Carboxylic AcidsChapter 18

Carboxylic Acids

• In this chapter, we study carboxylic acids, another class of organic compounds containing the carbonyl group.• The functional group of a carboxylic acid is a

carboxyl group, which can be represented in any one of three ways.

CO2HCOOHC-OHO

Nomenclature

IUPAC names• For an acyclic carboxylic acid, take the longest carbon

chain that contains the carboxyl group as the parent alkane.• Drop the final -e from the name of the parent alkane

and replace it by -oic acid.• Number the chain beginning with the carbon of the

carboxyl group.• Because the carboxyl carbon is understood to be

carbon 1, there is no need to give it a number.

Nomenclature

• In these examples, the common name is given in parentheses.

• An -OH substituent is indicated by the prefix hydroxy-; an -NH2 substituent by the prefix amino-.

3-Methylbutanoic acid(Isovaleric acid)

Hexanoic acid(Caproic acid)

OH

O

OH

O1 1

63

OH

OOHH2N COOH

5-Hydroxyhexanoic acid

15

4-Aminobenzoic acid

Nomenclature• To name a dicarboxylic acid, add the suffix -dioic acid to

the name of the parent alkane that contains both carboxyl groups; thus, -ane becomes -anedioic acid.• The numbers of the carboxyl carbons are not indicated

because they can be only at the ends of the chain.

O

HOOH

O

Butanedioic acid(Succinic acid)

Ethanedioic acid(Oxalic acid)

Hexanedioic acid(Adipic acid)

Propanedioic acid(Malonic acid)

HO OH

O

OOH

O

OH

O

O

HO

O

HO

1 1

1 1

2 3

4 6OH

O

HO15

O

Pentanedioic acid(Glutaric acid)

Nomenclature

CH3COOHHCOOH

CH3CH2COOHCH3(CH2)2COOHCH3(CH2)3COOHCH3(CH2)4COOHCH3(CH2)6COOHCH3(CH2)8COOHCH3(CH2)10COOHCH3(CH2)12COOHCH3(CH2)14COOHCH3(CH2)16COOHCH3(CH2)18COOH

DerivationCommon Name

IUPAC Name(acid)Structure

Greek: arachis, peanutGreek: stear, solid fatLatin: palma, palm treeGreek: myristikos, fragrantLatin: laurus, laurelLatin: caper, goatLatin: caper, goatLatin: caper, goatLatin: valere, to be strongLatin: butyrum, butterGreek: propion, first fatLatin: acetum, vinegarLatin: formica, ant

arachidicstearicpalmiticmyristiclauric

capriccapryliccaproicvalericbutyricpropionicaceticformic

eicosanoicoctadecanoichexadecanoictetradecanoicdodecanoicdecanoicoctanoichexanoicpentanoicbutanoicpropanoicethanoicmethanoic

Nomenclature

For common names, use, the Greek letters alpha (a), beta (b), gamma (g), and so forth to locate substituents.

C-C-C-C-OHO

OHH2N

O

OHOH

O

(-Aminobutyric acid; GABA)2-Hydroxypropanoic acid4-Aminobutanoic acid

4 3 2

1

4

1

2

(-Hydroxypropionic acid;lactic acid)

Physical Properties

H3C C

O

O

H

CH3C

O

O

H- +

+ -

hydrogen bondingbetween two molecules

Physical PropertiesCarboxylic acids are more soluble in water than are alcohols, ethers, aldehydes, and ketones of comparable molecular weight.

CH3COOHCH3CH2CH2OHCH3CH2CHO

CH3(CH2)2COOHCH3(CH2)3CH2OHCH3(CH2)3CHO

acetic acid

1-propanolpropanal

60.5

60.158.1

1189748

16388.1butanoic acid1-pentanol 88.1 137

103pentanal 86.1

Structure NameMolecularWeight

Boiling Point (°C)

Solubility(g/100 mL H2O)

infinite

infinite

16infinite

2.3slight

larger Ka increased [H3O+] stronger acid

A– + H3O+

[HA][A–] [H3O+]

Ka =

HA + H2O

larger Ka increased [H3O+] stronger acid

A– + H3O+

[HA][A–] [H3O+]

Ka =

HA + H2O

increased [H3O+] stronger acid

A– + H3O+

[HA][A–] [H3O+]

larger Ka

Ka =

HA + H2O

larger Ka increased [H3O+] stronger acid

A– + H3O+

[HA][A–] [H3O+]

Ka =

HA + H2O

RCOOH + H2O RCOO– + H3O+

[RCOOH][RCOO–] [H3O+]

Ka =

RCOOH + H2O RCOO– + H3O+

[RCOOH][RCOO–] [H3O+]

Ka =

acids > phenols ~ thiols > water ~ alcohols

Ka % ionized [H3O+], M pH

~1 107 ~100 ~0.1 1.00

1.8 10–5 1.3 1.3 10–3 2.88

3.3 10–10 0.0036 3.6 10–6 5.44

2.5 10–11 0.0016 1.6 10–6 5.80

1.3 10–16 0.0001 1.0 10–7 7.00

HCl

HOAc

PhOH

EtSH

EtOH

HOH

Comparative acidities of 0.1 M aqueous solutions of representative acids HA

1.8 10–16 0.0001 1.0 10–7 7.00

Fatty AcidsTable 18.3 The Most Abundant Fatty Acids in Animal Fats, Vegetable Oils, and Biological Membranes.

