The carboxy hydrogen and carbon are deshielded. Hydrogens on a carbon next to a carbonyl group are slightly deshielded. The effect diminishes rapidly with increasing distance from the carbonyl. The hydroxyl proton resonates at very low field ( δ = 10-13 ppm ). Its chemical shift varies strongly with concentration, solvent, and temperature because of its involvement in hydrogen bonding. δ = 10-13 ppm
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1/27/12 3 The carboxy group shows two important IR bands. Stretching frequencies for both the carbonyl group and the hydroxy substituent are seen in the IR spectra of carboxy groups. The O-H bond exhibits a very broad band at 2500-3300 cm -1 , lower than for alcohols because of strong hydrogen bonding. Acidic and Basic Character of Carboxylic Acids 19-4 Carboxylic acids are relatively strong acids . Carboxylic acids have much lower pK a values than do alcohols . The lowered pK a values are due to the electron withdrawing effect of the positively polarized carbonyl carbon, and the resonance stabilization of the carboxylate group.
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1/27/12 4 Two of the three resonance forms of the carboxylate ion are equivalent, leading to a symmetrical ion with equal carbon- oxygen bond lengths (1.26 Å), midway between a carbon-oxygen double bond (1.20 Å) and a carbon-oxygen single bond (1.34 Å). On Chalk Board Electron-withdrawing substituents increase the acidity of carboxylic acids. The inductive effect of electron withdrawing groups close to the carboxy group causes an increase in acidity. Three electron withdrawing groups on the α -carbon sometimes results in acidity near that of some inorganic acids.
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1/27/12 5 The dioic acids have two pK a values. In ethanedioic and propanedioic acids the first pK a is lowered by the electron withdrawing effect of the second. In higher dioic acids, both pK a values are close to monocarboxylic acids.
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