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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.