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

18.1 Carboxylic Acid Nomenclature

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"

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

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

18.2 Structure and Bonding

Formic Acid is Planar CO H HO 120 pm 134 pm

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

18.3 Physical Properties

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

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

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.

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

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

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

 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

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 –

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

18.5 Substituents and Acid Strength

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

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

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

Effect of electronegative substituent decreases as number of bonds between it and carboxyl group increases. pKapKa 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

18.6 Ionization of Substituted Benzoic Acids

Hybridization Effect pKapKa 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.