18.7Salts of Carboxylic Acids

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Carboxylic Acids are Deprotonated by Strong Bases

Equilibrium lies far to the right; K is ca. 1011.For low molecular weight acids, sodium and potassium carboxylate salts are soluble in water.

strongeracid

weakeracid

RCOH + HO– RCO– + H2O

OO

Unbranched carboxylic acids with 12-18 carbonsgive carboxylate salts that form micelles in water.

Micelles O

ONasodium stearate

(sodium octadecanoate)

CH3(CH2)16CO

O

Na+–

Micelles O

ONa

polarnonpolar

Micelles O

ONa

polarnonpolar

Sodium stearate has a polar end (the carboxylate 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.

Figure 18.5: A micelle

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.

18.818.8Dicarboxylic AcidsDicarboxylic Acids

Dicarboxylic Acids

One carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separation.

Oxalic acid

Malonic acid

Heptanedioic acid

1.2

2.8

4.3

COH

O

HOC

O pKa

HOCCH2COH

OO

HOC(CH2)5COH

O O

18.918.9Carbonic AcidCarbonic Acid

Carbonic Acid

HOCOH

O

CO2 + H2O HOCO–

O

H+ +

99.7% 0.3%

overall K for these two steps = 4.3 x 10-7

CO2 is major species present in a solution of "carbonic acid" in acidic media.

Carbonic Acid

HOCO–

O

–OCO–

O

H+ +

Ka = 5.6 x 10-11Second ionization constant:

18.1018.10Sources of Carboxylic AcidsSources of Carboxylic Acids

side-chain oxidation of alkylbenzenes (Section 11.12)

oxidation of primary alcohols (Section 15.9)

oxidation of aldehydes (Section 17.15)

Synthesis of Carboxylic Acids: Review

18.1118.11Synthesis of Carboxylic AcidsSynthesis of Carboxylic Acids

by theby theCarboxylation of Grignard ReagentsCarboxylation of Grignard Reagents

Carboxylation of Grignard Reagents

RXMg

diethylether

RMgXCO2

H3O+

RCOMgX

O

RCOH

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

R

MgX

C

O••

••

–diethylether

O ••••

MgX+–

R C

O••

•• ••

O ••••

H3O+

••

R C

OH••

••O ••

Carboxylation of Grignard Reagents

Example: Alkyl Halide

CH3CHCH2CH3

(76-86%)

1. Mg, diethyl ether

2. CO2

3. H3O+

CH3CHCH2CH3

Cl CO2H

Example: Aryl Halide

(82%)

1. Mg, diethyl ether

2. CO2

3. H3O+

CH3

CO2HBr

CH3

18.1218.12Synthesis of Carboxylic AcidsSynthesis of Carboxylic Acids

by theby thePreparation and Hydrolysis of NitrilesPreparation and Hydrolysis of Nitriles

Preparation and Hydrolysis of Nitriles

RX RCOH

O

The reactions convert an alkyl halide to a carboxylic acid having one more carbon atom than the starting halideA limitation is that the halide must be reactive toward substitution by SN2 mechanism.

– ••••C NRC ••N

SN2

H3O+

heat+ NH4

+

Example

NaCN

DMSO(92%)

CH2Cl

CH2CN

(77%)

H2OH2SO4

heatCH2COH

O

Example: Dicarboxylic Acid

BrCH2CH2CH2Br

NaCN H2O

(77-86%)NCCH2CH2CH2CN

H2O, HCl heat

(83-85%)HOCCH2CH2CH2COH

OO

via Cyanohydrin

1. NaCN

2. H+

CH3CCH2CH2CH3

O

CH3CCH2CH2CH3

OH

CN

(60% from 2-pentanone)

H2O

HCl, heat

CH3CCH2CH2CH3

OH

CO2H