Carboxylic Acids

Bettelheim, Brown Campbell and Farrell

Chapter 18

Introduction to Carboxylic Acids

• Carboxylic acids

• Derivatives of carboxylic acids– Anhydrides, Esters, and Amides– Made by reacting a carboxyl acid group with

another molecule. H2O is formed in each reaction

RCOHO

RCOR'O

RCOCR'O O

RCNH2

O

RC-OHO

H-OCR'O

RC-OHO

H-OR' RC-OHO

H-NH2

A carboxylic acid An esterAn anhydride An amide

Carboxylic Acids

• The functional group of a carboxylic acid is a carboxyl groupcarboxyl group, which can be represented in any one of three ways

CO2HCOOHC-OHO

Naming Carboxylic Acids

• IUPAC names– Take longest carbon chain that contains the

carboxyl group as the parent alkane– Change the final -ee from the name of the parent

alkane to -oic acidoic acid– Number the chain so that the carboxyl group

carbon is number 1– Carboxyl carbon is understood to be carbon 1,

so we don’t need to include the number in the name

Nomenclature

– Examples: (common name shown 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

Dicarboxylic Acids– Add the suffix -dioic aciddioic acid to the name of the parent

alkane that contains both carboxyl groups– Carboxylic acid groups must be at ends of chain,

so we do not need to number them

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)

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

– Common names use the Greek letters alpha (), beta (), gamma (), etc. 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

• The carboxyl group contains three polar covalent bonds; C=O, C-O, and O-H– Polarity of carboxyl group determines the

major physical properties of carboxylic acids

Physical Properties– Highly polar group– Two hydrogen bonds can form between groups – Two carboxyl groups create a dimer that behaves

as a higher-molecular-weight compound – Much higher boiling points than other types of

organic compounds of comparable molecular weight

H3C C

O

O

H

CH3C

O

O

H- +

+ -

hydrogen bondingbetween two molecules

Physical Properties– More soluble in water than comparable alcohols,

ethers, aldehydes, and ketones

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

Fatty Acids (carboxylic acids)• Fatty acids: long chain carboxylic acids

– derived from animal fats, vegetable oils, or phospholipids of biological membranes.

– Over 500 have been isolated from various cells and tissues.

• Generally 12 and 20 carbons in an unbranched chain—with even number of carbons

• May be unsaturated:– cis isomer predominates; trans isomers are rare.– Unsaturated fatty acids have lower melting points than

saturated fatty acids.

Fatty Acids

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)7 CH=CH(CH2 )7COOH

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

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

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

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

* 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 Acids• Unsaturated 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 Acids

• Saturated fatty acids are solids at room temperature– Hydrocarbon chains can 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

• 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

Fatty Acids• Fatty acid:Fatty acid: an unbranched-chain carboxylic acid derived

from hydrolysis of animal fats, vegetable oils, or membrane phospholipids– Usually unbranched chain with 10-20 carbons– EVEN number of carbons– May be saturated or unsaturated (C=C)– Unsaturated generally have cis double bonds– Unsaturated fatty acids have lower melting points than their

saturated fatty acids– Most abundant are palmitic acid (16:0), stearic acid (18:0),

and oleic acid (18:1)

Notation of fatty acids

Numbers in parentheses show number of carbons and double bonds

(carbons, double bonds)

Palmitic acid (16:0) has 16 carbons and no double bonds

Oleic acid (18:1) has 18 carbons and 1 double bond

Fatty AcidsCOOH

COOH

COOH

Unsaturated Fatty Acids

Saturated Fatty Acids

20:418:3

18:218:116:1

20:018:016:014:012:0

Carbon Atoms/Double Bonds*

MeltingPoint(°C)

Common Name

-49-11

-5161

7770635844

Arachidonic acidLinolenic acidLinoleic acidOleic acidPalmitoleic acid

Arachidic acidStearic acidPalmitic acidMyristic acid

Lauric acid

Reactions of Carboxylic Acids

Chapter 18: Carboxylic Acids• Acid-Base Properties

– Ionization and pH– Reaction with base

• Esterification (Reaction with Alcohol)• Reduction (NaBH4 or LiAlH4 )• Decarboxylation

Chapter 19: Derivatives of Carboxylic Acids • Reaction with Acids: Anhydride formation• Reaction with Amines: Amide formation

Acidity of Carboxylic Acids• Carboxylic acids are weak acids

– Ka generally in range of 10-4 to 10-5 for most unsubstituted aliphatic and aromatic carboxylic acids

– pKa is pH at which half of acid has lost its H

– pKa range is 4 - 5

CH3COHO

H2O CH3CO-

OH3O

+

[CH3COO-][H3O+]

[CH3COOH]= 1.74 x 10-5Ka =

pKa = 4.76

++Example:

