Lecture 20- Carboxylic Acids

General

Organic Chemistry Two credits Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. Carey Third Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Lecture 20

Chapter 12

Carboxylic Acids

Carboxylic Acid Nomenclature

systematic IUPAC names replace "-e" ending of alkane with "oic acid" O HCOH

Systematic Name methanoic acid

O CH3COH O CH3(CH2)16 COH

ethanoic acid octadecanoic acid

common names are based on natural origin rather than structure O HCOH

Systematic Name Common Name methanoic acid

formic acid

ethanoic acid

acetic acid

octadecanoic acid

stearic acid

O CH3COH O CH3(CH2)16 COH

Systematic Name Common Name

O CH3CHCOH OH

2-hydroxypropanoic acid O

CH3(CH2)7

(CH2)7COH C

H

lactic acid

C H (Z)-9-octadecenoic acid

oleic acid

Structure and Bonding

Formic Formicacid acidisisplanar planar

Formic Formicacid acidisisplanar planar

O

H C

120 pm H

O 134 pm

Electron ElectronDelocalization Delocalization

R

C

•• • O•

•• O ••

R

+ C

•• •– O• ••

•• O ••

H

H

Electron ElectronDelocalization Delocalization

R

C

•• • O•

•• O ••

R

+ C

•• •– O• ••

•• O ••

H

R

C

•• •– O• ••

+ O •• H

stabilizes carbonyl group

H

Physical Properties

Boiling BoilingPoints Points O

OH

O OH

bp

31°C

80°C

99°C

141°C

Intermolecular forces, especially hydrogen bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight

Hydrogen-bonded Hydrogen-bondedDimers Dimers

O

H

O CCH3

H3CC O

H

O

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.

Solubility SolubilityininWater Water carboxylic acids are similar to alcohols in respect to their solubility in water form hydrogen bonds to water

H O

H

O

H3CC

H O

H

O H

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

Carboxylic Carboxylicacids acidsare areweak weakacids acids but carboxylic acids are far more acidic than alcohols O CH3COH

CH3CH2OH

Ka = 1.8 x 10-5 pKa = 4.7

Ka = 10-16 pKa = 16

Free FreeEnergies Energiesof ofIonization Ionization CH3CH2O– + H+

∆ G°= 64 kJ/mol ∆ G°= 91 kJ/mol

O CH3CO– + H+

∆ G°= 27 kJ/mol

O CH3CH2OH

CH3COH

Salts of Carboxylic Acids

Carboxylic Carboxylicacids acidsare areneutralized neutralizedby bystrong strongbases bases O RCOH + stronger acid

O HO–

RCO– +

H2O weaker acid

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

Micelles Micelles unbranched carboxylic acids with 12-18 carbons give carboxylate salts that form micelles in water O ONa sodium stearate (sodium octadecanoate) O – CH3(CH2)16 CO Na+

Micelles Micelles O ONa nonpolar

polar

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

AAmicelle micelle

Micelles 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.

Substituents and Acid Strength

Substituent Substituent Effects Effects on on Acidity Acidity standard of comparison is acetic acid (X = H) O X

CH2COH

Ka = 1.8 x 10-5 pKa = 4.7

Substituent Substituent Effects Effects on on Acidity Acidity O X

CH2COH

X

Ka

pKa

H

1.8 x 10-5

4.7

CH3

1.3 x 10-5

4.9

CH3(CH2)5

1.3 x 10-5

4.9

alkyl substituents have negligible effect

Substituent Substituent Effects Effects on on Acidity Acidity O X

CH2COH

X

Ka

pKa

H

1.8 x 10-5

4.7

F

2.5 x 10-3

2.6

Cl

1.4 x 10-3

2.9

electronegative substituents increase acidity

Substituent Substituent Effects Effects on on Acidity Acidity O X

CH2COH

electronegative substituents withdraw electrons from carboxyl group; increase K for loss of H+

Substituent Substituent Effects Effects on on Acidity Acidity O X

CH2COH

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

effect of substituent decreases as number of bonds between X and carboxyl group increases

Ionization of Substituted Benzoic Acids

Hybridization Hybridization Effect Effect Ka

pKa

COH O

6.3 x 10-5

4.2

CH

COH O

5.5 x 10-5

4.3

C

COH

1.4 x 10-2

1.8

O

H2C HC

sp2-hybridized carbon is more electronwithdrawing than sp3, and sp is more electronwithdrawing than sp2

Ionization Ionizationof ofSubstituted SubstitutedBenzoic BenzoicAcids Acids X

O COH

Substituent H CH3 F Cl CH3O NO2

ortho 4.2 3.9 3.3 2.9 4.1 2.2

effect is small unless X is electronegative; effect is largest for ortho substituent

pKa meta 4.2 4.3 3.9 3.8 4.1 3.5

para 4.2 4.4 4.1 4.0 4.5 3.4

Sources of Carboxylic Acids

Synthesis Synthesis of of Carboxylic Carboxylic Acids: Acids: Review Review side-chain oxidation of alkylbenzenes oxidation of primary alcohols oxidation of aldehydes

Synthesis of Carboxylic Acids by the Carboxylation of Grignard Reagents

Carboxylation Carboxylation of of Grignard Grignard Reagents Reagents O RX

Mg diethyl ether

RMgX

converts an alkyl (or aryl) halide to a carboxylic acid having one more carbon atom than the starting halide

CO2

RCOMgX H3O+ O RCOH

Example: Example: Alkyl Alkyl Halide Halide

CH3CHCH2CH3 Cl

1. Mg, diethyl ether 2. CO2 3. H3O+

CH3CHCH2CH3 CO2H (76-86%)

Example: Example: Aryl Aryl Halide Halide 1. Mg, diethyl ether CH3 Br

2. CO2 3. H3O+

CH3 CO2H (82%)

Synthesis of Carboxylic Acids by the Preparation and Hydrolysis of Nitriles

Preparation Preparation and and Hydrolysis Hydrolysis of of Nitriles Nitriles RX

•• – C

SN2

N ••

RC

N ••

H3O+ heat

O RCOH + NH4+

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, i.e. best with primary, then secondary…… tertiary gives elimination

Example Example NaCN

CH2Cl

CH2CN

DMSO (92%)

O CH2COH (77%)

H2O H2SO4 heat

Reactions of Carboxylic Acids: A Review and a Preview

Reactions Reactionsof ofCarboxylic CarboxylicAcids Acids

Acidity Reduction with LiAlH4 Esterification Reaction with Thionyl Chloride

Acid-Catalyzed Esterification

Acid-catalyzed Acid-catalyzed Esterification Esterification (also called Fischer esterification) O

H+

COH + CH3OH O

COCH3 + H2O Important fact: the oxygen of the alcohol is incorporated into the ester as shown.