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Chapter 18 Carboxylic Acids

Chapter 18 Carboxylic Acids. Carboxylic Acid Nomenclature. O. HCOH. O. CH 3 COH. O. CH 3 (CH 2 ) 16 COH. Table 18.1 . systematic IUPAC names replace "-e" ending of alkane with "oic acid". Systematic Name. methanoic acid. ethanoic acid. octadecanoic acid. O. HCOH. O. CH 3 COH. O.

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Chapter 18 Carboxylic Acids

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  1. Chapter 18Carboxylic Acids

  2. Carboxylic Acid Nomenclature

  3. O HCOH O CH3COH O CH3(CH2)16COH Table 18.1 • systematic IUPAC names replace "-e" ending of alkane with "oic acid" Systematic Name methanoic acid ethanoic acid octadecanoic acid

  4. O HCOH O CH3COH O CH3(CH2)16COH Table 18.1 • common names are based on natural origin rather than structure Systematic Name Common Name methanoic acid formic acid ethanoic acid acetic acid octadecanoic acid stearic acid

  5. O CH3CHCOH O OH CH3(CH2)7 (CH2)7COH C C H H Table 18.1 Systematic Name Common Name 2-hydroxypropanoicacid lactic acid (Z)-9-octadecenoicacid oleic acid

  6. Structure and Bonding

  7. Formic acid is planar

  8. Formic acid is planar O H C O 120 pm H 134 pm

  9. – •• •• R O R O + •• •• •• C C O O •• •• •• •• H H Electron Delocalization

  10. – – •• •• •• R O R O R O + •• •• •• •• •• C C C + O O O •• •• •• •• •• H H H Electron Delocalization • stabilizes carbonyl group

  11. Physical Properties

  12. OH O O OH 141°C Boiling Points • Intermolecular forces, especially hydrogen bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight bp 31°C 80°C 99°C

  13. O O H CCH3 H3CC O H O Hydrogen-bonded Dimers • Acetic acid exists as a hydrogen-bonded dimer in the gas phase. The hydroxyl group of each molecule is hydrogen-bonded to the carbonyl oxygen of the other.

  14. Hydrogen-bonded Dimers • Acetic acid exists as a hydrogen-bonded dimer in the gas phase. The hydroxyl group of each molecule is hydrogen-bonded to the carbonyl oxygen of the other.

  15. H O O H H3CC H O H O H Solubility in Water • carboxylic acids are similar to alcohols in respect to their solubility in water • form hydrogen bonds to water

  16. Acidity of Carboxylic Acids • Most carboxylic acids have a pKa close to 5.

  17. O CH3COH Carboxylic acids are weak acids • but carboxylic acids are far more acidic than alcohols CH3CH2OH Ka = 1.8 x 10-5 pKa = 4.7 Ka = 10-16 pKa = 16

  18. O CH3CO– + H+ O CH3COH Free Energies of Ionization CH3CH2O– + H+ DG°= 64 kJ/mol DG°= 91 kJ/mol DG°= 27 kJ/mol CH3CH2OH

  19. O RC O d+ – •• •• O O •• •• •• RC RC •• – O O •• •• •• •• Greater acidity of carboxylic acids is attributedstabilization of carboxylate ion by inductive effect of carbonyl group – resonance stabilization of carboxylate ion

  20. Figure 19.4: Electrostatic potential maps ofacetic acid and acetate ion Acetic acid Acetate ion

  21. Substituents and Acid Strength

  22. O CH2COH X Substituent Effects on Acidity standard of comparison is acetic acid (X = H) Ka = 1.8 x 10-5pKa = 4.7

  23. O CH2COH X X Ka pKa H 1.8 x 10-5 4.7 1.3 x 10-5 4.9 CH3 CH3(CH2)5 1.3 x 10-5 4.9 Substituent Effects on Acidity • alkyl substituents have negligible effect

  24. O CH2COH X X Ka pKa H 1.8 x 10-5 4.7 2.5 x 10-3 2.6 F Cl 1.4 x 10-3 2.9 Substituent Effects on Acidity • electronegative substituents increase acidity

  25. O CH2COH X Substituent Effects on Acidity • electronegative substituents withdraw electrons from carboxyl group; increase K for loss of H+

