Chapter 18 Carboxylic Acids

1. Chapter 18Carboxylic Acids

2. 18.1Carboxylic Acid Nomenclature

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

4. O HCOH O CH3COH O CH3(CH2)16COH Table 18.1 (page 778) • 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 (page 778) Systematic Name Common Name 2-hydroxypropanoic acid lactic acid oleic acid (Z)-9-octadecenoic acid

6. 18.2Structure and Bonding

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

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

9. 18.3Physical Properties

10. 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 (1 atm) 31°C 80°C 99°C

11. 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.

12. 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.

13. 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

14. 18.4Acidity of Carboxylic Acids • Most carboxylic acids have a pKa close to 5.

15. O CH3COH Carboxylic acids are weak acids • but carboxylic acids are far more acidic than alcohols CH3CH2OH pKa = 4.7 pKa = 16

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

17. O – RC O + – •• •• 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

18. Figure 18.3(b). Electrostatic potential maps ofacetic acid and acetate ion Acetic acid Acetate ion

19. 18.5 Substituents and Acid Strength

20. O CH2COH X Substituent Effects on Acidity standard of comparison is acetic acid (X = H) pKa = 4.7

21. O CH2COH X X X pKa pKa Alkyl groups have negligible effect Electronegative groups increase acidity H H 4.7 4.7 4.9 2.6 CH3 F CH3(CH2)5 Cl 2.9 4.9 Substituent Effects on Acidity

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

24. CH3CH2CHCO2H 2.8 Cl CH3CHCH2CO2H 4.1 Cl 4.5 Effect of electronegative substituent decreasesas number of bonds between it and carboxyl group increases. pKa ClCH2CH2CH2CO2H

25. 18.6Ionization ofSubstituted Benzoic Acids

26. pKa O 4.2 COH O 4.3 COH H2C CH O 2.0 COH HC C Hybridization Effect sp2-hybridized carbon is more electron-withdrawing than sp3, and sp is more electron-withdrawing than sp2

27. O X COH Table 18.3 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

28. Hammett Equation Acidityof Substituted Benzoic Acids

29. s Values

30. 18.7Salts of Carboxylic Acids

31. O O Carboxylic Acids are Deprotonated by Strong Bases + + RCOH HO– RCO– H2O strongeracid weakeracid Equilibrium lies far to the right; K is ca. 1011. For low molecular weight acids, sodium and potassium carboxylate salts are soluble in water.

32. O ONa Micelles Unbranched carboxylic acids with 12-18 carbonsgive carboxylate salts that form micelles in water. sodium stearate(sodium octadecanoate) nonpolar polar

33. O Micelles ONa polar nonpolar Sodium stearate has a polar end and a nonpolar "tail“. The polar end is hydrophilic ("water-loving”). The nonpolar tail is hydrophobic ("water-hating”). In water, many stearate ions cluster together to form spherical aggregates; carboxylate ions are on the outside and nonpolar tails on the inside.

34. Figure 18.5: A micelle

35. 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 : emulsion Grease (not ordinarily soluble in water) dissolves in the interior of the micelle and is washed away with the dispersed micelle.

36. 18.8Dicarboxylic Acids

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

38. 18.9Carbonic Acid

39. O O + HOCO– H+ HOCOH overall K for these two steps = 4.3 x 10-7 Carbonic Acid + H2O CO2 99.7% 0.3% CO2 is major species present in a solution of "carbonic acid" in acidic media

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

41. 18.10Sources of Carboxylic Acids

42. Naturalproducts Formic, acetic, butyric, tartaric, citric, oleic, malic… Synthesis of Carboxylic Acids: Review side-chain oxidation of alkylbenzenes (Section 11.12) oxidation of primary alcohols (Section 15.10) oxidation of aldehydes (Section 17.15)

43. 18.11Synthesis of Carboxylic Acidsby theCarboxylation of Grignard Reagents

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

45. •• •• O O •• •• diethylether R C R + O •• •• MgX MgX •• – •• H3O+ O •• R C OH •• •• Carboxylation of Grignard Reagents – C O •• ••

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

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

48. 18.12Synthesis of Carboxylic Acidsby thePreparation and Hydrolysis of Nitriles

49. O – N C •• •• RC N RCOH •• Preparation and Hydrolysis of Nitriles H3O+ RX + NH4+ heat SN2 converts an alkyl halide to a carboxylic acid having one more carbon atom than the starting halide limitation is that the halide must be reactive toward substitution by SN2 mechanism

50. CH2Cl CH2CN H2O H2SO4 O heat CH2COH (77%) Example NaCN DMSO (92%)