Unsaturated Fatty Acids

Saturated Fatty Acids

20:4

18:3

18:2

18:1

16:1

20:0

18:0

16:0

14:0

12:0

Carbon Atoms:Double Bonds*

Melting Point(°C)

Common NameStructure

-49

-11

-5

16

1

77

70

63

58

44

arachidonic acid

linolenic acid

linoleic acid

oleic acid

palmitoleic acid

arachidic acid

stearic acid

palmitic acid

myristic acid

lauric acid

CH3(CH2)1 2COOH

CH3(CH2)1 0COOH

CH3(CH2)1 4COOH

CH3(CH2)1 6COOH

CH3(CH2)1 8COOH

CH3(CH2)7CH=CH(CH2 )7COOH

CH3(CH2)5CH=CH(CH2 )7COOH

CH3(CH2)4 (CH=CHCH2 )2(CH2)6COOH

CH3CH2 (CH=CHCH2 )3(CH2)6COOH

CH3(CH2)4 (CH=CHCH2 )4(CH2)2COOH

* The first number is the number of carbons in the fatty acid; the second is the number of carbon-carbon double bonds in its hydrocarbon chain.

Fatty AcidsUnsaturated fatty acids generally have lower melting points than their saturated counterparts.

COOH

COOH

COOH

COOH

Stearic acid (18:0)(mp 70°C)

Oleic acid (18;1)(mp 16°C)

Linoleic acid (18:2)(mp-5°C)

Linolenic acid (18:3)(mp -11°C)

Fatty AcidsSaturated fatty acids are solids at room temperature.• The regular nature of their hydrocarbon chains allows

them to pack together in such a way as to maximize interactions (by London dispersion forces) between their chains.

COOH

COOH

COOH

COOH

COOH

Fatty Acids

In contrast, all unsaturated fatty acids are liquids at room temperature because the cis double bonds interrupt the regular packing of their hydrocarbon chains.

COOH

COOH

COOH

COOH

COOH

Soaps

• Natural soaps are sodium or potassium salts of fatty acids.• They are prepared from a blend of tallow and

palm oils (triglycerides).• Triglycerides are triesters of glycerol.• The solid fats are melted with steam and the

water insoluble triglyceride layer that forms on the top is removed.

21

Soaps

Preparation of soaps begins by boiling the triglycerides with NaOH. The reaction that takes place is called saponification (Latin: saponem, “soap”). Boiling with KOH gives a potassium soap.

22

Soaps

Figure 18.2 In water, soap molecules spontaneously cluster into micelles, a spherical arrangement of molecules such that their hydrophobic parts are shielded from the aqueous environment, and their hydrophilic parts are in contact with the aqueous environment.

23

Soaps

Figure 18.3 When soaps and dirt, such as grease, oil, and fat stains are mixed in water, the nonpolar hydrocarbon inner parts of the soap micelles “dissolve” the nonpolar substances.

24

Soaps

• Natural soaps form water-insoluble salts in hard water.

• Hard water contains Ca2+, Mg2+, and Fe3+ ions.

25

Detergents

The problem of formation of precipitates in hard water was overcome by using a molecule containing a sulfonate (-SO3

- ) group in the place of a carboxylate (-CO2

-) group.• Calcium, magnesium and iron salts of sulfonic acids,

RSO3H, are more soluble in water than are their salts of fatty acids.• Following is the preparation of the synthetic detergent,

SDS, a linear alkylbenzenesulfonate (LAS), an anionic detergent.

26

Detergents

• Among the most common additives to detergents are foam stabilizers, bleaches, and optical brighteners.

27

Decarboxylation

• Decarboxylation: The loss of CO2 from a carboxyl group.• Almost all carboxylic acids, when heated to a very high

temperature, will undergo thermal decarboxylation.

• Most carboxylic acids, however, are resistant to moderate heat and melt and even boil without undergoing decarboxylation.• An exception is any carboxylic acid that has a carbonyl

group on the carbon b to the COOH group.

O

RCOH RH CO2decarboxylation +

high temperature

Decarboxylation

• Decarboxylation of a b-ketoacid.

• The mechanism of thermal decarboxylation involves (1) redistribution of electrons in a cyclic transition state followed by (2) keto-enol tautomerism.

OH

OO O

CO2Acetone3-Oxobutanoic acid

(Acetoacetic acid)

+warm

O OH

O

OH

C

O

O

OCO2

+

enol ofa ketone

(A cyclic six-membered transition state)

(1) (2)

Decarboxylation• An important example of decarboxylation of a b-ketoacid in

biochemistry occurs during the oxidation of foodstuffs in the tricarboxylic acid (TCA) cycle. Oxalosuccinic acid, one of the intermediates in this cycle, has a carbonyl group (in this case a ketone) b to one of its three carboxyl groups.

COOHCOOH

O

HOOC COOH

O

HOOC CO2+

Oxalosuccinic acid

only this carboxyl has a C=O beta to it.

-Ketoglutaric acid