  26. O CH2COH X Substituent Effects on Acidity • effect of substituent decreases as number of bonds between X and carboxyl group increases X Ka pKa H 1.8 x 10-5 4.7 Cl 1.4 x 10-3 2.9 ClCH2 1.0 x 10-4 4.0 ClCH2CH2 3.0 x 10-5 4.5

  27. Ionization ofSubstituted Benzoic Acids

  28. Ka pKa O 6.3 x 10-5 4.2 COH O 5.5 x 10-5 4.3 COH H2C CH O 1.4 x 10-2 1.8 COH HC C Hybridization Effect • sp2-hybridized carbon is more electron-withdrawing than sp3, and sp is more electron-withdrawing than sp2

  29. O X COH Ionization of Substituted Benzoic Acids • effect is small unless X is electronegative; effect is largest for ortho substituent pKa Substituent ortho meta para H 4.2 4.2 4.2 CH3 3.9 4.3 4.4 F 3.3 3.9 4.1 Cl 2.9 3.8 4.0 CH3O 4.1 4.1 4.5 NO2 2.2 3.5 3.4

  30. Salts of Carboxylic Acids

  31. O O Carboxylic acids are neutralized by strong bases • equilibrium lies far to the right; K is ~ 1011 • as long as the molecular weight of the acid is not too high, sodium and potassium carboxylate salts are soluble in water + + RCOH HO– RCO– H2O strongeracid weakeracid

  32. O O CH3(CH2)16CO Micelles • unbranched carboxylic acids with 12-18 carbonsgive carboxylate salts that form micelles inwater ONa sodium stearate(sodium octadecanoate) – Na+

  33. O Micelles ONa • sodium stearate has a polar end (the carboxylate end) and a nonpolar "tail" • the polar end is "water-loving" or hydrophilic • the nonpolar tail is "water-hating" or hydrophobic • in water, many stearate ions cluster together to form spherical aggregates; carboxylate ions on the outside and nonpolar tails on the inside polar nonpolar

  34. O Micelles ONa polar nonpolar

  35. Figure 19.5 A micelle

  36. Micelles • The interior of the micelle is nonpolar and has the capacity to dissolve nonpolar substances. • Soaps clean because they form micelles, which are dispersed in water. • Grease (not ordinarily soluble in water) dissolves in the interior of the micelle and is washed away with the dispersed micelle.

  37. Dicarboxylic Acids

  38. O O HOC COH O O HOCCH2COH O O HOC(CH2)5COH Dicarboxylic Acids pKa Oxalic acid 1.2 • one carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separation Malonic acid 2.8 Heptanedioic acid 4.3

  39. Carbonic Acid

  40. O HOCOH Carbonic Acid + H2O CO2 99.7% 0.3%

  41. O O HOCO– HOCOH Carbonic Acid + + H2O H+ CO2

  42. O O HOCO– HOCOH Carbonic Acid • CO2 is major species present in a solution of "carbonic acid" in acidic media + + H2O H+ CO2 overall K for these two steps = 4.3 x 10-7

  43. O HOCO– O –OCO– Carbonic Acid Second ionization constant: Ka = 5.6 x 10-11 + H+

  44. Sources of Carboxylic Acids

  45. Synthesis of Carboxylic Acids: Review • side-chain oxidation of alkylbenzenes (Chapter 11) • oxidation of primary alcohols (Chapter 15) • oxidation of aldehydes (Chapter 17)

  46. Synthesis of Carboxylic Acids by the Carboxylation of Grignard Reagents

  47. O RCOMgX O RCOH Carboxylation of Grignard Reagents • converts an alkyl (or aryl) halide to a carboxylic acid having one more carbon atom than the starting halide Mg CO2 RMgX RX diethylether H3O+

  48. •• •• O O •• •• diethylether R C R + O •• •• MgX MgX •• – •• O •• R C OH •• •• Carboxylation of Grignard Reagents d– C O •• •• H3O+

  49. CH3CHCH2CH3 CH3CHCH2CH3 Example: Alkyl Halide 1. Mg, diethyl ether 2. CO2 3. H3O+ Cl CO2H (76-86%)

  50. CH3 CH3 Br CO2H Example: Aryl Halide 1. Mg, diethyl ether 2. CO2 3. H3O+ (82%)

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