1. Chapter 18 Carboxylic Acids Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 18.1 Carboxylic Acid Nomenclature

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

4. Table 18.1 Systematic NameCommon Name methanoic acidformic acid ethanoic acidacetic acid octadecanoic acidstearic acid Common names are based on natural origin rather than structure. O HCOH O CH 3 COH O CH 3 (CH 2 ) 16 COH

5. Table 18.1 Systematic NameCommon Name 2-hydroxypropanoic acidlactic acid (Z)-9-octadecenoic acid or (Z)-octadec-9-enoic acid oleic acid O CH 3 CHCOH OH O (CH 2 ) 7 COH C C HH CH 3 (CH 2 ) 7

6. 18.2 Structure and Bonding

7. Formic Acid is Planar CO H HO 120 pm 134 pm

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

9. 18.3 Physical Properties

10. 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°C80°C99°C OH 141°C OH O O

11. 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. H 3 CC OH O CCH 3 O H O

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. Carboxylic acids are similar to alcohols in respect to their solubility in water. They form hydrogen bonds to water. Solubility in Water H 3 CC OH O O H O H H H

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

15. But carboxylic acids are far more acidic than alcohols. Carboxylic Acids are Weak Acids CH 3 COH O CH 3 CH 2 OH pK a = 4.7pK a = 16

16.  G°= 91 kJ/mol  G°= 27 kJ/mol  G°= 64 kJ/mol Free Energies of Ionization CH 3 CH 2 O – + H + CH 3 CH 2 OH CH 3 COH O CH 3 CO – + H + O

17. Greater Acidity of Carboxylic Acids is Attributed Stabilization of Carboxylate Ion by Inductive effect of carbonyl group Resonance stabilization of carboxylate ion RC O O ++ – O O – RC O O –

18. Figure 18.3(b): Electrostatic Potential Maps of Acetic Acid and Acetate Ion Acetic acidAcetate ion

19. 18.5 Substituents and Acid Strength

20. standard of comparison is acetic acid (X = H) Substituent Effects on Acidity X CH 2 COH O pK a = 4.7

21. Substituent Effects on Acidity X H F Cl pKapKa 4.7 2.6 2.9 Electronegative groups increase acidity X H CH 3 CH 3 (CH 2 ) 5 pKapKa 4.7 4.9 Alkyl groups have negligible effect X CH 2 COH O

22. Substituent Effects on Acidity Electronegative substituents withdraw electrons from carboxyl group; increase K for loss of H +. X CH 2 COH O

23. Effect of electronegative substituent decreases as number of bonds between it and carboxyl group increases. pKapKa 2.8 4.1 4.5 CH 3 CH 2 CHCO 2 H Cl CH 3 CHCH 2 CO 2 H Cl ClCH 2 CH 2 CH 2 CO 2 H

24. 18.6 Ionization of Substituted Benzoic Acids

25. Hybridization Effect pKapKa 4.2 4.3 1.8 COH O H2CH2C CH COH O O HC C sp 2 -hybridized carbon is more electron- withdrawing than sp 3, and sp is more electron-withdrawing than sp 2.