Carboxylic Acids

Chapter 20 2

Carboxylic AcidsThe functional group of carboxylic acids

consists of a C=O with -OH bonded to the same carbon.

Carboxyl group is usually written -COOH.Aliphatic acids have an alkyl group bonded

to -COOH.Aromatic acids have an aryl group.Fatty acids are long-chain aliphatic acids.

=>

Carboxylic Acids Carboxylic acid, compounds of the type

RCOOH such as Acetic acid ( present in vinegar), Malonic acid and Benzoic Acid.

CH3COOHH2C

COOH

COOH

COOH

Acetic acid Malonic acid Benzoic acid

4

Common NamesMany aliphatic acids have historical names.Positions of substituents on the chain are

labeled with Greek letters.

1-Carboxylic Acid Nomenclature

CH3CH2CH2CHCOOH

BrCH3CHCH2COOH

CH3

bromovaleric acid -methylbutyric acid

isovaleric acid

COOH

COOH COOH COOH

CH3

CH3CH3

benzoic acid

o-toluic acid m-toluic acid p-toluic acid

special names

Chapter 20 6

IUPAC NamesRemove -e from alkane (or alkene) name,

add -oic acid.The carbon of the carboxyl group is #1.

CH3CH2CHC

Cl

OH

O

2-chlorobutanoic acid

PhC

HC

H

COOH

trans-3-phenyl-2-propenoic acid (cinnamic acid)

=>

parent chain = longest, continuous carbon chain that contains the carboxyl group alkane, drop –e, add –oic acid

HCOOH methanoic acid

CH3CO2H ethanoic acid

CH3CH2CO2H propanoic acid

CH3

CH3CHCOOH 2-methylpropanoic acid

BrCH3CH2CHCO2H 2-bromobutanoic acid

8

Dicarboxylic AcidsAliphatic diacids are usually called by their

common names (to be memorized).For IUPAC name, number the chain from the

end closest to a substituent.Two carboxyl groups on a benzene ring

indicate a phthalic acid.

HOOCCH2CHCH2CH2COOH

Br

3-bromohexanedioic acid -bromoadipic acid =>

dicarboxylic acids:

HOOC-COOH oxalic acid

HO2C-CH2-CO2H malonic acid

HO2C-CH2CH2-CO2H succinic acid

HO2C-CH2CH2CH2-CO2H glutaric acid

HOOC-(CH2)4-COOH adipic acid

HOOC-(CH2)5-COOH pimelic acid

CO2H

CO2H

CO2H

CO2H

CO2H

CO2Hphthalic acid isophthalic acid

terephthalic acid

CCOOHH

CCOOHH

CCOOHH

CHHOOC

maleic acid fumaric acid

Chapter 20 11

Naming Cyclic AcidsCycloalkanes bonded to -COOH are named as

cycloalkanecarboxylic acids.Aromatic acids are named as benzoic acids.

COOH

CH(CH3)2

2-isopropylcyclopentanecarboxylic acid

COOH

OH

o-hydroxybenzoic acid(salicylic acid)

=>

2-Physical properties1- Boiling pointIntermolecular forces, especially hydrogen

bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight.

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.

2-2-Solubility in WaterSolubility in WaterCarboxylic acids are similar to alcohols in respect to their solubility in water

Form hydrogen bonds to water

HH33CCCC

OO HH OO

OOHHOO

HH

HH

HH

3-Acidity of carboxylic acidsMost carboxylic acids have a pKa close to 5.Carboxylic acids are weak acids but carboxylic

acids are far more acidic than alcohols.

The greater difference in ionization energy (∆ G) of ethanol and acetic acid reflects a greater stabilization of acetate ion relative to ethoxide ion.

Greater acidity of carboxylic acids is attributedstabilization of carboxylate ion by inductive effect of carbonyl group

And by resonance stabilization of carboxylate ion.

Salts of Carboxylic Acids

salts of carboxylic acids:

name of cation + name of acid: drop –ic acid, add –ate

CH3CO2Na sodium acetate or sodium ethanoate

CH3CH2CH2CO2NH4 ammonium butyrate

ammonium butanoate

(CH3CH2COO)2Mg magnesium propionate

magnesium propanoate

4-Substituent and acid strength Alkyl substituents have negligible

effect

Electronegative substituents increase acidity

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

Effect of substituent decreases the acidity as number of bonds between X and carboxyl group increases.

COOH

OCH3

COOH COOH

NO2

COOH

NO2

COOH

NO2

p-methoxy benzoic acid m-nitro p-nitro o-nitro

pKa = 4.46 pKa = 4.19 pKa = 3.47 pKa = 3.41 pKa = 2.16

Ionization of Substituted Benzoic AcidsHybridization Effectsp2-hybridized carbon is more electron-withdrawing

than sp3, and sp is more electron-withdrawing than sp2

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

5-Synthesis of carboxylic acids1- Oxidation of alkylbenzene

2- Oxidation of primary alcohols

3- Oxidation of aldehydes

4-Carboxylation of Grignard reagents

5- Hydroylsis of nitriles

6- Reactions of Carboxylic acids1- Formation of Acyl chlorides

2-Lithium aluminium hydride reduction

3-Esterification

Mechanism of Esterification

Intramolecular ester formation (Lactones)

4-α-Halogenation

This reaction of α- bormination of carboxylic acid is called Hell- Volhard- Zelinasky reaction

5- Decarboxylation of carboxylic acids