Acetic Acid

Acidity of RCOOH

– Highly electronegative substituents, such as -OH, -Cl, and -NH3

+, near the carboxyl group increase the acidity of carboxylic acids

– Pull electron density away from carboxyl group– Both dichloroacetic acid and trichloroacetic acid

are stronger acids than H3PO4 (pKa 2.1)CH3COOH ClCH2COOH Cl2CHCOOH Cl3CCOOHFormula:

pKa:

Name:

Increasing acid strength2.86

Chloroaceticacid

0.70

Trichloroaceticacid

1.48

Dichloroacetic acid

Acetic acid4.76

Ionization versus pH

• The form in which a carboxylic acid exist in an aqueous solution depends on the solution’s pH

• Very important in biological systems

R-C-OHO OH-

H+ R-C-OHO

R-C-O-O

H+

OH-

R-C-O-O

at pH 2.0or lower

at pH 8.0or higher

+

at pH = pKa = 4.0 - 5.0both forms are present in

approximately equal amounts

Reaction With Bases• All carboxylic acids, whether soluble or

insoluble in water, react with strong bases (NaOH, KOH) to form water-soluble salts

– Also form water-soluble salts with ammonia and amines (weak bases)

COOH NaOHH2O

COO- Na

+H2O+ +

Benzoic acid(slightly soluble in water)

Sodium benzoate(60 g/100 mL water)

COOH NH3H2O

COO- NH4

++

Ammonium benzoate(20 g/100 mL water)

Benzoic acid(slightly soluble in water)

Reaction With Bases

– Carboxylic acids react with sodium bicarbonate and sodium carbonate to form water-soluble sodium salts and carbonic acid, H2CO3

– Carbonic acid then decomposes to give water and carbon dioxide gas CO2

CH3COOH NaHCO3H2O

CH3COO- Na

+CO2 H2O+ + +

Acetic acid Sodium acetate

Reduction of Carboxylic Acids

Fischer Esterification• Fischer esterificationFischer esterification is commonly used to make

esters– Carboxylic acid is reacted with an alcohol in the

presence of an acid catalyst, such as concentrated sulfuric acid

– Fischer esterification is reversible– Can drive reaction in either direction by altering

experimental conditions (Le Chatelier’s principle)

CH3C-OHO

H-OCH2CH3

H2SO4CH3COCH2CH3

OH2O

Ethanoic acid(Acetic acid)

++

Ethyl ethanoate(Ethyl acetate)

Ethanol(Ethyl alcohol)

Fischer Esterification

– Alcohol adds to the carbonyl group of the carboxylic acid to form a tetrahedral carbonyl addition intermediate

– Intermediate then loses H2O to form an ester

CH3CO

OH

OCH2CH3

H H2SO4CH3C

O-HOCH2CH3

OH

H2SO4CH3COCH2CH3

OH2O+

+

A tetrahedral carbonyladdition intermediate

Soaps• Natural soaps are prepared by boiling lard

or other animal fat with NaOH, in a reaction called saponificationsaponification (Latin, sapo, soap)

Sodium soaps

1,2,3-Propanetriol(Glycerol; Glycerin)

A triglyceride(a triester of glycerol)

+

saponification+CH

CH2OCR

CH2OCR

CHOH

CH2OH

CH2OH

RCO 3NaOH

3RCO- Na

+

O

O

O

O

Soaps• In water, soap molecules spontaneously cluster

into micellesmicelles, a spherical arrangement of molecules such that their hydrophobichydrophobic parts are shielded from the aqueous environment, and their hydrophilichydrophilic parts are in contact with the aqueous environment.

Soaps• Soaps clean by acting as emulsifying agents

– Long hydrophobic hydrocarbon chains cluster so as to minimize their contact with water

– Polar hydrophilic carboxylate groups remain in contact with the surrounding water molecules

– These two forces cause soap molecules to form micelles

Soaps• When soap is mixed with dirt (grease, oil,

and fat stains), soap micelles “dissolve” these nonpolar, water-insoluble molecules.

Soaps

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

• Hard waterHard water contains Ca(II), Mg(II) and Fe(III) ions.

++

A sodium soap(soluble in water as micelles)

Calcium salt of a fatty acid (insoluble in water)

2CH3(CH2)1 4COO-Na+ Ca

2 + [CH3 (CH2)14COO-]2 Ca

2+2Na

+

Detergents• Can overcome problem of precipitates in by

using a molecule containing a -SO3- group

(sulfonic acid group) in the place of a -CO2-

group.– Calcium, magnesium and iron salts of sulfonic

acids, RSO3H, are more soluble in water than salts of fatty acids.

– Synthetic detergents can be synthesized from SDS, a linear alkylbenzene sulfonate (LAS), an anionic detergent.